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© Copyright 2008
Rail Safety and Standards Board Limited
Issue One December 2008
Railway Group Guidance Note
Rail Safety and Standards Board
Evergreen House
160 Euston Road
London NW1 2DX
Guidance on the Use of Satellite Navigation
Published by
GE/GN8578
GN
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Guidance on the Use of Satellite Navigation
Issue record
Issue
Date
Comments
One
December 2008
Original document
Superseded documents
This Railway Group Guidance Note does not supersede any other Railway Group
documents.
Supply
The authoritative version of this document is available at www.rgsonline.co.uk.
Uncontrolled copies of this document can be obtained from Communications, Rail Safety
and Standards Board, Evergreen House, 160 Euston Road, London NW1 2DX,
telephone 020 7904 7518 or e-mail enquiries@rssb.co.uk. Railway Group Standards and
associated documents can also be viewed at www.rgsonline.co.uk.
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Guidance on the Use of Satellite Navigation
Contents
Section
Description
Part 1
1.1
1.2
1.3
1.4
Introduction
Purpose of this document
Copyright
Approval and authorisation of this document
Advice for readers
Part 2
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9
2.10
2.11
2.12
Overview
Scope
System boundary
Structure of this document
Related documents
Technology overview
The railway context
Locator architecture and interfaces in the railway environment
Onboard architecture
Onboard interfaces
Physical architectures
Providing for locator upgrading
Existing standards in satellite navigation services
7
7
8
9
9
10
10
12
14
14
15
15
16
Part 3
3.1
3.2
3.3
3.4
3.5
3.6
Guidance on the Use of GPS
The Global Positioning System (GPS)
Matters requiring attention when implementing GPS
Augmentation techniques
Locator quality of service
The quality of service parameters
The future – GNSS improvements and developments
17
17
17
20
25
27
29
Part 4
4.1
4.2
4.3
4.4
Guidance on Classes of Locator Requirements
Quality of service parameters
Service class guidance
The three classes of locator
Functional architecture
30
30
30
32
33
Part 5
5.1
5.2
5.3
Choice of Equipment
Introduction
Augmentation choices – summary
Achieving application quality of service
38
38
38
40
Part 6
6.1
6.2
6.3
6.4
6.5
6.6
Design and Installation − Good Practice Guide
Introduction
Implementation process
System integration
Equipment installation
EMC
System approval
43
43
43
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45
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50
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Appendix A
A.1
A.2
Technology Overview
Overview of positioning technologies
Augmentation services
52
52
62
Appendix B
B.1
Interface to a Train Data Bus
Interface ‘C’: Position and speed reporting
75
75
Appendix C
Summaries of Some Applications of GNSS
76
Definitions and Explanations
Abbreviations and Acronyms
78
82
References
86
Tables
Table 1
Table 2
Table 3
Table 4
Table 5
Table 6
Table 7
Figures
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14
Figure 15
Figure 16
Figure 17
Figure 18
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Summary of GPS augmentations performance
Summary of classes of locator requirements
Indicative characteristics of various combinations of GNSS components
Equipment location options
RGSs relevant to system approval
Quality and performance of gyro sensors
Summary of some GNSS applications
Satellite navigation related interfaces to onshore and onboard
applications
Satellite navigation applications and benefits
Satellite navigation equipment sub-functions
Onboard locator interfaces and communications architecture
Effect of railway environment upon GPS performance
Examples of locator augmentation options
Statistical nature of accuracy
Illustration of Class C functional architecture
Illustration of Class B functional architecture
Illustration of Class A functional architecture
Typical installation of a GPS antenna
Typical installation of a GPS receiver for OTMR function
Generic view of satellite navigation services: standalone
Generic view of satellite navigation services: external augmentation
Generic view of satellite navigation services: on-board augmentation
(hybridisation)
Generic view of satellite navigation services: complementary
radionavigation
Galileo institutional arrangements
Horizontal accuracy of IMU following GNSS failure
25
31
40
49
51
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77
8
12
13
14
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28
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36
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Guidance on the Use of Satellite Navigation
Part 1
1.1
Introduction
Purpose of this document
This document gives guidance on good practice for the specification, purchase,
implementation and installation of satellite navigation technology in support of applications
requiring train position and speed. The guidance is provided for train operators, train
builders and service providers.
1.2
Copyright
Copyright in the Railway Group documents is owned by Rail Safety and Standards Board
Limited. All rights are hereby reserved. No Railway Group document (in whole or in part)
may be reproduced, stored in a retrieval system, or transmitted, in any form or means,
without the prior written permission of Rail Safety and Standards Board Limited, or as
expressly permitted by law.
Rail Safety and Standards Board (RSSB) members are granted copyright licence in
accordance with the Constitution Agreement relating to Rail Safety and Standards Board
Limited.
In circumstances where Rail Safety and Standards Board Limited has granted a particular
person or organisation permission to copy extracts from Railway Group documents, Rail
Safety and Standards Board Limited accepts no responsibility for, and excludes all liability
in connection with, the use of such extracts, or any claims arising therefrom. This
disclaimer applies to all forms of media in which extracts from Railway Group Standards
may be reproduced.
1.3
Approval and authorisation of this document
The content of this document was approved by:
Control Command and Signalling Standards Committee on 2 October 2008
This document was authorised by RSSB on 23 October 2008.
1.4
Advice for readers
1.4.1
This guidance note enables suppliers of positioning systems to understand the
railway’s use of these technologies, in order to serve the rail market more
effectively.
1.4.2
The railway industry has identified a number of location based services, based on
the ready availability of navigation data. These services can improve operating
efficiency and later may improve safety, and can enable such diverse
applications as on-board Passenger Information Services (PIS), Selective Door
Operation (SDO) and vehicle tracking. Once sufficiently accurate and reliable
services are available, this application set expands to include train control
applications.
1.4.3
This guidance note is intended to indicate best practice. In order to make best
use of the information, readers should consider how they intend to use satellite
navigation, and for what applications, and then apply their initial ideas to the
outlined framework of deployment.
1.4.4
Alternatively, readers can use this guidance note to gain an overview of the
technologies involved, and how equipment may be selected to achieve different
levels of service, and then assess what is most practical for their circumstances.
1.4.5
Since this guidance note is not intended to set mandatory standards, readers
should consider it to be a framework which enables better judgement of what can
be expected from satellite navigation systems and how this can be effectively
achieved.
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1.4.6
There are already several separate initiatives aimed at providing satellite
navigation facilities for operational applications on trains and, while there is a
need to give each of these initiatives the freedom to innovate, there is also the
risk that, in the absence of any co-ordinating initiatives, unnecessary
incompatibilities could develop. Such a step would thereby limit the value of
these initiatives.
1.4.7
A common approach to satellite navigation should better enable the development
of applications that use navigation information. If standard communications
services are available, both on-train and between a train and its operator,
application designers should be free to focus on the applications themselves and
justify those applications on the marginal benefits of each application, rather than
the application having to bear separately the full costs of a supporting
infrastructure.
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Part 2
2.1
Overview
Scope
2.1.1
The scope of this document is the on-train arrangements that support the use of
satellite-based positioning technologies. It includes onboard augmentation and
external augmentation techniques (introduced in 2.5) and supporting data.
2.1.2
Although the technology presented is Global Positioning System (GPS) based,
the guidance will be applicable to other Global Navigation Satellite Systems
(GNSS) such as GLONASS (the system of the Russian Federation) and Galileo
(the future European system).
2.1.3
Digital technologies, specifically satellite navigation, satellite communications and
broadband are increasingly available for implementing train systems. They are
expected to be key enablers for the modernisation of the British railway system.
Applications include vehicle positioning and speed measurement to support
customer information, customer services, internet and entertainment services.
The precision is capable of exceeding that traditionally available to railway traffic
management, and may over time lead to improved network capacity when
applied to traffic control.
2.1.4
This document provides guidance to duty holders to facilitate the achievement of
a level of system performance in these applications commensurate with their
needs. It provides the foundation for duty holders to establish best practice;
hence using these positioning technologies to meet the needs of the rail market
more effectively.
2.1.5
The goal of the successful integration into railway traffic management and control
of commercially available navigation and positioning technologies is to provide
cost benefit to the railway. To support this goal, this guidance note addresses
use and fitment within the railway environment.
2.1.6
This guidance note does not mandate or recommend any particular arrangement
for the provision of positioning technologies, but sets out the issues and criteria
which direct the choices to be made. It provides a framework within which
innovation may be undertaken, yet avoiding the risk of unnecessary
incompatibilities.
2.1.7
To facilitate installation and effective life-cycle management of the equipment, the
guidance provides for:
a)
One antenna being shared between as many radio frequency applications
as is reasonably practicable
b)
One positioning unit (termed a locator) accommodating the requirements for
as many applications as is reasonably practicable, with a definition of the
common interfaces. This provides for upgrading of the locator by
straightforward replacement, with minimum impact on the applications.
2.1.8
This approach limits the proliferation of multiple equipments providing similar
functions with the potential to compromise one another’s performance.
2.1.9
Guidance on the use of data communication technologies is set out in
GE/GN8579: Guidance on Digital Wireless Technology for Train Operators. This
is relevant where the locator output is communicated to the trackside.
2.1.10
The installation of these systems is subject to the requirements of Railway Group
Standards.
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2.2
System boundary
2.2.1
The boundary to the guidance that is the subject of this document is set out in
Figure 1.
Key:
Interface guidance in the scope of this document
Not in the scope of this document
Covered by GE/GN8579
Guidance on requirements and implementation in the scope of this document
Guidance on implementation in this document
Not in the scope of this document
Data communications between
train and trackside
Signals from
satellite navigation
systems
Onboard applications
Onshore applications
1 Aerial (as far as practicable)
Antenna
Splitter
Satellite
navigation
unit
1 Locator Unit
Figure 1 Satellite navigation related interfaces to onshore and onboard applications
2.2.2
Page 8 of 86
Section 2.9 sets out guidance applicable to the following data interfaces across
the boundary:
a)
The signals from GPS and augmentation service providers. Augmentation is
introduced in 2.5
b)
The interfaces between the locator unit and train systems.
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2.3
Structure of this document
2.3.1
The structure of this guidance note is top-down. The earlier parts address the
system issues concerning the implementation of satellite navigation technology,
and the later parts set out guidance on the equipment. It is comprised of six parts
and three appendices.
2.3.2
The guidance includes a classification of locator requirements. This supports the
management of life-cycle issues that are typical of these technologies, such as
the adoption of commercial non-bespoke products, upgrading and mid-life
replacement of equipment.
2.3.3
This guidance note is structured as follows:
Part 1
General information
This part sets out the background and purpose of this document and to whom it
applies.
Part 2
Introduction to satellite navigation on the railways
This part explains the scope of the guidance. It introduces the concept of a
locator.
Part 3
Guidance on the application of satellite navigation
With the support of Appendix A, this part sets out the technologies and their use.
The system options available for use in a locator based upon satellite navigation
are also set out.
Part 4
Classes of service
This part sets out a framework to categorise the performance requirements to
facilitate the specification of standard products for the railway.
Part 5
Equipment selection
This part outlines how equipment should be selected for different applications.
Part 6
Practical issues
This part sets out the practical issues of implementation and installation.
2.3.4
2.3.5
2.4
The appendices comprise:
a)
Appendix A, which sets out an overview of satellite navigation and the
supporting technologies and services
b)
Appendix B, which sets out guidance on the interface to a train data bus
c)
Appendix C, which sets out a summary of some applications.
Set out at the end of the document are:
a)
Definitions and explanations
b)
Abbreviations and acronyms
c)
References.
Related documents
RSSB research project T671 provided the source material for this guidance note.
RSSB research report T510 (expected mid-2009). The performance of GPS data in the
railway environment. This investigates the variation of GPS data over time.
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RSSB research report T740. The analysis of GPS data from the railway network. This
sets out the quality of the reception of GPS signals over the British railway network as
experienced on the Network Rail data recording trains.
The Galileo project GRAIL deliverables (expected late-2008). This project investigates the
use of GNSS to support applications related to ETCS. It includes a broader review of other
applications in less detail.
GE/GN8577 Guidance on the Application of Selective Door Operation Systems. Applicable
digital communication and positioning technologies are addressed.
GE/GN8579 Guidance on Digital Wireless Technology for Train Operators. The interface
between a locator and communications is addressed.
2.5
Technology overview
2.5.1
The use of satellite navigation
2.5.1.1 Satellite navigation systems have been available for some time. Originally
intended for military use, they are now available to the public and applied
intensively in the aviation, maritime and road transport sectors. With a need for
only limited terrestrial infrastructure they have the potential to be extremely cost
effective for determining position, speed, heading and time.
2.5.2
2.5.1.2
The most well-known global satellite navigation system is GPS (see Appendix A,
A.1.3). This is a ubiquitous navigation system with a consistent and predictable
performance.
2.5.1.3
When used by the general public, GPS is apparently easy to use. However,
there are a number of subjects where, in order to obtain satisfactory and
consistent performance in a professional context, attention is required. Examples
are the type of antenna, and the manner in which a locator behaves when
satellites are obscured. A professional user should understand the behaviour of
GPS signals in the railway environment and how to relate this to the needs of the
application. A range of techniques, collectively known as augmentation, are
available to limit the consequences of this behaviour.
2.5.1.4
The behaviour of the GPS signals and the augmentations is predictable. An
informed approach to the specifications of requirements ensures that projects and
their applications are not put at risk from unexpected GPS characteristics. This
guidance note provides the foundations for this informed approach.
GPS, augmentation and quality of service
2.5.2.1 Part 3, together with Appendix A, sets out guidance on satellite navigation
technology, its behaviour and the consequences, and the augmentation choices
available to mitigate the consequences when they are unacceptable. Part 3
concludes by defining a set of parameters that characterise this behaviour.
2.5.2.2
2.6
Part 4 sets out the use of these parameters to define three classes of locator
requirements for general use within the railway. A safety-critical extension for
future use is also provided for.
The railway context
2.6.1
Page 10 of 86
Satellite navigation technologies have developed at a very rapid pace over recent
years and are offering higher performance and functionality with reducing costs.
The railway industry is taking advantage of the development of satellite
navigation and has introduced these technologies in a number of applications.
The consequence is that trains are becoming equipped with more than one GPS
system. Train systems integration is becoming problematic, as there are
limitations of antenna location, data communication, data preparation and GPS
performance that have to be taken into account to optimise installation and train
maintenance through the life cycle.
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2.6.2
Satellite navigation technology, augmented by other position determination
methods, where appropriate, is seen as being suitable for a large range of the
applications, many of which are already in service. This range includes (amongst
others):
a)
The provision of time synchronisation for onboard systems
b)
Passenger Information Systems (PIS)
c)
Fleet / freight tracking
d)
On-Train Monitoring Recorder (OTMR)
e)
Passenger Load Determination (PLD)
f)
Automatic Passenger Counting (APC)
g)
Personal Digital Assistant (PDA)
h)
Traffic control and management
i)
Advisory speeds for energy management
j)
Energy metering
k)
Electronic ticketing
l)
Electronic seat reservations
m) Closed Circuit Television (CCTV) time and position watermarks
2.6.3
n)
Selective Door Operation (SDO)
o)
Fault logging
p)
Engine efficiency
q)
Track monitoring
r)
Odometer calibration
s)
Odometer for European Rail Traffic Management System (ERTMS).
Figure 2 sets out a schematic of the functional architecture that examples of
applications (on the left side of the figure) and railway stakeholders (on the right
side of the figure) can share. It is implicit in this figure that some applications
require a data communications link with the trackside, for example, depots and
control centres. GE/GN8579 sets out guidance on the implementation of shared
data connections between the train and the trackside.
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Guidance on the Use of Satellite Navigation
S
T
A
K
E
H
O
L
D
E
R
WLAN, GSM
GPRS, 3G …
Entertainment/WiFi
Figure 2 Satellite navigation applications and benefits
2.6.4
2.7
The trend is for the future commercially available equipment to provide greater
cost benefit for the railway. Guidance is therefore set out to indicate the
standardisation and methodology needed for the future use and fitment of
positioning technologies within the railway environment.
Locator architecture and interfaces in the railway environment
2.7.1
Page 12 of 86
A locator unit determines time, the train location, train speed and the heading at a
predetermined rate. These are communicated to the dependent applications
through one or more data interfaces.
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Key:
Not in the scope of this document
Guidance on implementation in the scope of this document
Guidance on data interface standards in the scope of this document
Antenna(s)
Signals to / from
systems sharing
antenna
Splitter
Interface B
Cabling
Power
supply
LOCATOR
Power from
train supply
Interface C
Satellite navigation
outputs in standard
format in the train data
bus protocol
Interface A
Figure 3 Satellite navigation equipment sub-functions
2.7.2
Figure 3 sets out the basic equipment required for an on-board satellite
navigation based location. The basic equipment comprises:
a)
A combined omnidirectional (active) antenna, shared with other systems (for
example, GSM-R) as far as practicable. It should be noted that some
applications can use two GPS antennas
b)
A signal splitter for sharing radio signals with those other systems, with an
appropriate amplifier
c)
A locator unit, fitted close to the antenna, giving a standardised output and
taking standard inputs from train systems, where appropriate
d)
A power supply unit.
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2.7.3
Figure 4 develops Figure 3 with more detail. It shows the data communication
arrangements in a generic architecture. This uses a single locator unit. This
provides data to recipient functions over a data bus arrangement. For centralised
applications, for example traffic regulation, the information is required at the
trackside. The data communication to these trackside functions is shown through
a communications gateway, for which the guidance is set out in GE/GN8579.
Interface ‘B’
Driver
Information
Locator
Unit
Interface ‘A’
Data bus
Comms
Gateway
Position and
Event
Reporting
Notes:
1.
2.
3.
Passenger
Information
OTMR
Shaded boxes represent possible users of time and
positioning data
An older train may not be fitted with a train data bus
Interface C, power supply, not shown
GE/GN8579
Figure 4 Onboard locator interfaces and communications architecture
2.8
2.9
Onboard architecture
2.8.1
This guidance addresses the functional components of a GPS-based locator.
Figure 4 sets out example applications in their functional sense. The
implementation of the applications requires the physical implementation of these
functions to be allocated to physical units.
2.8.2
A design judgement should be made to support the modularity of the equipment,
that gives preference to a single locator when this simplifies the integration of the
onboard systems. The allocation of functions between the locator and an
application should be decided by the user with the support of the designer
authorities. Criteria which should be part of the decision are:
a)
The responsibilities for data preparation when in service
b)
The scopes of supply where more than one supplier is contracted
c)
The systems already on the train and their architectures.
Onboard interfaces
2.9.1
Page 14 of 86
Figure 4 sets out the locator unit having standardised interfaces. These are:
Interface A:
Train data bus
Interface B:
Data from the antenna(s)
Interface C:
Power supply.
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2.10
2.11
2.9.2
The use of standard interfaces gives clarity and stability to application
developers. With performance and functional requirements they provide a basis
for interoperability over the network.
2.9.3
Clear guidance on antenna specifications and interfaces facilitates the integration
of onboard systems.
2.9.4
Interface A is the subject of guidance set out in Appendix B.
2.9.5
Interface B is determined by the antenna characteristics. Criteria that determine
this are the locator input requirements, the augmentation services chosen, if any,
and the requirements on the splitter (see Figure 3) if any.
2.9.6
Interface C is dependent upon the train power supply arrangements, for which
guidance is set out in 6.4.5.
2.9.7
On new build, the applications should share a common locator box and
communicate over the train data bus, unless a case to implement more than one
locator can be made. This also provides the interface to the communications
facilities between the train and the trackside, which should conform to the
guidance set out in GE/GN8579.
2.9.8
Bespoke interfaces may be required for particular applications, especially when
retro-fitting to existing rolling-stock.
Physical architectures
2.10.1
Physical architectures should be based on the guidance set out for interfaces and
data communications. Part 5 and Appendix B contain the main guidance.
2.10.2
Effective application requires a considered choice of modules and their functions.
These are determined in part by the specifications of the Commercial Off-TheShelf (COTS) equipment available, and in part by the needs of the application(s).
2.10.3
Physical architectures should support upgrading.
Providing for locator upgrading
2.11.1
During the lifetime of a train, technology improvements to the provision of
navigation data are ongoing, and the cost / performance ratio indicators of
equipment can be expected to decrease well within the economic life of any
rolling-stock. This enables the cost-effective provision of more demanding
services. To facilitate the retro-fitting of such improved equipment to trains, the
upgrade process should be reduced to ‘box swapping’, and possibly cards.
Cables and antennas should be retained whenever possible.
2.11.2
Equipment should be scaleable to evolving application requirements, and a clear
upgrade path supports the use of satellite navigation for increasing and more
demanding applications through the life of the rolling-stock.
2.11.3
Figure 4 sets out a locator unit receiving GPS data from an antenna and
providing positioning, speed and time data to a data bus. The data bus
distributes this data to a number of users, for example:
a)
There could be a position and event reporting unit, which converts the
locator output into reports as a service to other applications
b)
A communications gateway for transmitting the position, speed and event
reports to the trackside
c)
Onboard applications, such as passenger information, on train monitoring
recording and driver displays.
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2.12
2.11.4
To provide for upgrading, open standards should be used in preference to
proprietary standards or the development of bespoke solutions. This encourages
the development of more cost-effective COTS equipment and standard upgrade
paths.
2.11.5
When considering upgradeability, an architecture should be considered that can
accept a choice of augmentations successively, for example the selected satellite
navigation antenna installed for GPS could be chosen to receive Galileo and the
European Geostationary Navigation Overlay Service (EGNOS) signals as well.
Therefore, only a receiver upgrade would be required. As another example, an
interface and software for a future Inertial Measurement Unit (IMU) could be
included.
2.11.6
Upgrading can be accomplished either by swapping the locator unit, and the
communications unit, as set out in GE/GN8579, if necessary, or by the swapping
or addition of cards within a unit provided with the necessary internal interfaces.
2.11.7
The common locator unit should be specified to the most stringent requirements
of the dependent applications and aligned with one of the classes set out in
Part 4.
2.11.8
A form and fit envelope specification within which the common locator unit can be
packaged should be considered.
2.11.9
When considering life-cycle costs, the contribution of data generation and its
modification and assurance, should be included.
Existing standards in satellite navigation services
2.12.1
The standards currently in place for satellite navigation open public services are
highly unlikely to change without having backwards compatibility. There are longterm government commitments and international co-operation agreements
between the US, EU and other political institutions to provide the necessary
stability.
2.12.2
The standards for satellite navigation interfaces in place for current systems and
in development for future systems are:
2.12.3
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a)
The format of the GPS signal-in-space is a given, set out in the GPS ICD [1]
b)
Open standards are available for the formats and signal structures of the
publicly available external augmentation systems (see Part 3); these
standards include RTCM SC-104 versions 2.3 [2] and 3.0 [3],
ITU-M823-3 [4] and the EGNOS ICD [5]
c)
Open standards for safety-critical applications are being updated by
international organisations, such as the International Electrotechnical
Commission (IEC) to take account of satellite positioning developments,
such as Galileo
d)
The standards for non-public (commercial) augmentation systems (see
Part 3) are proprietary and the signals are encrypted.
Other industries have adopted the following standards to define satellite receiver
outputs:
a)
An open standard exists and is widely used – receiver outputs are defined in
NMEA 0183 [6] which is a serial communications one-talker-many-listeners
protocol
b)
Manufacturer-specific proprietary standards also exist.
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Part 3
Guidance on the Use of GPS
This part sets out the satellite navigation services available and the behaviour of their signals. The
performance of these services has particular characteristics, and these strongly influence the quality of
service required from the augmentations used by a locator. These provide the means to control the
variability of the satellite signals to obtain a desired level of quality of service from the locator. The
section concludes by introducing and explaining the parameters chosen to describe quality of service.
Because it is well-known, and is at the time of publication the only viable satellite navigation system,
GPS is introduced first. The issues that affect a successful implementation of GPS are also set out.
Augmentation (introduced in 2.5) and its consequences (for example, additional radio receivers, the
use of inertial devices, or more complex data according to the option) are also set out.
The enhancements foreseen to GPS and the introduction of other satellite systems are set out under
the generic term GNSS.
3.1
The Global Positioning System (GPS)
3.1.1
Appendix A, A.1.2 sets out the basic facts of the GPS system. GPS has a
number of features and, for the railway user, the following statements are
important:
a)
GPS is free at the point of delivery. There is no contract for the provision of
service, nor are there guarantees on the navigation performance that can be
obtained
b)
The performance of GPS is statistical in nature; therefore, the attainable
accuracy varies with time for the reasons set out in 3.2.2.6. To be
meaningful, any statement of accuracy performance should be accompanied
with a level of confidence usually related to the Gaussian distribution, for
example 95% or a σ range. Unless otherwise stated, an accuracy statement
assumes complete visibility of the sky and is usually given with a 95%
confidence. This means that for an average of 5% of the time the accuracy
is less than, and can for short periods of time be much less than, the
accuracy quoted. When there is partial obscuration of the satellites, the
confidence level degrades
c)
Although rare, the presence of system faults and control errors can induce
errors of several hundred metres for a limited period of time.
3.2
Matters requiring attention when implementing GPS
3.2.1
The effects of GPS signal interruptions
3.2.1.1 At switch on, or after a long obscuration, the locator does not have all the
information it needs to process the satellite messages. It takes several minutes
to obtain this information from the routine messages. This is known as the
Time To First Fix (TTFF). Assisted GPS has been developed to reduce this
problem, mainly for the mobile phone market. Appendix A, A.2.1.1, sets out this
augmentation.
3.2.1.2
3.2.2
Following a temporary loss of signal (for example, while in a tunnel) it may take
several seconds to re-acquire the signals and obtain a new position – this is
known as the Time To Fix (TTF). This occurs even when a sufficient number of
satellites are in view. The cause is the manner in which the receivers decode the
data messages. Some trade-off with sensitivity is possible, and simulation can be
used by suppliers to tune their receivers to the railway environment (see 6.2).
Factors affecting accuracy
3.2.2.1 The accuracy achievable varies continuously. The factors that contribute to this
variation are set out in this section.
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3.2.2.2
Ionosphere and troposphere errors:
The satellite signals have to traverse the ionosphere and troposphere. The
electrical properties of these outer parts of the earth cause delay to the signals
and these result in ranging errors. They are also continually varying. The use of
dual frequency receivers is a means to reduce these errors. In future satellite
systems (see Appendix A, A.1.3.2, A.1.4.3 and A.2.2.1) this may be the routine
processing mode. However, the use of the GPS L2 signal is limited to carrier
phase and Doppler processing, as the codes are not available at present to the
open service. This processing is available only in high-end commercial receivers.
3.2.2.3
Satellite clocks:
Time is a fundamental parameter to GPS signal processing. Each satellite has
an atomic clock on board which is extremely accurate. Even so, the variability
between them, although small, results in errors that are a major source of the
inaccuracies experienced.
3.2.2.4
Ephemeris:
Part of the GPS navigation messages contains the ephemeris data that informs
the locator of the position of each satellite for ranging purposes. This cannot be
exact, and the errors can occasionally become significant, especially when
control errors occur.
3.2.2.5
Satellites in view:
A locator without augmentation requires a minimum of four satellites in view to
provide a complete solution, providing position in three axes, plus speed, heading
and time. The satellites are not geo-synchronous and therefore their position in
the sky changes. They can be considered to be quasi-static for a period of about
10 minutes, as follows:
3.2.2.6
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a)
The constellation of satellites is distributed evenly around the earth. The
number in view from the UK at any one time is typically about six and,
routinely, can be nine or more
b)
The number of satellites in view can be reduced by cuttings, foliage,
buildings and in stations, either resulting in decreased accuracy or a loss of
GPS service coverage altogether
c)
The signals are always lost in tunnels, resulting in a loss of service coverage
d)
With certain augmentations and applications (of which the railway has the
potential to be one) the minimum requirement can reduce to two satellites in
view, and one on its own could be useful.
Dilution of precision:
a)
The locator solutions are derived usually by applying ranging techniques to
each satellite in view. The accuracy performance varies with the
geometrical arrangement of the satellites in view with respect to the user.
For example, high accuracy can be achieved if the satellites are evenly
distributed around the user, whereas accuracy can be much lower if the
visible satellites are in the same area of the sky. This happens, typically, for
less than one hour per day. This effect is called the dilution of precision
(DOP). The need for control of the effects of DOP is one of the reasons to
employ augmentation
b)
For guidance, a value of DOP of three or lower is routine and considered
acceptable. For short periods of time the DOP value can exceed 10
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c)
3.2.2.7
When accuracy is quoted, it is prudent to assume, unless otherwise stated,
that the applicable value of DOP is 1, and that the achievable accuracy at
any moment (in the absence of other influences on inaccuracy) is given by
the quoted accuracy multiplied by the prevailing value of DOP.
Multipath effects:
The locator receives GPS signals direct from the satellites, and when tall
buildings or other reflecting surfaces are nearby, also the reflected signals.
These can lead to significant errors. To control them it is necessary to include in
the locator specific processing. Two such techniques are Receiver Autonomous
Integrity Monitoring (RAIM) (see 3.3.1.6b), and carrier phase measurements (see
Appendix A, A.2.2.1).
3.2.2.8
Satellite faults:
Satellite faults, for example errors in the satellite clocks, are detectable by
differential augmentation and receiver augmentation. These faults are rare, but
not rare enough that safety-critical applications can be considered without
employing such fault detection techniques.
3.2.2.9
Jamming and spoofing:
The received signals are very low power and can be vulnerable to unintentional
electromagnetic interference and deliberate jamming. The detection of this type
of error, where seen to be a threat, can be controlled by the same techniques that
detect other sources of inaccuracy and mitigate the effects.
3.2.3
Specifying accuracy requirements
3.2.3.1 This section sets out a process that can be used to assess the accuracy
requirements of an application.
3.2.3.2
It is necessary to assess whether accuracy is important to an application.
Although accuracy is desirable, an understanding of what is necessary should be
achieved.
3.2.3.3
In cases where a geographic database is accessed for information on widely
separated locations, the accuracy of the location is often not critical because
there is no scope for their confusion.
3.2.3.4
Where the location relates to a geographical feature, the accuracy is determined
by the distances apart of such features to ensure that the correct one is identified,
for example a railway route, or a road.
3.2.3.5
Consideration should be given to the frequency with which a wrong location could
be acceptable. This in part is determined by the consequences of an error, the
mitigations, and the possibility that errors are mitigated by a characteristic of the
application.
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LOCATION ACCURACY (m)
Dense foliage next to line
Reduced accuracy
due to lower visibility
(Dilution of Precision – DOP)
High station platform gantry
Loss of service coverage
due to low visibility
Tunnel
Reduced accuracy
due to reflected signals
(multipath effect)
Continued loss of service coverage
outside tunnel as signals are re-acquired
(Time to First Fix – TTFF)
13
Expected accuracy performance
under ideal open skies environment
Complete loss of service coverage
inside tunnel
0
Figure 5 Effect of railway environment upon GPS performance
3.3
Augmentation techniques
3.3.1
The purpose of augmentation
3.3.1.1 Figure 5 illustrates how the performance of an application based solely upon GPS
location could be affected by the normal railway environment, and demonstrates
the effects on both accuracy and service coverage.
3.3.1.2
Satellite signals on their own give sufficient service quality from the locator for
some applications, but without augmentation do not support applications which
require, for example, a high level of service coverage, or consistent high accuracy
in the areas of satellite visibility.
3.3.1.3
GPS augmentation should be considered where:
3.3.1.4
a)
A quality of service coverage is required, or
b)
A consistent accuracy is required, or
c)
A level of confidence in the position and speed data provided by the locator
is required. This is described as integrity. It reduces the probability that an
unknown inaccuracy is present, and is applied to safety-related applications.
The higher the integrity, the lower the probability of an unacceptable
undetected error.
The main augmentation choices are:
a)
Differential augmentation. Ground stations are used to monitor the accuracy
of the GPS signals being received and transmit to users corrections that
improve the accuracy of their locators. The corrections can be broadcast
from either:
i)
Satellites in space, or
ii)
Over terrestrial radio networks
Appendix A, A.2.1 sets out these techniques
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3.3.1.5
3.3.1.6
b)
Hybridisation with other sensors in the locator
c)
More sophisticated processing of the GPS signal-in-space
d)
Map-matching techniques.
Accuracy:
a)
If an application does not require consistent accuracy, or if the consequence
of inaccuracy is benign, then augmentation for accuracy should not be
required. This could be the case of event reporting at spot locations,
especially where the visibility of the sky is good. For guidance, acceptable
inaccuracies of over 50 m may not require augmentation
b)
Where consistent accuracy is required over time and / or space,
augmentation should be considered. For many applications it may be
sufficient to know when an accuracy target is not being achieved, or to have
an estimate of the likely prevailing error. Each case should be judged, but,
for guidance, consistent target accuracies below 50 m could require
augmentation, especially if required where the sky is only partially visible.
Target accuracies below 5 m probably require sophisticated locators to
obtain consistent performance. There will also be rogue values. The
objective is to assess how important it is to be aware of them, how often they
can be accepted and whether any mitigations are required
c)
To some extent, all augmentation methods contribute to improving accuracy
and service coverage, but some are more appropriate than others according
to the requirements. Advice should be obtained from a competent source
when setting out the requirements. Section 3.3.1 summarises the common
augmentation techniques.
Integrity monitoring:
Integrity is provided by two main techniques:
a)
The augmentation ground stations are able to estimate the achievable
accuracy and transmit a warning when a defined threshold is exceeded. In
addition, faulty satellites can be identified. This threshold is currently one
specified by the aviation sector
b)
Locators can use the redundancy available when more than four satellites
are in view and search for the best solution. This technique is called RAIM,
and is set out in Appendix A, A.2.2.2. The techniques can be extended to all
sensors used by a locator.
3.3.2
Space-based augmentation
3.3.2.1 GPS differential corrections from space are broadcast from other satellite
systems. They provide good accuracy performance, but visibility of these other
satellites is required. If the GPS satellites suffer obscuration at the locations of
interest, then it is probable that these other satellites will also be frequently
obscured. Hence, they are not usually a solution to the difficulties of service
coverage. The details of space-based augmentation are set in Appendix A,
A.2.1.2 and A.2.1.3.
3.3.3
Terrestrial augmentation
3.3.3.1 Terrestrial transmission of differential corrections do not suffer from the same
difficulties as satellite visibility, and the signals should be capable of reception on
a train. However, for dependable reception, as well as the signal strength along
the railway, the susceptibility to the railway electro-magnetic environment should
also be assessed. The details of terrestrial differential augmentation are set out
in Appendix A, A.2.1.11 and A.2.1.12.
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3.3.4
Hybridisation with other sensors on a train
3.3.4.1 Hybridisation with other sensors can, in principle, be implemented with almost
any source of location, speed and acceleration data. For railway applications the
most convenient devices are usually inertial devices and tachometers. They can
be used individually or together. They are effective at extending the area of
service coverage, the degree of effectiveness being determined by the size of the
obscured area, the duration required for nominal operation and the accuracy
requirement. For indeterminate periods of obscuration, if continuous operation of
the locator is required, the accuracy requirement should allow for degradation
over time. Appendix A, A.2.2.4, sets out the detail of the use of tachometers, and
A.2.2.5 sets out the detail of the use of inertial devices.
3.3.4.2
3.3.5
Other sensors are much better processed by the data fusion in the locator rather
than the application. However, these sensors may already be used in the
application. In this case it is necessary to ensure that errors are not accumulative
in the final output. Although Figures 8, 9 and 10 allow for both possibilities, it is
necessary to ensure that sensor errors are not accumulated within the locator
output.
Processing of the GPS signals
3.3.5.1 More sophisticated processing of the signal-in-space is becoming increasingly
available in COTS equipment, as follows:
a)
The use of dual frequency receivers should become the standard in future
GNSS equipment, with GPS providing this service as standard as the
current satellites in the constellation are replaced
b)
Carrier phase processing is producing remarkable accuracies and, although
it is sensitive to indirect transmission paths, the processing is becoming
increasingly robust
c)
The use of Doppler information provides solutions that are independent of
the ranging process.
Appendix A, A.2.2.1, sets out more detail relating to these techniques.
3.3.6
Use of map-matching
3.3.6.1 Map-matching techniques can also be employed. Because the train is on a train
route, or a track, the use of coordinates in the solution reduces the dependence
on the number of satellites in view and usefully contributes to the control of errors
associated with hybridisation. The detail of the use of track coordinates is set out
in Appendix A, A.2.2.3. Although the use of only two dimensions in a map is a
possibility, the solution is improved by the use of the three dimensions.
3.3.6.2
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There are two types of map:
a)
A locations database. This contains data to support an application, for
example where to make a particular announcement, which information to
display or which doors to enable. This type of database is implicit in many
applications and, even when confident that amendments and errors can be
managed effectively, requires close management attention. In non-safetyrelated applications it is quite acceptable, but in safety-related applications
the management of the data should meet the SIL required.
b)
Track coordinates. This data supports the data fusion process. As set out in
Appendix A, A.2.2.3, there is confidence that the train is on a track. If
available, map-matching with track coordinates is recommended because
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there is a useful reduction in the minimum number of satellites required to
make a fix, and the effectiveness of RAIM is improved. When using track
coordinates:
i)
The procedure to create one currently is substantial − it should become
much easier in the future
ii)
It should be provided as a package, with a procedure that ensures
amendments, error identification and correction are undertaken
effectively
iii)
An algorithm that ensures that the track is correctly identified at
initialisation, and that afterwards the train’s path is not normally lost, is
desirable, but does not yet exist
iv)
Standard formats for map data are not yet agreed.
3.3.7
Implementation of augmentation in COTS products
3.3.7.1 Many COTS products include one or more augmentation methods. The
procurement cost at the locator level is not usually significant in relation to its lifecycle costs. However, these do include costs such as, for example, antenna
installation, data preparation and management, that can be significant.
3.3.8
Augmentation summary
3.3.8.1 Table 1 sets out the main points of the principal augmentation techniques in
terms of the locator quality of service parameters, service coverage, accuracy
and integrity.
3.3.9
Augmentation and Appendix A
3.3.9.1 Appendix A, A.2, sets out more detail on augmentation as follows:
a)
b)
c)
GPS information services external to the train:
i)
Assisted GPS (A-GPS) (see A.2.1.1)
ii)
Open Space-Based Augmentation Services (SBAS) (see A.2.1.2). In
Europe the only open service is EGNOS (see A.2.1.3)
iii)
Commercial space-based augmentation services (see A.2.1.10)
iv)
Terrestrial augmentation services (see A.2.1.11)
Onboard augmentation in the locator:
i)
Receiver autonomous integrity monitoring (RAIM), and its extension to
all sensor requirements (see A.2.2.2)
ii)
Track coordinates − map-matching techniques as an additional sensor,
for prediction, and to support applications (see A.2.2.3)
iii)
Additional sensors in the on-board locator (see A.2.2.4 and A.2.2.5).
This is known as hybridisation
iv)
More complex GPS signal processing in the on-board locator (see
A.2.2.1)
Other radio-based positioning systems (see A.2.1.12).
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Augmentation service
(with GPS)
No augmentation
Service coverage
(with GPS)
GPS signal not available in
cuttings or stations where view of
satellites is obscured
Accuracy and integrity
(with GPS)
13 m accuracy at 95%
confidence
No integrity, unless RAIM is used
Open space-based
augmentation system
EGNOS
Commercial SpaceBased Augmentation
Systems (SBAS)
Terrestrial differential
GPS (DGPS)
Low-grade inertial
systems
GPS and EGNOS signal not
available in tunnels, cuttings or
stations where view of satellites
is obscured
At present, difficult to receive on
trains via geostationary satellite.
Data can alternatively be
received via terrestrial
communications (for example
SISNeT), but no standard
arrangement for the railway is
yet in place
Not available in tunnels, cuttings
or stations where view of
satellites is obscured.
Communication arrangements
with trains are undefined
Not available in tunnels, cuttings
or stations where view of
satellites is obscured. The
DGPS signal could be subject to
interference on the railway
An inertial measurement unit
(IMU) gives a limited navigation
availability when visibility of the
satellites is lost. Low-grade
systems only maintain the
accuracy of the positioning up to
approximately 20 seconds after
the satellite signals are lost,
before drift progressively
degrades it. Precise performance
not yet known in rail environment
The user needs visibility of the
sky where the service is required.
Subject to DOP variations
1 to 2 m accuracy at 95%
confidence in ‘clear skies’. Actual
accuracy is less in a rail
environment
A level of signal-in-space (SIS)
integrity ‘guaranteed’
The user needs visibility of the
sky where the service is required.
Subject to DOP variations
1 to 2 m accuracy at 95%
confidence
No integrity, although quality of
service reports are available
>99% availability, subject to
satellites being visible
1 to 2 m accuracy at 95%
confidence
Integrity not quantified
The user needs visibility of the
sky where the service is required.
Subject to larger DOP variations
Maximum of 13 m accuracy at
95% confidence (as per GPS),
but drift starts immediately
No integrity unless RAIM is used
Limited availability with rapidly
degrading accuracy for a short
period when satellites are
obscured
Subject to DOP variations
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Augmentation service
(with GPS)
Medium- and high-grade
inertial systems
Map-matching
Service coverage
(with GPS)
Navigating by the IMU capability
when satellites are not visible
depends on the quality of the
inertial system. Medium-grade
systems can maintain the
accuracy of the positioning for up
to approximately one minute,
whereas high-grade systems
have equivalent performance
over several hours
5 to 10 m accuracy at 95%
confidence (dependent on map
accuracy and correctness)
The user needs visibility of the
sky where the service is
required. Subject to DOP
variations
RAIM provides some SIS
integrity
RAIM improves confidence in the
signal regardless of the level of
accuracy
RAIM
Not available in tunnels, cuttings
or stations when number of
satellites in view is limited
May be of some help in detecting
multipath
Accuracy and integrity
(with GPS)
Maximum of 13 m accuracy at
95% confidence (as per GPS),
but drift starts immediately
Integrity available by crosschecking GPS and IMU
Better, but degrading accuracy,
for longer periods (a few minutes)
when satellites are obscured
Limited navigating by the IMU
available in tunnels, cuttings or
stations when view of satellites is
obscured
Depends on use of prediction
algorithms suitable for the railway
Better than 10 m accuracy at
95% confidence
Integrity subject to sufficient (>5)
satellites being visible
The user needs visibility of the
sky where the service is required.
Subject to larger DOP variations
Principle can be applied to all
sensors
Table 1 Summary of GPS augmentations performance
3.4
Locator quality of service
3.4.1
The term quality of service is used to describe the characteristics specified at the
output of a locator. These characteristics describe the quality of the performance
of the locator in the railway environment and in the areas of coverage required.
Where service is needed, a defined minimum accuracy is usually required, and
where the application is safety-related a guaranteed probability of undetected
error or failure is also required (called integrity).
3.4.2
Quality of service is set out by three principal parameters: service coverage,
accuracy and integrity. Each of these parameters is statistical in nature, and
there is no absolute guarantee of a particular level of performance, but rather a
level of confidence that the equipment operates at or above the required
performance. The parameters are interrelated. For example, specifying a high
accuracy has a negative impact on integrity, and a low accuracy can make
integrity more readily achievable. These are set out in more detail in 3.4.
3.4.3
The meanings of these terms in other transport sectors can be different. In
official references, the definition of the performance of the GNSS signals-in-space
conforms to aviation usage. Under the description of ‘Required Navigation
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Parameters’, the aviation sector defines quality of GNSS service parameters that
are similar to, but not the same as, the use of the words ‘availability’, ‘continuity’
and ‘integrity’ in the railway sectors. The definitions in this document, and the
use of the terms, conform to railway usage.
3.4.4
Although the GPS signals have a specified continuity property, this is completely
masked by the variable coverage experienced in the railway environment. The
first decision for a railway application should be to decide upon the service
coverage required from the locator.
3.4.5
Different applications require different qualities of service. For example, the
simplest applications (such as an on-board automatic passenger information
system) only need a basic level of service, getting occasional input from the
locator unit as a 'trigger' to indicate that the train is within a defined geographical
range of a given point. Other applications may require a continuous stream of
highly accurate position information to be available from the locator unit. This
could be the case for trackside functions where the data needs to meet time
delay requirements.
3.4.6
The augmentation techniques introduced above and set out in Appendix A, A.2,
when combined correctly, provide a locator that meets the quality of service
requirements of the application and its geographic area of use.
3.4.7
Often it is possible to achieve the same quality of service with different
combinations of augmentation. Figure 6 sets out a typical set of alternatives.
First, augmentations are added to improve service coverage where visibility of the
satellites is obscured. This is followed by a subsequent step to add augmentation
options to obtain improved and more consistent accuracy. The integrity
performance of a basic GPS system can be obtained by augmentation to add the
necessary integrity monitoring and / or redundancy.
ILLUSTRATIVE
OR
Increasing performance
One-step upgrade
Two-step upgrade
Increase availability
of position report
Increase accuracy,
integrity and availability
GPS
INS
GPS ++ basic
basic IMU
INS
Increase accuracy
and integrity of
position report
GPS
IMU
GPS ++ INS
INS
++ SBAS
SBAS
OR
GPS
GPS
++ basic
+ IMUINS
GPS
basic
INS
++ terrestrial
terrestrial
augmentation
augmentation
OR
GPS
GPS
++ basic
+ IMUINS
GPS
basic
INS
++ RAIM
RAIM
OR
GPS
GPS ++ basic
basic
GPSINS
+ IMU +
INS ++
map
matching
simple
simple map
map
matching
matching
Figure 6 Examples of locator augmentation options
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3.5
The quality of service parameters
3.5.1
Service coverage
3.5.1.1 Service coverage provided by the locator refers to the proportion of the railway
network on which a train’s position can be determined at the required level of
accuracy and integrity. Section 3.2.2 sets out the need to consider the service
coverage required by an application. It is the characterisation of a lack of service
coverage that is the main factor in determining whether an application can
operate with GNSS alone, or whether augmentation is required.
3.5.2
3.5.1.2
It is in the nature of the railway and GNSS processing that some temporary loss
of service coverage should be accepted. The extent of the acceptability is
defined by the application.
3.5.1.3
Depending upon the needs of the application, service coverage can be specified
either:
a)
As the specific geographic area(s) where coverage is required
b)
The maximum distance that can be run before coverage is recovered
c)
The maximum time that can pass before coverage is recovered.
Accuracy
3.5.2.1 Accuracy is the most commonly referenced performance parameter.
Sections 3.2.2 and 3.2.3 set out guidance on assessing the accuracy needs that
apply to all service classes. It is a meaningless parameter without an associated
measure of acceptable statistical variation, as set out in Figure 7. The accuracy
of GNSS and the supporting technologies vary continually in time. An indication
of the instantaneous accuracy can be given by the DOP. The actual error
present at any moment cannot be determined absolutely, for example, because
of the presence of multipath effects, and varying durations of obscuration from
the signals-in space. Accuracy requirements can also require the use of
augmentation techniques.
3.5.2.2
It is the minimum accuracy required that should be specified. For most of the
time, much better accuracy should be obtained. For a small amount of time it is
inevitable that the required minimum accuracy may not be obtained. The
application should be assessed to determine the consequences of inadequate
accuracy and whether measures are required to compensate. In many
applications the occasional operating inconvenience arising from inaccuracy
should be acceptable. In others, an indication of marginal or inadequate
accuracy could be required so that other measures can be put in place. Where,
despite these measures, undetected inaccuracy could lead to safety concerns, a
degree of integrity should be specified to limit the rate at which undetected
inaccuracy or undetected lack of coverage can occur.
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Time series of position measurements
Statistical distribution of accuracy
Measured position
± 2σ
± 2σ
95% of
measurements
Time
Measured position
True position
Measured position
+2σ
-2σ
True
position
Figure 7 Statistical nature of accuracy
3.5.3
Integrity
3.5.3.1 Integrity describes the probability that a failure or error is present at the output of
a locator, and it has not been detected. It is a measure that is applied when the
application is safety related or safety critical. There are three components to
integrity:
a)
Integrity risk (usually expressed as a probability of an undetected failure)
that is to say, that a fault is present somewhere in the system and is not
detected and, as a consequence, the data at the output of the locator is not
trustworthy
b)
Alert limit (also known as the threshold value) is the maximum allowable
error in the measured position before an alarm is triggered. The threshold
value should be greater than the nominal accuracy of the locator unit in
order to avoid excessive false alarms
c)
Time to alarm is the time elapsed between the occurrence of the failure in
the system and its presentation to the user. The failure can be due to an
excessive inaccuracy being detected (defined by the alert limit), or that a
particular satellite or sensor is untrustworthy.
3.5.3.2
Where integrity is required, a form of augmentation is also required. In Europe,
the GPS signals are monitored by EGNOS, as set out in A.2.1.3. The integrity
information is not readily available to trains at present. However, it is readily
available to control centres, which can take appropriate action if an integrity
failure is detected.
3.5.3.3
In the locator, integrity can be provided by RAIM, as set out in
Appendix A, A.2.2.2. The use of Doppler and carrier phase measurements can
also be used, as set out in Appendix A, A.2.2.1. These measures are necessary
should errors due to multipath effects be considered to be unacceptable.
3.5.3.4
The monitoring of GNSS signals to confirm integrity is only acceptable for safetyrelated tasks if the monitoring itself meets a defined level of integrity.
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3.5.4
3.6
Other quality of service parameters
3.5.4.1 In addition to the main parameters set out in 4.1.1.1, there are other quality of
service parameters. Applications should be considered on a case-by-case basis
for:
a)
Availability / reliability: these terms are used in this guidance to describe the
intrinsic performance of the on-board hardware
b)
Time To First Fix (TTFF): the time taken between switching the locator on, or
resuming visibility of the satellites following a prolonged period of
obscuration or being out of service, and obtaining the first reliable position
and speed reports from the locator
c)
Time To Fix (TTF): the time taken between resuming visibility following a
shorter period of obscuration (typically less than two hours) and obtaining
the first reliable position and speed reports from the locator
d)
Fix rate: the frequency at which the locator unit is able to provide position
solutions meeting accuracy and integrity requirements.
The future − GNSS improvements and developments
3.6.1
Standalone satellite navigation services are those that can be used to determine
position, velocity and time, without recourse to any other system. The two
systems planned, in addition to GPS, providing coverage in the UK are:
a)
The European Union’s proposed system, Galileo – see Appendix A, A.1.4
b)
The Russian Federation’s (GLObal NAvigation Satellite System) GLONASS
– not included in this document, due to uncertainty of its availability over the
short-term.
3.6.2
Appendix A, A.1.2.1, sets out the use of multiple satellite systems. Other than
GPS, these are not available for service at the time of publication. The use of
multiple systems is attractive for applications which require integrity, because a
number of sources of error become more easily detectable. The effects of DOP
are also much reduced.
3.6.3
GLONASS is expected to be the first to become available, but the timetable is not
known. Although the modulation scheme is not the same as GPS, there are
combined GPS / GLONASS receivers on the market, particularly for high-end
applications.
3.6.4
The European system Galileo is set out in Appendix A, A.1.4. The signals are
similar to GPS, and are expected to provide a wider range of augmentation and
integrity functions integrated within the system, rather than being additional as in
the case of GPS. In particular, dual frequency receivers should become
standard. This should enable ionosphere and troposphere induced errors to be
reduced routinely. Dual GPS / Galileo receivers are already available on the
market. However, this programme continues to suffer delay. In the meantime,
the GPS constellation is being steadily improved by new satellites offering open
dual frequency signals.
3.6.5
The third system could be Chinese. The details of the constellation and the
technical details are uncertain, but the programme is going forward. The Indian
government also intends to create an independent system.
3.6.6
In the longer term, beyond 2015, advances in atomic clocks and inter-satellite
links should remove errors, due to the independent clocks currently in use. This
should remove a principal source of error in GPS and, together with dual
frequency receivers, enable sub-metric accuracies to be routinely available.
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Part 4
Guidance on Classes of Locator Requirements
Classes of locator requirements are defined in this part, in order to provide a simple way of
classifying types of locator based upon satellite navigation and augmentation technologies.
These classes apply to the quality of the signal at the output from the locator, not the GPS
signal alone. This part provides:
a)
The derivation of the service classes presented, using the quality of service
parameters presented in Part 3 that describe the locator performance
b)
A functional view of generic functional architectures that provide the three classes,
presented from C to A
c)
Reference to the technical solutions available.
4.1
Quality of service parameters
4.1.1
General
4.1.1.1 For the railway, Part 3 sets out three quality of service parameters: service
coverage, accuracy and integrity. These parameters refer to the quality of the
data at the output of the locator unit and are used here with the definitions set out
in this document. The aviation and marine transport sectors have defined service
levels derived from their navigation requirements. These are reflected in the
required navigation parameters that are defined for locators in these domains,
and that are part of the specifications for future GNSS systems.
4.1.2
The interrelationship between service coverage, accuracy and integrity
4.1.2.1 A lack of service coverage at or above the threshold specified for the application
has an immediate effect upon accuracy and integrity. It may be necessary for the
locator to estimate when the accuracy has fallen below a specified threshold.
There are two strategies:
4.1.2.2
4.2
a)
Suspend the applications and functions concerned
b)
Modify the confidence intervals for position and speed data, which enables
some continued operation.
Accuracy specifications and integrity specifications have an inverse relationship.
Excluding the hardware integrity performance, high accuracy and low integrity,
and low accuracy and high integrity, have an equivalence in terms of locator
performance.
Service class guidance
4.2.1
This guidance note sets out three classes of locator requirements, each defined
by a level of performance (service coverage, accuracy and integrity). The
rationale for defining a small number of classes is:
a)
To encourage users to identify locator requirements according to the class
appropriate for their applications
b)
To encourage the supply market to focus on locator products appropriate for
the railway environment that align with these classes.
A proliferation of bespoke products would be expected if a specific locator were
to be designed for each application. This would result in higher costs.
4.2.2
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The functional architecture of each class is derived from a generic functional
architecture so that development from Class C to Class A can be seen as an
upgrading exercise on a functional view. However, it is not the purpose to
constrain innovation or the physical implementation.
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4.2.3
The economics of these technologies are such that, if an application has
requirements whose performance exceeds those provided by the standard class
product, rather than modify the product, it could be preferable to adopt the COTS
product of a higher class.
4.2.4
Table 2 sets out the relationship between the classes in terms of the quality of
service parameters and augmentation options.
Class
Service coverage
Accuracy
Integrity
C
Limited to areas with an
assured view of the
satellites, with marginal
improvement at the
boundary if
augmentation for
accuracy or integrity is
used
Augmentation can be
applied for accuracy
purposes. Typical
choices are differential
augmentation and the
use of speed
measurement data,
usually derived from a
tachometer
Integrity monitoring of
the GPS and other
data used can provide
some integrity
B
Service coverage is
increased by means of
augmentation using
inertial hybridisation
The augmentation can
be applied to obtain
consistent accuracy.
Track coordinates
could be beneficial
where satellites are
obscured
Integrity monitoring of
the GPS and other
data used can provide
some integrity
A
Service coverage can
be increased by means
of augmentation using
inertial hybridisation
The augmentation can
be applied to obtain
consistent accuracy.
Track coordinates
could be beneficial
where satellites are
obscured
Augmentation is
applied for integrity
purposes. Typical
choices would be
EGNOS to prove the
signal-in-space, and
RAIM or carrier phase
techniques to control
multipath. The locator
itself may include
integrity measures
Table 2 Summary of classes of locator requirements
4.2.5
Rationale and decision process
4.2.5.1 The three classes of locator requirements are differentiated by the three quality of
service parameters: service coverage, accuracy and integrity. From the simplest
architecture, a GPS receiver with no augmentation, to the most elaborate, the
classification presented provides a logical process to assess the augmentation
that economically and practically supports the performance required from a
locator. In 4.3.4.1 and 4.3.4.2, a provision is made for a fourth Class A+ to
support safety-critical applications in due course. It is emphasised that the
classes apply to the output of the locator.
4.2.5.2
The primary characteristic of the railway environment is the signal obscuration,
due to tunnels, canyon effects, stations and foliage. Therefore, the first
assessment of an application to be made defines the acceptable level of service
coverage. Ideally, the application requirements would be defined such that no
augmentation is required, but this is often not possible. Discontinuous
applications would normally have the least demanding requirements, though
there are exceptions.
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4.2.5.3
Having determined the need or not for augmentation, for reasons of service
coverage, the next assessment to be made concerns the accuracy requirement.
Beyond a certain threshold of accuracy, even if augmentation is not required for
reasons of service coverage, a form of augmentation may be required to meet the
accuracy requirement.
4.2.5.4
A persistent source of error in the railway environment is multipath. This is most
likely to occur where there is partial obscuration of satellites or high-rise buildings
border the railway, where it is more difficult to provide protection. The effect of
multipath is also more severe when stationary or moving slowly. Where integrity
is required, or accuracy requirements are high, the presence of multipath should
be at least detectable and, if necessary, its effect limited.
4.2.5.5
The final assessment addresses integrity. The scope of this guidance note does
not extend to SIL3 and SIL4 applications, as the use of GNSS-based locators to
obtain these levels of integrity is in its infancy. There is confidence that GNSSbased locators are able to provide SIL1 and SIL2 levels of integrity, for which
augmentation is usually necessary. Applications requiring an assured level of
integrity are likely to be the most demanding on locator design.
4.3
The three classes of locator
4.3.1
Class C
4.3.1.1 Class C sets out the most basic type of satellite navigation receiver that excludes
inertial hybridisation. Its distinguishing feature is that it suffers from obscuration
and loss of service coverage. When based upon COTS products the accuracy
performance is usually low, and integrity is not specified. Augmentation to obtain
a desired accuracy requirement is provided for. There are occasional examples
of applications that are managed such that the locator is required only in areas of
open sky, but also requires an assured accuracy with integrity. Such an
application is the Locoprol project, led by Alstom, Belgium.
4.3.2
4.3.1.2
Map-matching techniques are feasible, as is differential augmentation, subject to
the guidance set out in Part 3 and Appendix A.
4.3.1.3
Although the hardware is usually COTS, the software modules need careful
selection. Features such as the predictive modes, often implemented when
signals are weak, may not be suitable for a railway application without
modification or being disabled. Matters of concern should include the number of
channels that can operate in parallel, TTF, and the treatment of weak signals.
Class B
4.3.2.1 Class B includes augmentation, typically by hybridisation, to increase service
coverage. Accuracy is provided to meet the needs of the application, but proof of
integrity would not normally be possible unless the accuracy requirements are
extremely undemanding. Based essentially on lower price COTS products, the
performance in the absence of GNSS signals is limited in time. Map-matching
techniques are feasible, as is differential augmentation, subject to the guidance
set out in Part 3 and Appendix A.
4.3.2.2
4.3.3
Subject to the verification of features being suitable for the railway application, the
software is usually that which is commercially available. This means that
techniques to control multipath effects, such as carrier phase, dual frequency
receivers and Doppler measurements are unlikely to be available as standard
marketed features.
Class A
4.3.3.1 Class A provides a specified safety integrity up to SIL2. The implications of
integrity requirements on the software and the supporting functionality of the
locator are such that the use of lower cost COTS products would have no
purpose. In achieving integrity, there is a compromise to be made between the
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functionality adopted to obtain the necessary diversity and fault detection, and the
need to aim for simplicity to facilitate the proof of safety. The limitation to the
achievable SIL is essentially due to single channel hardware whose SIL limit is
SIL2, or at best low SIL3. SIL4 locator software is feasible, but not known to be
commercially available outside of bespoke applications.
4.3.4
Class A+
4.3.4.1 A Class A+ is defined for integrity performance up to SIL4. This is, presently,
futuristic. The necessary functionality is available, and in time should provide
better service coverage, accuracy and integrity processing at less cost. The
problem is to contain the cost of the safety platform on which these are
implemented to enable such locators to be economic. To maintain an open
market it is desirable to have techniques that make bespoke safety platforms
from a given supplier unnecessary. The viability of these techniques as yet
remains unproven.
4.3.4.2
4.4
Functional architecture
4.4.1
4.4.2
Logically, the concept can also be applied to Classes C and B, to create C+ and
B+ sub-classes. These would define higher integrity performance within the
service coverage limits, as in the Locoprol example. Generally, the cost of
providing specific software and perhaps hardware to do this for a restricted
market, would be such that a Class A+ product with a wider market is likely to be
more cost effective.
The guidance on augmentation set out in Appendix A, A.2, should be understood
before reading this section.
Class C locator
4.4.2.1 The distinguishing feature of the Class C locator architecture is that it is
dependent for positioning upon a GPS receiver. Figure 8 indicates that a range
of augmentations are possible. These improve accuracy and could be used to
support an integrity requirement, but do not significantly improve service
coverage. As an example, the use of track coordinates would reduce the effect of
obscuration, but it is not a solution to the areas of no coverage. The use of a
tachometer could provide a limited extension of coverage. Assured accuracy
where satellite visibility is inadequate would require the use of a Class B locator.
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Antenna
Antenna
LOCATOR
Differential
augmentation
GPS
receiver
Pseudorange
Pseudorange for n
Phase
Track
coordinates
Train speed
data
Location
database
satellites. (Note 1)
Time
Data fusion
GPS
processor
(Note
2)
Position
Position
Speed
Velocity
Time
Application
Notes: 1. May also include phase and Doppler information for each satellite (see Appendix A,
A.2.2.1.
2. Kalman filtering is the usual technique, but others are used.
3. Dashed lines and boxes are optional augmentations to achieve the required
quality of service.
4. The shaded boxes would normally be used unless the application has no need of
them.
Figure 8 Illustration of Class C functional architecture
4.4.2.2
Page 34 of 86
To enable the required quality of service to be obtained from the locator, the
diagram includes GPS augmentation options, that is to say, differential
corrections, use of phase and Doppler information and perhaps other sensors (a
train always has a tachometer and therefore train speed data can be considered
for use even in Class C architectures). However, these cannot be considered as
usually within the capabilities included in low-value COTS locator products, and
their use has cost implications.
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4.4.3
Class B locator
4.4.3.1
This class supports applications with a service coverage that includes areas
with limited or no visibility of the sky. To improve service coverage beyond
that achievable with Class C, the Class B locator architecture includes the
Class C architecture and also a form of other augmentation options, usually
an IMU. The quality of the IMU determines the accuracy versus service
coverage performance of the locator. This, to a limited extent, can be traded
against a low-integrity requirement. Figure 9 indicates the same range of
augmentations as Figure 8 to obtain the required quality of service.
Antenna
Differential
augmentation
Track
coordinates
Train speed
data
Location
database
LOCATOR
IMU
GPS
receiver
Pseudorange for n
satellites. (Note 1)
Time
Rate
information
Calibration
Data fusion
(Note 2)
Position
Position
Speed
Velocity
Time
Application
Notes: 1. May also include phase and Doppler information for each satellite (see
Appendix A, A.2.2.1).
2. Kalman filtering is the usual technique, but others are used.
3. Dashed lines and boxes are optional augmentations to achieve the required
quality of service.
4. The shaded boxes would normally be used unless the application has no need of
them.
Figure 9 Illustration of Class B functional architecture
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4.4.4
Class A locator
4.4.4.1 Based on the Class B locator architecture, Class A is able to deliver a SIL1 or a
SIL2 integrity performance. To enable the required quality of service to be
obtained, Figure 10 indicates the same augmentations options as Figures 8
and 9. Here, if used, these support the integrity requirement. The quality of the
IMU determines the accuracy versus service coverage characteristic of the
locator for the integrity requirement. Unjustifiably high accuracy should never be
required.
Antenna
Differential
augmentation
Track
coordinates
Train speed
data
LOCATOR
Integrity
Pseudorange for n
satellites. (Note 1)
Time
GPS
receiver
(Note 5)
IMU
Rate
information
Calibration
Data fusion
with RAIM (Note 2)
Position
Position
Speed
Velocity
Time
Integrity
Location
database
Application
Notes: 1. Should include phase and Doppler information for each satellite (see Appendix A,
A.2.2.1.
2. Kalman filtering is the usual technique, but others are used.
3. Dashed lines and boxes are optional augmentations to achieve the required
quality of service.
4. The shaded boxes would normally be used unless the application has no need of
them.
5. Dual frequency receiver should be considered (see Appendix A, A.2.2.1.2).
Figure 10 Illustration of Class A functional architecture
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4.4.4.2
The particular characteristics of Class A are:
a)
Integrity in the presence of multipath is demonstrated. Multipath is a
significant source of risk in the railway environment to the extent that, if
adequate controls and mitigation are in place in the receiver, the use of
differential augmentation for integrity purposes contributes little to the
attainment of the required integrity level. It does however contribute to
attaining the accuracy requirement. A high-accuracy and service-coverage
requirement requires an IMU of adequate performance, which cannot be
considered as a usual offering included in lower value COTS products, and
their attainment has cost implications
b)
The response to a fault in the signal-in-space is to detect it and isolate its
consequence with a defined certainty. The use of RAIM and map-matching
are probably the most effective means of doing this
c)
Because in the railway the locator needs to deal with regular obscuration,
the absence of the satellite signals should not constitute a threat to safety.
The tachometer and IMU combination provide integrity for a limited period of
time, which is extended by the use of map-matching to track coordinates
(preferably in three dimensions). Once this limit is reached, operating
procedures should manage the continuing degradation
d)
Jamming and spoofing of GPS signals represent threats to the integrity of
the locator. Measures to ensure integrity should be able to detect them in
the same way as other threats are detected. Adequate measures should
enable these to be treated as an inconvenience, rather than a threat to SIL1
and SIL2 applications. The use of RAIM (applicable to all sensors), mapmatching with IMU and other sensors, should provide adequate detection of
erroneous signals, from whatever cause, as part of the data fusion process.
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Part 5
5.1
Choice of Equipment
Introduction
5.2
5.1.1
Parts 3 and 4 set out the means to describe a quality of service, to provide a
classification of the service required, and the supporting rationale.
5.1.2
This Part 5 provides guidance on a decision process to define the quality of
service required, and therefore a class of locator, that meets the needs of the
application(s). The use of a GPS-based locator is assumed.
5.1.3
This Part 5 also provides guidance for the user to understand the choices that
can be made by a designer in meeting the quality of service requirement,
because these can influence the integration of the on-board systems. For the
decision-making process to be successful it is necessary for the user and the
designer to be able to communicate and cooperate such that each can appreciate
the problems the other is addressing.
Augmentation choices − summary
5.2.1
Once the application has been defined in terms of acceptable service coverage,
accuracy and integrity, one of the following classes of architectures should be
chosen:
a)
Class C: Gaps in service coverage. It is a GPS receiver with no inertial
augmentation, usually a COTS-based product
b)
Class B: Higher service coverage. It is a Class C architecture that is
augmented with inertial devices. Accuracy is assured within the specified
obscuration limits, with no, or very low, levels of integrity
c)
Class A: High service coverage and with a specified integrity performance.
Up to SIL2 integrity should be possible. Class A is more likely to require
bespoke software because of the need to meet SIL standards.
5.2.2
The use of COTS products generally implies no intrinsic integrity of the output. In
each case, however, the inclusion of bespoke software can provide integrity
improvements by the use of algorithms processing a combination of differential
augmentation, map-matching, inertial data and data from other devices.
5.2.3
In Table 3, brief descriptions of some combinations of techniques are
summarised, with indicative performance and a rough order of magnitude cost
figures. These are not intended to be definitive, as costs are changing all the
time, with higher specification and / or lower cost equipment becoming available.
5.2.4
The figures exclude the cost of any certification that might be required for
Class A. In addition, they exclude packaging and installation costs. These
system combinations and levels of performance are currently feasible, with the
exception that maps of sufficient quality are in development rather than available
off-the-shelf.
System
combination
GPS standalone
Performance
13 m accuracy at 95%
confidence
Class C
>70% service coverage
Limitations
Not available in tunnels,
cuttings or stations when
view of satellites is
obscured
Possible cost
ranges unfitted
~£100 per receiver
Some SIS integrity
assured with the use of
RAIM
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System
combination
GPS augmented by
low- to mediumquality IMU
Class B
Performance
13 m accuracy at 95%
confidence
>90% service coverage,
depending upon accuracy
spread acceptable
Some SIS integrity
assured with the use of
RAIM
GPS augmented by
EGNOS
Class C, meeting a
higher accuracy
requirement
GPS augmented by
location database
Class C. Class A is
possible where the
integrity of the
location database is
assured
GPS augmented by
map-matching
~1 to 2 m accuracy at
95% confidence in open
skies – is less in an
obscured environment
Service coverage – see
Appendix A, A.2.1.3
Higher integrity
5 to 10 m accuracy at
95% confidence
>70% service coverage
Medium integrity with
RAIM
5 to 10 m accuracy at
95% confidence
Class C. Class A is
possible where the
integrity of the map
database is assured
GPS augmented by
EGNOS and mapmatching
>70% service coverage
Class C meeting a
higher accuracy
requirement.
Class A is possible
where the integrity of
the map database is
assured
GPS augmented by
commercial spacebased augmentation
systems and mapmatching
>60% service coverage
High accuracy
Class C
Medium integrity with
RAIM
~1 to 2 m accuracy at
95% confidence (best
case – see above)
High integrity
~0.1 m accuracy at 95%
confidence
<70% service coverage
Unquantified integrity
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Limitations
Possible cost
ranges unfitted
Navigating by the IMU
alone is available when
view of satellites is
obscured for a limited
period, the length of
which depends on the
quality of the IMU
(ranging from tens of
seconds for low quality
to several minutes for
medium quality)
Not available in tunnels,
cuttings or stations when
view of satellites is
obscured
~£500 to £5,000 per
locator, depending
on grade of INS
~£300 per receiver
Quoted accuracy may
not be achieved in
conditions of poor DOP
Not available in tunnels,
cuttings or stations when
view of satellites is
obscured. Dependent
on the creation and
management of location
databases
~£200 per receiver
Not available in tunnels,
cuttings or stations when
view of satellites is
obscured. Dependent
on the creation and
management of digital
route maps
~£200 per receiver
Not available in tunnels,
cuttings or stations when
view of satellites is
obscured
~£300 per receiver
Quoted accuracy may
not be achieved in
conditions of poor DOP
Not available in tunnels,
cuttings or stations when
view of satellites is
obscured
Route map costs
influenced by the
quality and level of
accuracy required
Route map costs
influenced by the
quality and level of
accuracy required
Not known, as
equipment and
services are
provided by
subscription on a
commercial basis
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System
combination
Performance
GPS augmented by
EGNOS, mapmatching and lowgrade inertial
systems
Class A is possible
where the integrity of
the map database is
assured
GPS augmented by
space based
augmentations and
medium-grade
inertial systems
~1 to 2 m accuracy at
95% confidence (best
case – see above)
>90% service coverage
depending upon accuracy
spread acceptable
Limitations
Inertial system allows
navigating by the IMU for
short periods (tens of
seconds) when GPS
signal is obscured
An integrity level
guaranteed
~1 to 2 m accuracy at
95% confidence
>95% service coverage,
depending upon accuracy
spread acceptable
Inertial system allows
navigating by the IMU
for a few minutes when
the GPS signal is
obscured
Possible cost
ranges unfitted
~£1,000 per unit
depending on
capability of IMU,
service provided
free of charge for
EGNOS
Route map costs
influenced by the
quality and level of
accuracy
~£1,000 to £10,000
per unit, depending
on the capability of
the IMU
Class B and Class A
Integrity possible
Table 3 Indicative characteristics of various combinations of GNSS components
5.3
Achieving application quality of service
5.3.1
The choice of augmentation is not only for the designer, but also for the user. It can have a
direct influence on the internal interfaces within a vehicle communication system; it affects
the choice of antennas and their quantity, and it can affect the position of equipment within
the vehicle. It also has implications for the responsibilities of the user during the system life
cycle, especially where data management is concerned. The guidance set out in this
section suggests a sequence of decisions that enable these issues to be reconciled with
the needs of the application(s).
5.3.2
GPS processing
5.3.2.1 In accepting a design of a locator to meet service coverage, accuracy and
integrity requirements, the user should ensure that the supporting design choices
on the subjects below are made on sound arguments:
a)
The choice of single frequency or dual frequency receivers
b)
The use of phase and Doppler information
c)
The use of RAIM
d)
The use of differential augmentation
e)
The design of the data fusion.
5.3.2.2
The techniques a) to d) are expected to become increasingly common in COTS
products. Their presence is always acceptable, although unnecessary complexity
should be avoided. To understand whether they are required, the user should
obtain from the locator designer the justification that demonstrates that the
required quality of service is achievable with the choices made.
5.3.2.3
COTS products are likely to include features and characteristics that are not
appropriate for the intended application. For example, dead reckoning when a
locator is obscured from the satellites could be based on logic not applicable to a
railway. Often, these features are not identified in the product specification. The
purchase of these products requires a carefully managed process where the
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product is shown to be fit for the application. Simulation, as set out in 6.2.1.4,
should be considered.
5.3.2.4
5.3.3
The antenna is the subject of guidance set out in Part 6.
Decisions relevant to all Classes A, B and C
5.3.3.1 Referring to the Figures 8, 9 and 10, the first choice concerns the use of a
tachometer. Traction units invariably have a tachometric device, and the driver’s
speedometer has one almost by definition. Unless there are interface difficulties
that are impractical to resolve, the data fusion should always include a train
speed input. The reverse logic is also feasible, that is to say, the locator is able
to monitor the wear of the wheels on the axle used to drive the tachometer (or
speed unit) and maintain an estimate of a correction factor. Because of the
uncertainties of the wheel / rail interface, there is a limit to which the prevailing
diameter can be estimated, but it remains useful to do this.
5.3.3.2
The second choice concerns the use of an IMU, that is to say, to decide if
Class C is sufficient. It can be used for:
a)
Service coverage. The output from the locator remains useful when in
obscuration up to a defined maximum period and with a defined confidence
b)
Detecting track geometry with more certainty than with GPS, and can be
used to match the location against an application database
c)
Improving accuracy and integrity.
5.3.3.3
The third choice concerns the use or not of differential augmentation. In the
circumstances prevailing at the date of publication of this guidance note, there is
yet no differential service suitable for the railway in real-time for most
applications. If integrity is also required, then the actual performance of EGNOS
(as the only choice available today) may be severely restricted depending upon
reception of EGNOS information from the geostationary satellites and the level of
GPS satellite obscuration. However, the guidance on differential augmentation
set out in Appendix A, A.2.1, should be consulted before a definitive decision is
made. When it becomes available to the railway, then it should certainly be used
to achieve accuracy.
5.3.3.4
The fourth choice concerns the use or not of track coordinates. The criteria to
apply here are:
a)
The availability of the data with sufficient quality. The quality should be
proven to be acceptable by means of simulation
b)
The process to amend the data as it is modified over time. There could be a
complete fleet of trains to update simultaneously.
5.3.4
Decision relevant to Classes A and B
5.3.4.1 Classes A and B require a choice of IMU to be made. Appendix A, A.2.2.5, sets
out guidance on the criteria that directs this choice.
5.3.5
Decisions relevant to Class A
5.3.5.1 Class A requires that the integrity of the position and speed solutions be assured
with a level of certainty. When available to the railway, the differential
augmentation is able to contribute to the assurance of integrity. Its use requires a
careful choice of locator architecture, including the manner in which the data
fusion is implemented. This complex subject is for the designer, not the user; but
the user should have at his disposal the evidence sufficient to support a claim by
a supplier that his design meets the user’s integrity requirements for the
application.
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5.3.6
5.3.5.2
The same attention to the integration of the IMU within the data fusion is also
required. There are several options, and the choice should be supported by a
cogent rationale. The integrity of the IMUs data in the absence of GPS signals
should be supported by a design concept. (One possibility is to use a
tachometer, which can simplify the IMU required.)
5.3.5.3
There are several ways to extract information from the GPS signals-in-space.
The usual method of ranging can be complemented by using the Doppler
information and phase information given by the carriers. There is also the option
of using a dual frequency receiver (see Appendix A, A.1.3.2, A.1.4.3 and A.2.2.1).
Given the relatively low integrity requirements of SIL1 and SIL2, the user should
avoid complexity and adopt the simplest solution possible. As these techniques
become more common and available in COTS products their cost reduces.
Where multipath is required to be effectively controlled their use is recommended,
subject to the cost of supply being acceptable. Long Range Kinematic (LRK),
Real-Time Kinematic (RTK) and phase techniques are unlikely to be essential
but, if available in a competitively priced COTS unit, they should be considered.
For SIL1 and SIL2 applications the precautions set out in BS EN 61508-1:2002
should be sufficient to ensure the acceptability of the hardware and software.
+
Comment relevant to Class A
5.3.6.1 The use of a safety platform to support a Class A+ locator is unlikely to be
economic. There is development of safety concepts to support SIL3 and SIL4
locators, but they are not yet in commercial use.
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Guidance on the Use of Satellite Navigation
Part 6
6.1
Design and Installation − Good Practice Guide
Introduction
6.1.1
This part sets out some of the specific and practical aspects that should be
considered when implementing satellite navigation technology.
6.2
Implementation process
6.2.1
System performance definition
6.2.1.1 It is the responsibility of the purchaser to map their own specific application
requirements onto the classes of service defined in Part 3 of this guidance note.
Particular attention should be paid to those operational environments in which the
performance of GPS alone may not be sufficient to meet application
requirements.
6.2.1.2
6.2.2
When specifying a satellite navigation system the full range of performance
requirements should be specified. Parameters that should be considered are as
follows:
a)
Accuracy, horizontal and vertical (if required)
b)
Service coverage
c)
Integrity
d)
The format and geodetic reference frame in which positional data is
provided.
6.2.1.3
Communications and antenna requirements should be taken into account when
determining the overall system performance requirements for the specific
application(s) being considered.
6.2.1.4
Wherever practicable, the performance of different locator solutions should be
simulated prior to any procurement. These simulations are available either via
the purchase of COTS products or commercial consultancy services. This can
provide a cost-effective alternative or complement to wide-area performance
trials.
Cost and upgrade path
6.2.2.1 As set out in 3.3, when different solutions are available, it is necessary to apply
selection criteria to determine the most appropriate solution. In addition to quality
of service, other criteria that should be applied are cost and upgradeability. The
trade-off should be made considering longer-term strategy, as well as short-term
drivers. For example, if future requirements call for a higher performance locator,
then an easily upgradeable solution – or even a higher performance option – is
generally preferable over a lower cost option.
6.2.2.2
When considering upgradeability, the solution selected should be such that the
different augmentation services considered can be added successively, for
example a satellite navigation antenna installed for GPS is likely to be able to
receive Galileo and EGNOS signals as well, and would only require a receiver
upgrade.
6.2.2.3
Life-cycle costs should include the data management arrangements.
6.3
System integration
6.3.1
Responsibility
6.3.1.1 When the implementation of a system is initiated, full consideration of the
requirements of the integration with existing vehicle systems, such as speed
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measuring, should be made. The responsibility for the integration should be
clearly stated within specification / contractual documentation.
6.3.2
System definition
6.3.2.1 Existing vehicle interfaces should be clearly defined. If this information is not
available it may be necessary to make on-vehicle measurements. The
responsibility to define the interface should be clearly stated within specification /
contractual documentation.
6.3.3
Risk assessment
6.3.3.1 An assessment of the risks of installing the system should be carried out. It
should include an assessment of the consequences of the failure of system
interfaces, such as the possible false energisation of vehicle wiring; analysis
tools, such as Failure Mode and Effects Analysis (FMEA) and Fault Tree Analysis
(FTA) should be used, as appropriate. A demonstration that the risk to the
existing vehicle systems is as low as reasonably practicable should be made.
6.3.3.2
6.3.4
System support and maintenance
6.3.4.1 The user’s maintenance objectives for the system should be set out at the start of
the project. Attention should be given to the management of all supporting data
required.
6.3.4.2
6.3.5
The risk assessment should also be extended to the use of the system, on an
application-by-application basis. This risk assessment should cover the various
failures that might occur, and reflects the statistical nature of the performance of
the locator unit. However, the risk assessment should also include explicit
consideration of the potential impacts of the vulnerability of the satellite navigation
system to unintentional electromagnetic interference, as well as the security
threat of jamming and / or spoofing.
Although the equipment should be maintained in line with manufacturers’
requirements, routine maintenance should not be required.
Role of design
6.3.5.1 It is essential that maintenance and testing contribute to meeting reliability targets
for a system in service. This is best achieved by placing maintainability at the
heart of the design process.
6.3.5.2
The design of the locator unit should include built-in test equipment and autodiagnoses to the extent that this is compatible with the COTS and maintainability
requirements. Where there is a conflict, a life-cycle cost model should be
analysed to determine which should take precedence.
6.3.5.3
When a fault is detected, the unit concerned should be identified and a fault code
assigned. This information should be made available to the staff responsible as
part of the train’s standard maintenance facilities.
6.3.5.4
Special attention should be paid to databases, in order to ensure that they are
within their period of validity, and corruption should be detected with a high
degree of confidence. Updating databases should be subject to verification to
ensure that amendments are correct. Procedures should be commensurate with
the level of safety required, and not more demanding, as this would lead to a
waste of resources.
6.3.5.5
Guidance on the principal functional elements of maintainability are set out
in 6.3.6.
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6.3.6
Built-in self-test
6.3.6.1 Systems should include automatic self-testing. This should operate at switch-on
and run continuously in the background. Examples are:
6.3.6.2
6.3.6.3
6.4
a)
Testing of system memory at start-up
b)
Verification that software versions are valid
c)
Verification that database versions are valid
d)
Verification that hardware interfaces are working. In particular there should
be indications that confirm the train consist data, so that the interfaces and
equipment present can be identified, and also indications that confirm the
serviceability of the train communications.
In the event of failures, the system should provide notifications as follows:
a)
To the train maintenance system
b)
To the train crew, if any action is required to enable the train to continue in
service.
Subsequent action, for example reporting failures to a central server, depends
upon the train’s maintenance system.
Equipment installation
The following is based on principles used on successful installations. It may not be
possible to comply with them completely, and the alternatives should be considered on a
case-by-case basis.
6.4.1
Antenna selection
6.4.1.1 Typical antennas provide near hemispherical coverage (that is to say,
160 degrees). Satellite signals are Right-Hand Circularly Polarised (RHCP) and
therefore a conical helix antenna or variation is suitable. Antenna designs vary
from helical coils to thin patch antennas.
6.4.1.2
6.4.2
In order to reduce multipath effects, a special type of antenna can be deployed
known as a ‘choke ring antenna’. The antenna comprises vertically aligned
concentric rings connected to the ground plane, whereby the multipath signals
incident on the antenna at the horizon and negative elevation angles are
attenuated. However, the cost of such an antenna may be prohibitive for many
applications.
Antenna position
6.4.2.1 It is preferable to share one antenna between all positioning applications, as far
as is practicable. There should be one locator system on board, the
requirements for which are determined from the applications. This avoids the
proliferation of multiple equipments serving the same function and (potentially)
compromising one another’s performance, and which complicates the systems
integration process. There are also limitations imposed on the number of
antennas by the vehicle body construction.
6.4.2.2
Ideally, the antenna should be located on the vehicle longitudinal centre line, in
order to maximise the line of sight of the satellites and should be as close to
horizontal as practicable. Although this is not critical to successful operation, the
antenna should be as close to the centre line as practicable.
6.4.2.3
To combat multipath effects, antenna positioning is of prime importance. The
antenna should be placed above the highest reflector to prevent reflected waves
from arriving from and above the horizon. Certain antennas require a ground
plane to increase gain at low elevation angles. However, the ground plane itself
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may diffract signals incident on the antenna at low elevation angles (see
also 6.4.3).
6.4.2.4
6.4.3
The antenna should be positioned as far as practicable from all potential sources
of Electromagnetic Interference (EMI). The minimum distances from particular
sources for a receive-only GPS antenna should be:
a)
GSM / GSM-R Antenna – 1 m
b)
CSR / SMA Antenna – 1 m
c)
National Radio Network (NRN) Antenna – 1 m.
6.4.2.5
These separation distances apply regardless of which service the antenna is
receiving – GPS, Galileo, EGNOS, other SBAS. An example is shown in
Figure 11.
6.4.2.6
Pantograph and associated switchgear: due to the broad spectrum of noise that
is generated during arcing, a minimum distance of 5 m is reasonable.
6.4.2.7
When the antenna position is being determined, consideration should be given to
the position of the receiver in order that the cable length between the two is kept
as short as is practicable.
6.4.2.8
Consideration of the future fitting of other antennas, for example GSM-R (both
voice and data) should be made. These are usually required to be fitted at each
cab-end of a multiple unit vehicle, and therefore fitting the satellite navigation
antenna at the inner end of a vehicle fitted with a cab facilitates the future fitting of
these other antennas. Equipment cupboards at the inner end tend to have more
space available.
6.4.2.9
When multiple units are joined together as one train, the role of the intermediate
antennas and systems should be assessed to determine whether they are
required to be in service.
Antenna installation and maintenance
6.4.3.1 Low-profile GNSS antennas are available that have a similar height to existing
cab radio antennas and, therefore, if they are positioned in a similar longitudinal
line, demonstrating that gauge is not infringed is straightforward. The choice of
antenna should be made so that it can be classified as ‘frangible’, as set out in
GE/GN8573: Guidance on Gauging.
6.4.3.2
Any ground plane requirements specified by the antenna supplier should be
observed. If the antenna is mounted on a non-metallic roof it may be necessary
to incorporate a ground plane within the mounting arrangement or apply a
metallic film on the roof underside. Alternatively, an antenna that does not
require a ground plane may be more practical.
6.4.3.3
To prevent the antenna and cabling from rising to an excessive potential, should
the overhead catenary come into contact with the antenna, any exposed
conductive part should be firmly bonded to the vehicle roof. This can be achieved
by locally removing the existing insulating roof paint and replacing it with a
conductive one, such as zinc primer.
6.4.3.4
The cable between the antenna and the receiver is a particular threat to
Electromagnetic Compatibility (EMC) and should be routed separately to all other
cables and not tied to them. Ideally, the minimum separation should be 150 mm
and, if it is necessary to cross over other wiring, the cables should be aligned
perpendicularly. The antenna cable should be as short as practicable.
6.4.3.5
Cable, compliant with manufacturers’ requirements and rolling stock
requirements, not consumer co-axial, should be used.
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6.4.3.6
The design of the installation should ensure that the level of sealing is still
maintained.
6.4.3.7
Ideally, the design of the installation only requires roof access, in order to
maintain or replace the antenna. If this is not practicable, then the ease of
access to the underside of the roof should be considered.
6.4.3.8
The antennas should be maintained and cleaned in accordance with
manufacturers’ instructions.
Figure 11 Typical installation of a GPS antenna
6.4.4
Receiver fitment standards
6.4.4.1 To ensure a reliable performance, all electronic equipment should be compliant to
BS EN 50155:2007 [7]. Included within this standard are the environmental
conditions that the equipment can experience. These are usually more onerous
than that are applied to standard commercial PC equipment and therefore should
be carefully considered, together with the actual equipment installation to
minimise equipment failure.
6.4.4.2
For retrofit applications, older vehicles usually have a control system based on
relays that have unsuppressed coils. This means that supplies and battery volt
connections can contain high-voltage direct transients and non-battery voltage
connection indirect transients. To ensure reliable operation, experience has
shown that, if electronic equipment is being retrospectively fitted to vehicles
produced earlier than approximately 1995, then meeting the requirements of
BRB/RIA Specification No 12 (1984) [8] should be considered, together with the
standards identified by the train builder. Testing of older vehicles not built to
BRB/RIA Specification No 12 should be considered.
6.4.4.3
Fitment requirements should include the general principles for on-train receiver
(and general equipment) design. These include:
a)
The use of enclosures suitable for rolling stock
b)
The use of reverse polarity protection
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6.4.5
6.4.6
c)
Maintenance and depot handling requirements, including electrical
protection on test equipment
d)
Polarisation of connectors and use of different connector sizes
e)
Avoidance of gold plating on frequent use connectors
f)
Provision of spare cables within any looms.
Power supply
6.4.5.1
The auxiliary supply on an Electric Multiple Unit (EMU) or electric locomotive is
derived from the traction supply and is subject to interruptions when the train
goes through neutral sections (25kV a.c. overhead) or 3rd rail gaps (750V d.c.).
To prevent the frequent loss of operation when this happens, the system should
be supplied from a vehicle battery-backed d.c. supply or its own Uninterruptible
Power Supply (UPS). The voltage ranges for batteries are 96-121V d.c. and
18-28V d.c. The battery terminal voltage for an electrical multiple unit when not
being charged is 96V; the battery terminal voltage for a diesel multiple unit when
not being charged is 24V. There are exceptions. For example, the Class 66 and
Class 67 freight locos and Eurostar passenger trains are 72V when not under
charge, which is a European ‘standard’. London Underground vehicles vary from
60V d.c. to 110V d.c. depending upon build era.
6.4.5.2
These traction supply interruptions can at times cause the auxiliary supply to
generate excessive surges, and therefore the effect on the equipment of those
surges specified in BRB/RIA Specification No 12 (1984) should be considered,
together with the standards identified by the train builder.
Position of the locator
6.4.6.1 To limit the length of cable to the antenna, a suitable position for the GPS
receiver might be the roof space or body-end equipment cupboards (passenger
vehicles). Ease of access for installation and maintenance should be provided.
6.4.6.2
When retrofitting equipment to existing vehicles, the usual locations available,
particularly on multiple units, are as set out in Table 4, together with specific
environmental threats to consider.
Location
Environmental consideration
Worst-case parameter
Equipment
cupboards
Heat sources
Max 55˚C
Ventilation requirements
Zero air flow
Roof space
EMI sources
Heat sources
Solar gain, temperatures in
excess of 50˚C have been
recorded in these locations
Due to the combination of
heat sources and solar
gain, the equipment should
be rated to meet the ‘T3’
category (70˚C) as set out
in EN 50155:2007
Ventilation requirements
Luggage
rack / stack
Leaking roofs
Passenger interference
Ventilation requirements
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Zero air flow
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Location
Environmental consideration
Underseat
Passenger interference, a
substantial enclosure, usually
steel, is required for this
location
Ventilation requirements
Liquid ingress from cleaning /
passengers
Worst-case parameter
Zero air flow
The equipment itself
should be sealed to
EN 60529:1992 IP65,
minimum
Table 4 Equipment location options
Figure 12 Typical installation of a GPS receiver for OTMR function
6.5
EMC
6.5.1
To ensure EMC, the equipment should not be located close to known sources of
EMI, such as the following:
a)
Rotating machines and associated chokes and cables
b)
Line filter chokes
c)
Traction converters, transformers and alternators
d)
HV cables (25kV locomotives and EMUs)
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6.5.2
6.6
e)
Pantographs and circuit breakers
f)
Large contactors and electro-pneumatic (EP) valves.
EN 50155:2007 specifies the minimum limits for both emissions and susceptibility
that are required to maintain electromagnetic compatibility with the existing
vehicle equipment. If the equipment is to be located in the vicinity of possible
sources of EMI, such as those set out in 6.5.1, then more onerous limits
regarding susceptibility should be applied.
System approval
6.6.1
The process for the approval of new vehicles and modifications to existing rolling
stock is set out in The Railways and Other Guided Transport Systems (Safety)
Regulations 2006 (ROGS) and the Interoperability Regulations, issued by the
Department for Transport. Duty holders issue their own internal guidance on
these regulations, and other documents are produced by Network Rail, the
Association of Train Operating Companies, vehicle owners and the Office of the
Rail Regulator (ORR).
6.6.2
RGSs that are typically relevant are set out in Table 5. However, this list is not
exhaustive and should be reviewed for each application.
Number
Title
Notes
GE/RT8015
Electromagnetic Compatibility
between Railway Infrastructure
and Trains
GM/RT2100
Structural Requirements for
Railway Vehicles
GM/RT2120
Requirements for the Control
of Risks Arising from Fires on
Railway Vehicles
GM/RT2149
Requirements for Defining and
Maintaining the Size of
Railway Vehicles
Warning Signs and Labels
Fitted to Electrical Equipment
on Rail Mounted Vehicles
Equipotential Bonding of Rail
Vehicles to Running Rail
Potential
The scope of work should for the most
part be dependent on the
characteristics of the system, and
clarification should be determined at
an early stage
Generally, for equipment that is less
than 5 kg, compliance can be obtained
‘by inspection’. However, for
equipment greater than this, structural
calculations should be provided to
demonstrate that the equipment and
mountings can withstand the specified
mechanical load cases
The choice of materials and how the
equipment is enclosed are important
factors. Generally, cables should
meet these requirements. However,
there are a range of data and antenna
cables that do
These requirements usually only apply
to any antenna installations. Further
guidance on antennas is set out in 6.4
This is dependent on the location,
access requirements, and electrical
protection
All exposed conductive parts that may
become live should be bonded to the
vehicle’s main structure and this is
most easily demonstrated by fitting a
wire link between the equipment and
chassis
GM/RT2300
GM/RT2304
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Number
GE/RT8270
Title
Notes
Assessment of Compatibility of
Rolling-Stock and
Infrastructure
This standard mandates requirements
and responsibilities for the route
acceptance of rail vehicles for
operation on Network Rail controlled
infrastructure, necessitated by the
introduction of a new vehicle,
modification to an existing vehicle,
change to the operation of the vehicle
or change to the infrastructure
Table 5 RGSs relevant to system approval
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Appendix A
A.1
A.1.2
A.1.2.1
Technology Overview
Overview of positioning technologies
A.1.1
This appendix sets out the organisational and technical background to the GNSS
services that are available now (GPS and EGNOS), and those planned to
become available in the coming years.
A.1.2
To indicate that the services are expected to be complementary, it introduces
how these could work together. A future upgrade to take advantage of the new
services should concern only the software in the equipment; the antenna and
cabling not needing modification. Indeed, there are some multi-service locators
already being marketed.
A.1.3
The current external GPS augmentation services, both open and commercial, are
set out.
General
Satellite navigation services
A.1.2.1.1 In order to structure the description of satellite navigation and associated
technology, it is useful to have a simple picture of the way that various services fit
together and can be combined. A basic illustration of this is set out in
Figures 13 to 16.
A.1.2.1.2 GLONASS is connected to the system with a dashed line, as its constellation of
satellites is incomplete. However, the Russian authorities are upgrading this
system and it is expected to have an important role in the future as a complement
to GPS.
GPS
GLONASS
Galileo
Onboard equipment
Aerial
Antenna
Map
Navigation
Locator
Position, velocity, time
(a) Standalone GNSS service
Figure 13 Generic view of satellite navigation services: standalone
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GPS
Galileo
GLONASS
External Augmentation
System
Antenna
Aerial
Aerial
Aerial
Onboard equipment
Antenna
Aerial
Receiver & Navigation
Locator
Processor
Map
Augmentation
Receiver
Receiver
Position, velocity, time
(b) External augmentation
Figure 14 Generic view of satellite navigation services: external augmentation
Galileo
GPS
Onboard equipment
Map
GLONASS
Aerial
Antenna
Locator
Onboard
system
Position, velocity, time
(c) Onboard augmentation
Figure 15 Generic view of satellite navigation services: on-board augmentation
(hybridisation)
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GPS
Galileo
Onboard
equipment
Map
GLONASS
Antenna
Aerial
Navigation
Locator
Processor
Complementary
Radionavigation
Service
Services
Antenna
Aerial
Complementary
Service Receiver
Position, velocity, time
(d) Complementary radionavigation service
Figure 16 Generic view of satellite navigation services: complementary
radionavigation
A.1.2.1.3 These figures show that there are a number of ways in which the satellite
navigation service can be used or processed on the vehicle:
a)
Standalone GNSS service (Figure 13) –
The signals from the satellite system(s) are received through a single
antenna and are processed on board the vehicle within a single receiver to
give basic position, velocity and time information, without the addition of any
external data other than that from maps or a locations database. At present,
the standalone service is restricted to GPS alone, although in the future
there should be additional services from Galileo and the rejuvenation of the
Russian GLONASS system. These systems can be used separately or in
combination
b)
External augmentation (Figure 14) –
The standalone service is combined with external (to the vehicle) signals
from a so-called GNSS augmentation service that improves the performance
of the basic service in some way, most often by offering improved accuracy
(through corrections) or integrity (through monitoring) at a fixed site. The
GNSS augmentation system does not offer a standalone service – it can
only function as long as the appropriate standalone service is also present.
A.1.2.1.4 On-board augmentation (Figure 15) –
The standalone service is combined with data from an on-board system, such as
a tachometer and / or an Inertial Navigation System (INS), perhaps with mapmatching, to improve service coverage, accuracy and integrity. This
augmentation is set out in A.2.2 and the following sections.
A.1.2.1.5 Complementary radionavigation services (Figure 16) –
The standalone service is combined with data from an external complementary
radionavigation service different to, and independent of, the GNSS core system.
A.1.2.1.6 Integration of the various services can be performed, either:
a)
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In the GNSS receiver so that a single navigation output is obtained, or
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b)
In a separate navigation processor such that separate navigation outputs
can be obtained from each of the independent systems being used.
A.1.2.1.7 Irrespective of the type of augmentations used for satellite navigation, it is
necessary to have some form of geographical database or map data to reference
data required by the application.
A.1.2.2
External augmentation services
A.1.2.2.1 Augmentation systems provide additional data that are combined with the
standalone GNSS systems to improve performance. The data that augments
GPS is transmitted over an independent transmission system.
A.1.2.2.2 The principal wide-area GPS augmentation system available or planned at the
European level is the European Geostationary Navigation Overlay Service
(EGNOS). This is set out in further detail in A.2.1.3.
A.1.2.2.3 Commercial GPS Space-Based Augmentation Systems (SBAS) with UK
coverage include:
a)
OmniSTAR
b)
SkyFix
c)
StarFire.
A.1.2.2.4 A description of these commercial wide-area augmentation services is set out
in A.2.1.10.
A.1.2.2.5 Local augmentation systems available in the UK include:
a)
Differential GPS (DGPS) available across the UK and Ireland (broadcast
from radio beacons by the General Lighthouse Authorities)
b)
The Ordnance Survey DGPS system, OS Net™ (not yet available in realtime)
c)
Assisted GPS (A-GPS) disseminated over, for example, Global System for
Mobile communication (GSM) or 3G mobile networks.
A.1.2.2.6 A description of these ground-based augmentation services is set out in A.2.
A.1.2.3
On-board augmentation
A.1.2.3.1 On-board augmentation systems are also known as receiver hybridisation. They
are classified into three groups:
a)
b)
Those that are focused on integrity checking alone, that is to say, the onboard system is used to cross-check the position generated by the GNSS
and identify fixes which appear to have larger errors than predicted. This
type of system can be classified into two types:
i)
Receiver Autonomous Integrity Monitoring (RAIM), where the GNSS
receiver itself determines the integrity of the navigation solution using
redundancy of visible satellites. This is the better solution, as the data
fusion processing is able to optimise the solution and detect doubtful
data
ii)
Vehicle Autonomous Integrity Monitoring (VAIM), where other on-board
sensors are used to cross-check the integrity of the GNSS navigation
solution. This is possible, but is not known to be advantageous
Integrated systems, where the GNSS system and the on-board system are
coupled to a greater degree than simply cross-checking, in order to improve
the overall navigation solution
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c)
Map-matching and map-aiding, not as part of the application, but as part of
the position solution.
A.1.2.3.2 These augmentations are set out in A.2.2 and the following sections.
A.1.3
A.1.3.1
Global Positioning System (GPS)
Institutional arrangements
A.1.3.1.1 GPS is a publicly-owned system of the USA. The 1996 Presidential Decision
Directive (PDD) NSTC-6 establishes national policy for management and use of
the USA Global Positioning System and related USA Government
augmentations.
A.1.3.1.2 The 2001 GPS SPS Specification defines levels of performance the USA
Government commits to provide to civil GPS users. This document:
a)
Identifies performance standards the USA uses to manage SPS
performance
b)
Standardises SPS performance parameter definitions and assessment
methodologies
c)
Describes historical SPS performance characteristics and ranges of
behaviour.
A.1.3.1.3 Other standards documents are:
A.1.3.2
a)
GPS Signal-In-Space Interface Control Documents: ICD-GPS-200C / IRN200C-005R1 for civil L1 / L2 signals and ICD-GPS-705 for civil L5 signals [1]
b)
The Federal Radio Navigation Plan (including the Federal Radio Navigation
Systems document) which:
i)
Presents the current federal policy and plan for common-use civil and
military radio navigation systems
ii)
Outlines the government’s approach for implementing new and
consolidating existing radio navigation systems
iii)
Provides government radio navigation system planning information and
schedules.
Technical overview
A.1.3.2.1 GPS comprises a constellation of (nominally) 24 orbiting Department of Defense
satellites to provide navigation, position, velocity and precision timing services to
users worldwide. Initial Operational Capability (IOC) of GPS was declared at the
end of 1993, with Full Operational Capability (FOC) being declared in 1995. GPS
provides services to both military and civilian users.
A.1.3.2.2 Planning of GPS began in the 1970s with the first satellites being launched in
1978. Applications development began in 1979, when there were four satellites
in orbit; there are now about 30 serviceable satellites in orbit. The overall GPS
programme to-date has comprised three major block programmes with a fourth
block programme ongoing. These are:
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a)
Block I, for proof of concept
b)
Block II / IIA, for development of the first operational constellation
c)
Block IIR, for replenishment of the constellation
d)
Block IIF, for follow-on development of new capabilities, launched in 2007.
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A.1.3.2.3 GPS provides two levels of service: The Precise Positioning Service (PPS) giving
full system accuracy to designated users (mainly the US military and its allies);
and the Standard Positioning Service (SPS) providing accurate positioning to all
users equipped with a suitable receiver.
A.1.3.2.4 Each GPS satellite broadcasts a navigation message on a
1575.42 MHz L-band carrier with spread spectrum modulation. This broadcast is
referred to as the L1 signal, and supports the SPS. The minimum SPS received
power is specified as -160.0 dBW.
A.1.3.2.5 The GPS satellite also transmits a second ranging signal, known as L2. L2, like
L1, has spread spectrum modulation and is transmitted at 1227.6 MHz.
A.1.3.3
The open service provided
A.1.3.3.1 The SPS provides accurate positioning free-of-charge to all users. There are no
service guarantees or liabilities provided. PPS is not available and is no longer
considered. However, the frequency L2 by virtue of its phase and Doppler
characteristics is usable, and high-performance surveying and navigation
receivers depend upon it.
A.1.3.3.2 The navigation data contained in the L1 signal are composed of satellite clock
and ephemeris data for the transmitting satellite, plus GPS constellation almanac
data, GPS to UTC (USNO) time offset information, and ionospheric propagation
delay correction parameters for use by SPS users.
A.1.3.3.3 The entire navigation message repeats every 750 seconds. Within this
750-second repeat cycle, satellite clock and ephemeris data for the transmitting
satellite are sent 25 separate times so they repeat every 30 seconds. As long as
a satellite indicates a healthy status, a receiver can continue to operate using
these data for the validity period of the data (up to four or six hours). The
receiver updates these data whenever the satellite and ephemeris information are
updated – nominally once every two hours.
A.1.3.3.4 The accuracy delivered by GPS varies as a function of the user’s latitude and
longitude, primarily due to constellation geometry. It varies over a one- to threeday cycle because of the relationships of the individual orbits. This effect is
termed the dilution of precision. Its value can vary from approximately unity to
over 10. The prevailing accuracy parameter is given by the nominal value
multiplied by the DOP value. Accuracy also varies over an 11-year period, due to
the solar sun-spot cycle.
A.1.3.4
User equipment
A.1.3.4.1 A GPS antenna and receiver are required; these are available at low cost and
can be either passive or active. Standalone receivers and engines of various
qualities, for integration into bespoke solutions can be used, as well as off-theshelf integrated applications. Hybrid GPS / EGNOS receivers are also available.
A.1.3.4.2 Receiver outputs providing navigation and time information are standardised;
therefore there are no potential compatibility problems, although some
specialised data is only available from receivers in manufacturer-specific formats.
A.1.3.4.3 Receivers have been used on trains in Britain and across Europe for over
10 years, and the antennas are within the operational restrictions given by
Britain’s smaller loading gauge.
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A.1.3.5
Performance
A.1.3.5.1 The following parameters are the published performance figures for GPS based
on a nominal 21 satellite constellation plus three spares. Actual performance
often, but not always, exceeds these values:
Horizontal accuracy:
13 m (95%)
Vertical accuracy:
22 m (95%)
Coverage:
Worldwide, up to 3000 km vertically
Availability (of SIS): 99% assuming: expected
horizontal error under 36 m (95%) and expected
vertical error under 77 m (95%)
Continuity (of SIS):
Information not available
Fix interval:
1-20 per second
Fix dimensions: 3-D positioning and time if four or
more satellites are available
2-D positioning and time when three satellites are
available
System capacity:
Unlimited
Ambiguity:
None
Integrity:
Not specified, but historical performance is available.
A.1.3.5.2 GPS system architecture includes many features such as redundant hardware,
robust software and rigorous operator training to minimise integrity anomalies. In
the event of an anomaly, for example a clock error or incorrect ephemeris, the
best response time could be about several minutes, which is insufficient for
certain applications. For such applications, augmentations such as RAIM in the
receiver may be required.
A.1.3.6
Future evolution
A.1.3.6.1 GPS is undergoing a process of continuous improvement with the aim of
transitioning to a future manifestation, termed GPS III.
A.1.3.6.2 From a civil perspective the most important developments are two new civil
signals at L2 and L5 (1176.45 MHz). The new civil L2 signals recently became
available on the first Block IIR-M satellites. An Initial Operational Capability (IOC)
is expected in 2009 and a Full Operational Capability (FOC) is expected in 2012.
A.1.3.6.3 The new civil L5 signals became available on the first Block II-F satellites
launched in 2007. L5 IOC is anticipated for 2012 and FOC for 2015.
A.1.3.7
Disadvantages of using GPS alone
A.1.3.7.1 If GPS is used as a standalone system, with no augmentation, the following
points should be taken into account:
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a)
There is absolute reliance on an uncontrolled third-party for service provision
b)
There are no service guarantees or liabilities provided
c)
There is undefined integrity in the SIS
d)
The signals are always lost in tunnels
e)
The signals are obscured in cuttings, by foliage and buildings and in stations
f)
It suffers from multipath effects, due to reflection
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g)
The reliability of the signal at a particular location varies periodically
according to the number of satellites in view and their geometry (this
decreases over time as more satellites are launched)
h)
The service delivered varies as a function of latitude and longitude, primarily
due to constellation geometry
i)
The service delivered also varies over an 11-year period, due to the solar
sun-spot cycle.
A.1.3.7.2 Some of these characteristics are typical of satellite systems and are not unique
to GPS – they are only worthy of consideration if GPS is used in isolation. When
available, the improved GPS and other GNSS implementations can only be better
than this initial GPS. An application which works with GPS today, obtains, in due
course, a noticeably improved performance.
A.1.3.7.3 General vulnerabilities of GPS are well-known and have been extensively
documented in:
A.1.4
A.1.4.1
a)
The Volpe Report on GPS Vulnerability (2001) [9]
b)
Helios Technology, European Union Radionavigation Plan [10]
c)
‘Civilian GPS systems and potential vulnerabilities’, Benshoof [11].
Galileo
Institutional arrangements
A.1.4.1.1 At the time of writing, the institutional arrangements supporting the Galileo
programme are as per the EC communication published in September 2007;
these are summarised in Figure 17.
Figure 17
Galileo institutional arrangements
A.1.4.1.2 Galileo is a joint initiative by the European Union and the European Space
Agency (ESA). The European Union, represented by the European Commission,
is responsible for the political dimension of Galileo and for setting objectives.
ESA is responsible for the technical definition, development and validation of
Galileo.
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A.1.4.1.3 The oversight role belongs to Council and Parliament and takes the form of:
a)
Political oversight, exercised directly by the Council and the European
Parliament, and
b)
Programme oversight in the form of a ‘European GNSS Programme
Committee’, in which representatives from the Member States assist in the
implementation of the programme and provide overall guidance on all
important aspects of the programme.
A.1.4.1.4 As the institution that is directly accountable to Council and Parliament, the
European Commission has overall programme management responsibility.
A.1.4.1.5 The Commission is accountable for the entire Galileo programme, and to that
effect:
a)
Has management and / or contractual control over all the subordinate
implementation levels
b)
Has access to both financial resources and to the political authorities, that
can provide the necessary arbitrage between all elements of the programme
c)
Acts as sponsor for the programme, overseeing all development,
procurement, operations and maintenance, and exploitation contracts
related to the system infrastructure.
A.1.4.1.6 The GNSS Supervisory Authority (GSA) is effectively an organisational tool of the
EC, insofar that:
a)
It acts as the accreditation authority, and is responsible for organising
certification and security aspects of the programme
b)
It is responsible for the long-term commercial exploitation of the Galileo and
EGNOS systems
c)
It advises and assists the programme manager on all aspects of the
programme.
A.1.4.1.7 As the co-initiator of the European GNSS programmes and the technical architect
of these programmes, the ESA acts as procurement agent and overall prime
contractor.
A.1.4.2
Technical overview
A.1.4.2.1 Galileo comprises a constellation of 30 satellites in three planes inclined at 56° to
the equator orbiting at an altitude of nearly 24,000 kilometres.
A.1.4.3
Services provided
A.1.4.3.1 Galileo is intended to provide five different services:
a)
Open access
b)
Safety-of-Life
c)
Public regulated
d)
Commercial
e)
Search and rescue.
A.1.4.3.2 These are generated from combinations of up to 10 different signals with
associated ranging codes and navigation data using broadcast and point-tomultipoint connectivity, and complement the GPS services to deliver enhanced
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benefits to users. All the Galileo satellites share the same nominal frequency,
making use of Code Division Multiple Access (CDMA) compatible with GPS.
A.1.4.3.3 Six of these signals, including three data-less channels, so-called pilot tones
(ranging codes not modulated by data), are accessible to all Galileo users on the
E5a, E5b and L1 carrier frequencies for Open Services (OS) and Safety-of-Life
(SoL) services.
A.1.4.3.4 Two of the signals on E6 with encrypted ranging codes, including one data-less
channel, are accessible only to some dedicated users that gain access through a
given Commercial Service (CS) provider.
A.1.4.3.5 Also, two signals (one in E6 band and one in E2-L1-E1 band) with encrypted
ranging codes and data are accessible to authorised users of the Public
Regulated Service (PRS).
A.1.4.4
User equipment
A.1.4.4.1 A Galileo antenna and receiver are required; these should be easily purchased
for a given level of service. They are expected to be readily available by the time
that the system is operational, as considerable funding has been dedicated to
development of the Galileo user segment.
A.1.4.4.2 Galileo operates in the same frequency band as GPS and it is, therefore,
relatively straightforward to develop combined receivers at a cost equivalent to, or
only slightly higher than, that of a single system receiver, especially if the receiver
processing is performed in software.
A.1.4.4.3 Combined receivers offering different combinations of Galileo, GPS, EGNOS and
other services are available.
A.1.4.5
Performance
A.1.4.5.1 The expected performance of the Galileo OS is set out below. Higher levels of
performance can be expected for the other services, that is to say, SoL, PRS, etc.
Horizontal accuracy:
15 m (95%, single frequency)
4 m (95%, dual frequency)
Vertical accuracy:
35 m (95%, single frequency)
8 m (95%, dual frequency)
Coverage:
Worldwide
Availability (of SIS): 99.8%
Fix interval:
1-20 per second
Fix dimensions:
3-D positioning and time if four or more satellites are
available
2-D positioning when three satellites are available
System capacity:
Unlimited
Ambiguity:
None
Integrity:
None for OS.
A.1.4.5.2 The combination of GPS and the Galileo OS offers users the opportunity to
benefit from both constellations via an interoperable receiver. Service availability
should be significantly enhanced, especially when the number of visible satellites
may be partially obscured. Each receiver should be able to position itself to
approximately twice as many satellites, therefore increasing the likelihood of
being able to achieve the required accuracy.
A.1.4.6
Future evolution
A.1.4.6.1 The timeframe 2013 is the earliest operational date for the OS.
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A.1.4.6.2 Looking forward, the development of a Galileo evolution plan has been approved
by EU Member States, although the associated activities do not yet have an
assigned budget.
A.1.4.7
Summary of augmentation services
A.1.4.7.1 The choice of augmentations has an impact on the locator unit complexity and
cost. The following augmentation types are set out in A.2. They can be used
singly or in combination.
A.2
Augmentation services
A.2.1
A.2.1.1
Augmentation external to the train
Assisted GPS (A-GPS)
A.2.1.1.1 The performance of conventional GPS (unaugmented) is highly unreliable when
surrounded by tall buildings (as a result of multipath), or when the satellite signals
are weakened by being indoors or under trees.
A.2.1.1.2 Furthermore, even under conditions of strong GPS satellite visibility, when first
turned on, some non-standard GPS units may not be able to download the
almanac and ephemeris information from the GPS satellites, rendering them
unable to function until a clear signal can be received continuously for up to one
minute. (See definitions section for an explanation of these terms.)
A.2.1.1.3 An Assisted GPS (A-GPS) receiver can address these problems in several ways:
a)
It first locates the user by means of a cell phone interface, which roughly
determines which cell site it is connected to
b)
It supplies orbital data for the GPS satellites to the cell phone, enabling the
cell phone to lock to the satellites when it otherwise could not, and
autonomously calculate its position
c)
It has improved knowledge of ionospheric conditions and other errors
affecting the GPS signal than the cell phone alone, enabling a more precise
calculation of position.
A.2.1.1.4 Some A-GPS solutions require an active connection to a data network to function
− in others it simply makes positioning faster and more accurate, but is not
mandatory.
A.2.1.1.5 A-GPS Summary:
A.2.1.2
a)
Signal assists the plain GPS processing by providing additional data
b)
Aims to eliminate ‘urban canyon' effects
c)
Requires visibility of GPS and is therefore subject to masking problems
d)
Does not require a separate antenna or receiver.
Augmentation services – SBAS
A.2.1.2.1 A Space-Based Augmentation System (SBAS) uses a network of reference
stations to determine GPS correction data. These corrections are broadcast in a
GPS look-alike signal, modulated with Wide Area Differential (WAD) corrections
and integrity data. The SBAS signal is sent from geostationary satellites that
provide dual coverage over the SBAS region.
A.2.1.2.2 The GPS look-alike signals are normally received through the GPS antenna and
not through a separate communications channel, to improve availability; the WAD
corrections improve accuracy; and the integrity messages improve integrity
(safety or quality of service). Since they are broadcast from satellites, they can
be disrupted in the same way as GPS signals.
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A.2.1.2.3 SBAS can be public (EGNOS) or commercial (OmniSTAR, SkyFix, StarFire). The
commercial services require subscription. It is clear what is being paid for and
users have recourse to a contract with a subscription service.
A.2.1.2.4 EGNOS Summary:
a)
Provides DGPS
b)
Requires visibility of GNSS and the EGNOS satellites, and is therefore
subject to masking problems
c)
Can use the same antenna
d)
Processing is now usually available in COTS, and in consumer receivers
EGNOS is standard.
EGNOS details are set out in A.2.1.3.
A.2.1.2.5 Commercial SBAS − Summary:
a)
Provides DGPS
b)
Requires visibility of GNSS and other satellites, and is therefore subject to
masking problems
c)
Can use the same antenna
d)
Is available only on subscription with proprietary processing.
The details of commercial SBAS are set out in A.2.1.10.
A.2.1.3
A.2.1.4
EGNOS
Institutional arrangements
A.2.1.4.1 EGNOS is the first step of Europe’s Global Navigation Satellite System (GNSS)
policy that culminates in Galileo. EGNOS has been developed by ESA, together
with both the European Commission (EC) and Eurocontrol.
A.2.1.4.2 The EGNOS system is dependent on there being a GNSS service in existence,
such as GPS, GLONASS or, in the future, Galileo. The EGNOS service does not
however require 100% availability or integrity of a GNSS service, since EGNOS is
designed to fill in the gaps in the GNSS service and warn of any integrity or
availability failures.
A.2.1.4.3 Although marketed as a multimodal system, EGNOS has been highly influenced
by the aviation process and has, therefore, been standardised for the aviation
sector. EGNOS has been developed over a seven-year period based on an ESA
System Requirements Document (SRD) that was written in December 1998 at a
time when the international standards had not been concluded. The resulting
SRD refers to:
a)
The International Civil Aviation Organisation (ICAO) Standards and
Recommended Practices, Draft version 7A (R16)
b)
The Radio Technical Committee – Aeronautical (RTCA) Minimum
Operational Performance Standards (MOPS) Change 3.
A.2.1.4.4 EGNOS is part of a global family of SBASs, including WAAS in the US, MSAS in
Japan, and planned systems such as GAGAN in India and BEIDOU in China.
These systems, although independent of each other, are all developed to the
same standards, are interoperable and should provide a global network of often
interlinking coverage.
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A.2.1.5
Technical overview
A.2.1.5.1 EGNOS is one of a number of wide-area SBAS being developed to augment the
USA Global Positioning System (GPS) and the GLONASS system. Each SBAS
broadcasts a GPS look-alike signal modulated with wide-area differential (WAD)
corrections and integrity data. This information is gathered from a dedicated
network of ground reference stations and is broadcast from dedicated
geostationary satellites that provide dual coverage over the SBAS region. The
additional GPS look-alike signals improve availability; the WAD corrections
improve accuracy, and the integrity messages improve integrity (safety or quality
of service).
A.2.1.5.2 EGNOS provides a Europe-wide, standardised and quality-assured correction
system to support GPS. Its high-compatibility with GPS means that a single
antenna and receiver can process both the GPS and EGNOS signals, eliminating
the need for a separate radio to receive differential corrections. This allows many
users to dispense with their current local-area differential or commercial services.
A.2.1.5.3 EGNOS is available over Europe and is due to enter full operation during 2009,
but the precise date is liable to change.
A.2.1.5.4 EGNOS has been designed to meet the demanding performance requirements
for landing aircraft, as well as having the performance potential to support a
number of mass-market applications:
a)
Accuracy is improved (relative to GPS or GLONASS) to about 2-3 m vertical
and 1-2 m horizontal through the broadcast of WAD corrections. The
specified worst-case accuracy is 7.6 m
b)
Integrity (safety) is improved both through the high degree of redundancy in
the system and by alerting users within 5.2 seconds if a fault with EGNOS,
GPS or GLONASS is detected
c)
Availability is improved by broadcasting GPS look-alike signals from three
geostationary satellites.
A.2.1.5.5 The actual performance achieved by EGNOS in an urban environment may not
reach the specified performance levels above. If the level of GPS stand-alone
performance is poor (for example, due to multipath or poor DOP), then EGNOS
may not result in a significant performance improvement.
A.2.1.5.6 In a typical rail environment, it is also likely that the EGNOS messages from the
geostationary satellite will be obscured by trees, tunnels, cuttings, etc. EGNOS
messages may alternatively be received via a web server and terrestrial
communications, but this may not enable the real-time determination of corrected
position.
A.2.1.5.7 To be effective, the integrity information should be received and acted upon
within the specified period. The life of the corrections is longer, and their
usefulness degrades over an undefined period that can be up to about
10 minutes.
A.2.1.6
Services provided
A.2.1.6.1 Of the multiple services that EGNOS provides, there are two basic services
defined to match the categorisation of the Galileo services:
a)
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The open service consists of signals for timing and positioning, freely
accessible from EGNOS satellites without any charge. This service is
accessible to any user equipped with an SBAS compatible receiver within
the EGNOS open service area. No guarantees are associated with this
service
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b)
From the time of qualification of the EGNOS system, after the initial
operations phase, a Safety of Life (SoL) service should be available to
transport applications. The SoL service is accessible to any user equipped
with an SBAS compatible certified receiver within the EGNOS SoL service
area.
A.2.1.6.2 The SoL service, accessible through the EGNOS geostationary satellites, can be
used in most safety-critical applications in the transport markets. For example,
the SoL service should be available in Europe for commercial aviation
applications, such as non-precision approaches with vertical guidance. The
specific application enablers should be developed to utilise the SoL service, such
as compatible certified receivers, flight procedures, etc.
A.2.1.7
User equipment
A.2.1.7.1 Most COTS GPS equipment available is provided with built-in EGNOS
functionality. Combined GPS / GLONASS / EGNOS receivers are also available.
A.2.1.7.2 Some combined receivers, which include Galileo, are also being marketed.
A.2.1.8
Performance
A.2.1.8.1 EGNOS performance parameters are set out below:
Horizontal accuracy: 1 to 2 m (Reported: note that the specification is to
provide 7.4 m)
Vertical accuracy:
3 to 4 m
Coverage:
ECAC area
Availability:
0.95
Continuity:
8×10-7 /approach
Integrity:
2×10-7 /approach
Fix interval:
Data rate is 250 bits per second
Fix dimensions:
Three-dimensional
System capacity:
Unlimited
Ambiguity:
None
Spectrum:
1559-1620 MHz service band.
A.2.1.9
Future evolution
A.2.1.9.1 The future evolution of EGNOS is uncertain – an evolution plan has been
developed by ESA but as yet there is no budget allocated. Politically, EGNOS is
viewed as a precursor to Galileo, although the two systems are completely
independent.
A.2.1.10
Commercial Space-Based Augmentation Systems (SBAS)
A.2.1.10.1 SBAS, include OmniSTAR, SkyFix and StarFire. These systems use networks
of reference stations to determine GPS correction data, and broadcast these
corrections via satellite links to users equipped with bespoke receivers, which
are much more complex than ordinary GPS receivers. These services are
provided on a subscription basis.
A.2.1.10.2 This type of system delivers an accuracy of better than a metre (at 95%
confidence), but without a guaranteed availability or level of integrity.
A.2.1.10.3 Eutelsat is an established satellite communications provider offering many
facilities, including mobile fleet tracking, which is widely used in the road
haulage industry. The satellites follow equatorial geostationary orbits. The
EutelTRACS™ facility was recommended in T043 for tracking facilities.
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A.2.1.11
Augmentation services – terrestrial
A.2.1.11.1 Terrestrial augmentations broadcast differential corrections and integrity
messages over a ground-based data link. Some terrestrial augmentations are
similar to SBAS in that they derive their augmentations on a regional basis,
whereas others are purely local. Both cases are dissimilar to SBAS in that they
do not use satellites. The signals from a single terrestrial data broadcast
station, for example a radiobeacon, generally provide corrections over a small
area. However, terrestrial augmentation services are usually organised on a
network basis such that whatever the user’s location, one or more broadcast
stations are visible (subject to physical obscuration and obstructions).
A.2.1.11.2 Terrestrial signals are not liable to the same disruptions as satellite signals. For
example, the propagation properties of the radio waves at some of the
frequencies used, means that line-of-sight to the broadcast station is not always
necessary.
A.2.1.11.3 DGPS provided through a terrestrial augmentation only gives augmentation and
therefore cannot be used in isolation from GPS.
A.2.1.11.4 Terrestrial augmentations may require a separate antenna and receiver.
Examples include the maritime radiobeacons (which provide DGPS) and the
eLORAN complementary system. Internet connections are becoming a
possibility, but these services are new and their integrity is as yet unknown.
A.2.1.11.5 Summary − DGPS:
a)
Has good propagation properties and good coverage of the UK
b)
Requires processing often
c)
Is not always present in COTS receivers
d)
Needs a separate antenna and receiver.
A.2.1.11.6 Summary − eLORAN:
A.2.1.12
a)
Should have good propagation properties and reasonable coverage of
the UK
b)
Operates separately to GNSS
c)
Needs a separate antenna and receiver.
Ground-based augmentation systems
A.2.1.12.1 The marine radiobeacon DGPS service provides the user with differential
corrections and integrity messages for the GNSS satellites in view, principally
GPS at present, but also including GLONASS in some cases. These
corrections are calculated locally and are only reliable over a limited distance
from the broadcast station, but provide the user with an accuracy of about a
metre (at 95% confidence), together with integrity information. This system
requires a special receiver to receive and process the augmentation data, but
most GPS receivers are capable of processing the corrections. Its reception
quality in the railway environment is unknown.
A.2.1.12.2 OSNet™ is a GPS-based augmentation system provided by the Ordnance
Survey. It uses the GPS satellite network and supplements the GPS signals
with a network of reference stations. This network is used to determine the
differential corrections to be applied to improve the accuracy of GPS. These
corrections are available free-of-charge for post-processing and are also
provided on a subscription basis in real-time over several communications
bearers, including the internet and GSM.
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A.2.1.12.3 OSNet™ can be used at different levels of accuracy to obtain different levels of
service. The highest level of accuracy available is in the order of centimetres
but this needs post-processing (that is to say, it is not usable in real-time) and
has no commitment to availability or integrity. It is reported to be unsuitable for
communications.
A.2.1.12.4 A potential future ground-based augmentation may be provided by eLORAN,
which is a commercial system that can use existing high-powered lowfrequency radio infrastructure to supply a Europe-wide augmentation signal. A
major advantage is that its power and frequency means that the eLORAN signal
is available in many situations where the GPS signal is marred or is unavailable.
Its reception quality in the railway environment is unknown.
A.2.2
A.2.2.1
On-board augmentation services
Processing of information contained in the GPS signal-in-space
A.2.2.1.1 The signal-in-space is able to provide the following information:
a)
Code-based pseudo-range measurements from carrier L1 (and L2C on
newer Block II-RM GPS satellites)
b)
Doppler measurements from carriers L1 and L2
c)
Carrier phase measurements from carriers L1 and L2.
A.2.2.1.2 The pseudorange data is derived from the signals modulated on the L1 carrier
form the routine processing offered by all GPS receivers. The use of data from
a second carrier enables the receiver to remove the majority of the error
contributions from the ionosphere and troposphere effects on propagation. It is
expected to be routinely available from the majority if not all navigation satellites
launched in the future. Existing GPS dual frequency receivers have tended to
be expensive, because the code is not routinely available on the older GPS
satellites and different processing was required. As replacement satellites are
launched and new constellations enter service, the codes will be available
enabling dual frequency receivers to be competitive in terms of cost and
performance.
A.2.2.1.3 The use of carrier phase-based positioning techniques is becoming more
common in COTS equipment and has the potential to provide very high
accuracy. Successful carrier phase positioning requires the resolution of the
number of integer carrier phase cycles between the user and each satellite in
view. To work successfully, it relies on a technique known as integer ambiguity
resolution. This is a crucial element of phase processing because, if the wrong
integer ambiguity is calculated, the resulting measurements of range between
user and satellite can be in error by at least 19 cm.
A.2.2.1.4 Use of carrier phase measurements for navigation and positioning requires
sensitive receiver equipment and complex processing. Continuous lock should
be maintained on visible satellites over a period of time to allow successful
ambiguity resolution. In the event of signal loss, the integer number of carrier
phase cycles between the user and satellite is effectively lost, causing what is
known as a ‘cycle slip’. Cycle slips corrupt the carrier phase measurement and
require the user receiver to recommence the process of ambiguity resolution
during which time the position solution will be in error. Detection and correction
of cycle slips becomes more onerous when the user receiver is located on a
dynamic platform.
A.2.2.1.5 Two kinematic techniques are in use:
a)
Real-Time Kinematic (RTK): GPS is a precise positioning technique that
provides high accuracies over short baselines (typically 10 km or less)
between a reference receiver and one or more mobile rover receivers.
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RTK is used in applications where precision is vital, as achieved
accuracies can be at the sub-centimetre level. The typical accuracy for
RTK is 1 cm ± 2 parts-per-million (ppm) horizontally and 2 cm ± 2 ppm
vertically
The RTK process involves the transmission of GPS signal corrections in
real-time from the reference receiver at a known location to the remote
rover receivers. This technique allows the mobile users to correct for the
effects of atmospheric delay, orbital errors and other variables in GPS
geometry. RTK utilises the more precise GPS carrier phase
measurements rather than the code phase typically used in regular
differential applications
b)
Long Range RTK: Ambiguity resolution issues, coupled with inaccurately
modelled satellite orbit errors and atmospheric effects, constrain the
baselines of regular RTK to tens of kilometres. However, new
developments in the modelling of ionospheric effects, coupled with an
ionosphere-free ambiguity resolution technique, have led to successful
long-range RTK. Long-range RTK has the capability to work over
baselines up to 100 km
The technique only differs from regular RTK in terms of the algorithms
used to establish the position solution. Long-range RTK is a subject
currently undergoing significant levels of academic research and is
therefore cutting edge in nature. Some commercial applications, such as
Magellen’s proprietary Long Range Kinematic (LRK) technique, claim
centimetre level accuracy over baselines of up to 40 km.
A.2.2.2
Receiver Autonomous Integrity Monitoring (RAIM)
A.2.2.2.1 RAIM utilises statistical detection theory to answer two questions: does a failure
exist and, if so, which is the failed satellite? The principles set out here can be
extended to include all sources of data used by a locator, not only the GPS
data. The term VAIM has been proposed for this.
A.2.2.2.2 A RAIM capability sufficient to answer the first question, but not the second, is
termed Fault Detection (FD), whereas a RAIM solution capable of addressing
both questions is termed Fault Detection and Isolation (FDI) or Fault Detection
and Exclusion (FDE).
A.2.2.2.3 The basic principle of RAIM is that it uses signals from all visible satellites to
determine consistency across position solutions determined from different sets
of satellites.
A.2.2.2.4 The first case, FD RAIM, requires the user to have at least five satellites in view.
With five satellites, it is possible to compare five different positions derived from
different sets of four satellites, each solution disregarding the signal from a
(different) visible satellite. Statistical checks are then applied to determine
whether the solutions are consistent. If the solutions are consistent, the solution
is accepted, whereas, if the solutions prove to be inconsistent, an integrity alarm
is given. Without prior knowledge it is not possible to determine which of the
five (one correct and four erroneous) solutions is correct, as the true position is
unknown and the RAIM check only determines inconsistency.
A.2.2.2.5 FDI and FDE RAIM need six satellites to be in view to determine the faulty
satellite and allow it to be removed from the position solution. FDI and FDE
RAIM use slightly different processes to remove the faulty satellite from the
position solution.
A.2.2.2.6 FDI RAIM compares the solutions available from all of the visible satellites, from
the maximum down to the minimum number (four). One satellite at a time is
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excluded from the solution. The solutions are checked for self-consistency, and
the faulty satellite identified uniquely and its navigation signal disregarded.
A.2.2.2.7 FDE compares the solutions available from the set of satellites used in the
detection suite. For example, if the receiver usually utilises four satellites to
determine its navigation solution and six are visible, two satellites are excluded
at a time and solutions for each minimum set of satellites compared for selfconsistency. Therefore, the algorithm may not determine precisely which
satellite is faulty, but does exclude the faulty satellite from the navigation
solution (in this example two satellites would be excluded − it not being
necessary to determine uniquely which one is faulty).
A.2.2.2.8 FDE has the advantage over FDI in that it can cope with multiple satellite
failures, as long as sufficient satellites are in view. Also, as FDE does not
identify uniquely the faulty satellite, the probability of errors is reduced
compared with FDI. However, the computational effort associated with FDE
increases rapidly with the number of satellites in view, as it needs to compare
all possible combinations. For example, for eight visible satellites, FDI would
make the following computations:
a)
A position solution from all eight satellites
b)
A position from all sets of seven satellites (eight solutions altogether) which
would all be inconsistent
c)
A position from all sets of six satellites (28 solutions altogether), of which
seven would be self-consistent and consistent with one solution from the
previous set.
A.2.2.2.9 This approach would be enough to identify and exclude the faulty satellite for as
long as it remained in view. Once the faulty satellite is identified, it can be
removed from future position calculations, which would then utilise the seven
reliable satellites.
A.2.2.2.10 FDE would, however, compute the solutions from all sets of four satellites (70
solutions altogether), of which 35 would be self-consistent. The other 35
solutions would be disregarded. As it is not known which satellite is faulty, it
would be necessary to repeat this process for every subsequent position
calculation, which would continue to take input from all eight satellites.
A.2.2.2.11 RAIM algorithms are implemented within GNSS receivers. A number of
standards exist for this but these standards are limited to the aviation sector.
The most relevant standards are:
a)
Technical Standards Order (TSO) C129a for FD RAIM, which requires the
implementation of specified failure detection algorithms, along with aiding
of the receiver using input from the aircraft’s barometric altimeter
b)
TSO C146 for a standalone receiver using FDE RAIM, which specifies the
FDE algorithms to be used, and also allows the use of space-based
augmentation systems such as EGNOS (see A.2.1.3 and the following
sections) as well as barometric aiding. A similar standard (TSO C145)
exists for receivers that are integrated into the flight management systems
of the aircraft.
A.2.2.2.12 Summary – RAIM:
a)
Is additional processing in receivers and statistically eliminates erroneous
signals
b)
Relies on a sufficient number of satellites being in view
c)
Does not itself provide any positional information.
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A.2.2.2.13 Summary – VAIM:
A.2.2.3
a)
Is a category covering on-board systems giving augmentation and
hybridisation
b)
Examples include tachometers, odometers and inertial devices
c)
Some may already be fitted to the train, therefore only an interface to the
locator unit is required.
Augmentation – map-matching
A.2.2.3.1 A database of valid solutions, for example a map of track coordinates, is also a
possible on-board augmentation. Such data can be critical to the performance
of the navigation system, for example:
a)
The accuracy of the overall system is only as good as the accuracy of the
weakest component, which could be the map, if used. For example, if the
accuracy of the satellite navigation system is of the order of centimetres,
yet the accuracy of the map is of the order of metres, then the augmented
accuracy of the overall navigation system can only be of the order of
metres
b)
The map and the satellite navigation system need to operate to the same
geodetic reference system. When data from different systems is fused
together, corrections should be made if the systems are operating in
different geodetic reference systems. For example, data co-ordinates in
the WGS-84 (GPS) datum require conversion prior to overlaying on the
standard Ordnance Survey (OS GB36) datum – these are freely available
from public sources.
A.2.2.3.2 Map-matching uses a different approach to analysing the input data. The
previous location of the train is combined with the current location estimate and
an on-board location database or digital map to determine whether or not the
current estimate is reasonable.
A.2.2.3.3 As an example of the use of map-matching for augmentation: if the map shows
that the train is travelling along a dead straight section of railway, then the
locator unit can eliminate any deviations in the received satellite signals,
because no transverse movement is possible. This can enhance the use of
odometer data when the satellite signal is unavailable. Since the digital map
shows the course of the railway line across the land, the known speed of the
train indicates the current position as calculated from the last (accurate) satellite
signal.
A.2.2.3.4 Map-matching techniques can be used to aid location determination when a
satellite signal is recovered (for example, on exit from a tunnel) because the
locator unit already knows approximately where the train should be – at the
geographical location of the tunnel exit.
A.2.2.3.5 However, the main disadvantage of relying on map-matching algorithms is
managing their validity. It is not possible for the locator unit to distinguish
between an erroneous satellite signal and an accurate signal being matched to
an erroneous map. This means that, for applications that should distinguish
between tracks, highly accurate maps should be available and should be
maintained to reflect changes in track layout, for example, due to engineering
work.
A.2.2.3.6 However, at route level, maps of the route (a series of discrete coordinates
along the route) can be compiled from recorded position reports from the trains
operating those routes. This level of map data is useful for basic map-matching
and is already being used on an ad hoc basis by some train operators. At an
even more gross level, low-cost locations databases, which contain the
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geographical locations of stations or other important points, would not require
expensive or frequent updates but are of more limited use.
A.2.2.3.7 Summary − map-matching:
A.2.2.4
a)
Needs up-to-date maps and location databases
b)
Needs the maps to be of the appropriate level of accuracy and resolution
for the applications
c)
Requires integrated processing (now common) for some applications
d)
Can be self-updating, in that errors detected during service can be used as
part of the map correction process at depot.
Tachometry / odometry
A.2.2.4.1 Tachometers and odometers measure distance and / or speed. All trains are
fitted with a form of tachometer. The principle is to count and sum up the
number of wheel rotations. It is not necessary to interface to a tachometer
directly. The train’s speed measurement system may have a suitable interface,
or the OTMR.
A.2.2.4.2 As the value of the measured distance involves the radius of the wheel, the
main error component results from inaccurate knowledge of the wheel radius.
The processing of tachometer information with the other sensors in a locator
enables the diameter and its variation, because of the conic form of the rim, to
be known.
A.2.2.4.3 A significant error, due to wheel slip and slide, should be considered. Because
of the fixed axle, one wheel, and sometimes both, are always sliding. In
addition, there are conditions of low adhesion where an axle can lag behind the
vehicle speed, even when there is no acceleration. When braking, the
opportunities for slide are evident, and this is often when most accuracy is
required. The error of an odometer is naturally only along the track and usually
increases with travelled distance. The processing of tachometer information
with the other sensors in a locator, enables the evolution of the error to be
modelled and corrected.
A.2.2.5
Augmentation – inertial devices
A.2.2.5.1 Inertial devices include accelerometers and gyro-based Inertial Measurement
Units (IMUs). The choice of one or both is a subject for design. The description
here presents the range of characteristics available and the principles of their
use. The presentation focuses on the use of IMUs, as they are most concerned
by these issues.
A.2.2.5.2 A full inertial navigation system consists of three mutually orthogonal
accelerometers and three single-degree-of-freedom (or two twin-degree-offreedom) gyroscopes.
A.2.2.5.3 This system can determine the motion of the train in all three axes, including
angular momentum, and should consequently allow the locator unit to calculate
accurately the train location during a period of loss of the GNSS signals.
A.2.2.5.4 High-performance systems can be very expensive, but cheaper ones may be
used depending on the levels of accuracy required. This is influenced by the
level of drift, which occurs over time and governs the effectiveness of IMUs; this
can be evaluated as the length of signal outage that can be tolerated while
staying within the given accuracy tolerance.
A.2.2.5.5 By sensing the forces acting on a body, acceleration can be extracted. The
vehicle's velocity can then be determined by integrating this acceleration with
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respect to time, and distance can be derived from a further integration. The
gyroscopes measure angular velocity with respect to inertial space and are the
dominant factor in determining the accuracy of the system. Attitude can then be
determined by integrating with respect to time again.
Quality
Drift
Low
> 100 deg / hr
Medium
1 - 100 deg / hr
High
< 1 deg / hr
Table 6 Quality and performance of gyro sensors
A.2.2.5.6 The quality of the IMU ultimately dictates its navigation performance. Table 6
sets out the performance of a single axis gyroscope at various levels of quality.
Typically, IMU gyroscopes of higher quality exhibit drift rates at the level of
fractions of a degree per hour, whereas lower quality sensors, such as MicroElectro-Mechanical Systems (MEMS), exhibit drift rates of hundreds of degrees
per hour.
Evolution of position error for different quality IMUs
High quality
Medium quality
Low quality
100
1 σ position error (m)
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
60
Outage time (seconds)
Figure 18 Horizontal accuracy of IMU following GNSS failure
A.2.2.5.7 Figure 18 illustrates the accuracy performance of various grades of IMU
following the loss of GNSS updates. These figures are based on the measured
performance of three different quality sensors (see reference 13 for full details).
The sensor performance was measured in a car travelling up to 70 mph, and is
therefore not exactly that which would be expected from a train-mounted unit,
due to different motion and dynamics, but can be taken as being reasonably
representative.
A.2.2.5.8 The difference in performance of the sensors indicates that a high-grade sensor
would enable rail users to coast through GNSS outages of greater than one
minute while continuing to meet the 10 m accuracy requirement, whereas use of
low-grade sensors would only compensate for GNSS outages of less than
around 15 seconds. These figures are only indicative, and trials of specific
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systems would be necessary to determine actual IMU performance on board a
train.
A.2.2.5.9 IMU and satellite navigation sensors are typically integrated at the user level
through a Kalman Filter. A Kalman Filter allows positions or measurements
from the satellite navigation receiver to be integrated with measurements from
other sensors that, on their own, may not be sufficient to allow a position fix to
be computed. Such integration can improve the positioning quality and also
ensures that position is continually available during periods of GNSS outage.
There are various ways to integrate IMU and GNSS through a Kalman Filter, for
example:
a)
Uncoupled mode: This combines the GNSS and other navigation sensors
such that the integrated solution does not affect either of the sensors and
there is no feedback to either of them. This is suitable for high-accuracy
sensors, where no calibration is needed. The filtered navigation system
has two independent navigation solutions, one based upon GNSS and the
other based upon IMU sensors. The solutions are combined to produce a
best solution for output or display
b)
Loosely coupled mode: Here the GNSS is the primary sensor that
calibrates the other navigation sensors. The accuracy of the system is
determined by the accuracy of the GNSS alone. The data can be fed back
at three levels:
c)
A.2.2.5.10
A.2.2.5.11
i)
Reference navigation solution: This design uses the inertial
instruments to propagate the GNSS receiver’s navigation state
between measurement updates, thereby reducing the uncertainty, or
process noise
ii)
Inertial aiding of GNSS tracking loops: The data from the integrated
navigation solution is used to reduce the bandwidth of the carrier
tracking loops in the GNSS receiver
iii)
Error-state feedback to the IMU: These GNSS-IMU systems calibrate
the inertial instruments when GNSS is available, but do not use the
inertial instruments to improve the operation of the GNSS receiver.
They are effective against a loss of GPS signal, but do not offer
increased resistance to multipath or improve signal reacquisition
Integrated or tightly coupled mode: Here the data integration is done at
GNSS measurement level, that is to say, pseudorange level, not position
level. There is only one feedback, utilising velocity to aid the GNSS
tracking loops. The reduced uncertainties allow for a less noisy state
estimate, a tighter constraint on measurements for rejection of multipath,
and improved reacquisition.
A wide range of inertial sensors are available, either as individual components
or integrated into Inertial Navigation Units (IMU) and also integrated IMU-GPS
systems. The price of the systems is proportional to the performance. Typical
prices are expected to be as follows:
a)
Gyroscopes are available from around $50 for very low performance
MEMS and up to $20,000 for very high-performance systems
b)
MEMS accelerometers, with drift rates of 2% to 5% of distance travelled
cost as little as $10 per unit
c)
Inertial units cost from around $400 for MEMS-based systems and up to
$80,000 for high-performance systems.
A range of integrated IMU-GPS systems exist with prices varying widely
depending on the technology used and performance achieved.
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A.2.2.5.12
A.2.2.6
a)
Measures acceleration
b)
When used as an augmentation, is an integrated system which has a
performance that depends on the quality of the IMU and the integration
method
c)
Operates independently of satellite navigation systems and also has
complementary properties, that is to say, it is autonomous and requires
no outside input (other than for initial calibration) and has very good
short-term stability, but with performance that degrades as the time since
the last calibration increases
d)
Can be the most expensive of the augmentation services, as quality of
instrument and cost are directly related to each other. IMU components
can cost between $50 and $100,000, depending on performance.
Interface to train systems
A.2.2.6.1
Section A.2.2.4 indicates that it can be convenient to interface to a source of
wheel rotation data, usually the speed measurement system or the OTMR.
A.2.2.6.2
A.2.2.7
Summary − IMU:
Another example of a particular railway augmentation used for on-board
passenger information and selective door control systems is the use of the
door release indication to confirm that a train has stopped in a station at the
correct location. This is a means of detecting that a train has stopped at a
station where GPS service coverage and / or accuracy is lower, due to station
canopies or other structures. It can be dependent upon the role of train staff
to operate the door release at the correct location.
Trackside or commercial communications
A.2.2.7.1
There are several new and existing types of trackside communication
equipment which can be used by trains. These include GSM-R, WiFi and
WiMAX.
A.2.2.7.2
A strong advantage of trackside services, particularly WiMAX, is that they are
able to provide a high-quality high-bandwidth service at all times, with none of
the loss-of-signal risks possessed by GNSS and SBAS. They all use known,
published standards, and equipment is easily available. However, they are
reliant on the provision of equipment (trackside or commercial) – causing
either a high set-up cost or a reliance on commercial timescales – and are
dependent on commercial organisations such as mobile phone operators.
A.2.2.7.3
GE/GN8579 sets out guidance on data communications arrangements.
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Appendix B
B.1
Interface to a Train Data Bus
Interface ‘C’: Position and speed reporting
B.1.1
This appendix supports the description of the target physical architecture set out
in Figure 4.
B.1.2
The internal architecture is characterised by interface ‘C’. This is the interface
from the locator, providing position, speed, time, and perhaps related
information, such as acceleration. The locator is also a time server, providing
time synchronisation services to applications.
B.1.3
The justification for the standardisation of interface ‘C’ comes from its role in
enabling many on-board applications as a source of standardised position,
speed and time. If interface ‘C’ were not standardised, then each system that
uses the locator services would have to be built or modified specifically for each
proprietary variant that exists. This would increase the cost of providing GNSSenabled applications on the railways and make the deployment of each new
service more difficult to justify.
B.1.4
Although the actual implementation of the locator unit can vary between
manufacturers, the format of the information it provides should not. It is
expected that this is to be the standardised GPS format set out in reference [6].
B.1.5
There are likely to be many different consumers of the locator information and
time synchronisation services on board a train, and a broadcast
communications service, such as an Ethernet, should be the basis for the
Interface ‘C’ standard. It is recommended that Topology Change Notification
(TCN) is not used, as it is not compatible with IP and has constraints on its use
(for example, for data downloads).
B.1.6
Both for the purposes of external data communications and the general
principle of application / network independence, interface ‘C’ is expected to
include the use of internet working using the industry Internet Protocol Standard
(IPS). This also allows the distribution of locator information and time
synchronisation services over additional networks on board a train via routers.
This is set out in more detail in GE/GN8579.
B.1.7
The Network Time Protocol (NTP) [12] provides an open and well-known
standard for the provision of time synchronisation services over intranets. It is
recommended that this is adopted as it is already available in COTS products.
B.1.8
Position and time information should be multicast to the systems that need it.
The multicast IPS service provides a standard means to enable this over an
internet. It is a protocol for transmitting IPS datagrams from one source to
many destinations in a network of hosts. Research has shown that only minor
modifications are required to an IPS network to add multicast routing support to
IPS. The resulting IPS multicast routing protocol, known as the Multicast
Transport Protocol (MTP), provides efficient delivery of datagrams from one
source to an arbitrary number of destinations throughout a heterogeneous
network.
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Appendix C
Summaries of Some Applications of GNSS
Table 7 characterises selected applications to illustrate the use of GNSS and augmentation to achieve
a given performance.
Quality of Service Requirements
1
Application
Accuracy
(Note 1)
Coverage
(Note 2)
Timing
Low
n/a
Timing
High
See last
column
Passenger
information
Low
High
Selective
door
operation1
Low
Geographically
dependent
Precision
stopping
High
Geographically
dependent
Traffic
regulation
Medium
Low
Integrity
(Note 3)
GNSS Solution
GNSS receiver alone with a
As required local clock having a specified
performance
ETCS and trains in general
could use GNSS as a common
timing source. The coverage
Discussed and integrity needs are not yet
in the next specified. The timing correction
column
scheme, continuous use of
GNSS, or occasional updating
of a secondary clock should be
chosen
A GNSS receiver with a
tachometer would be an
None
adequate locator, except in
underground railways
Except in underground railways,
a locator with some measures
against obscuration are
required. In the simplest case a
tachometer would be sufficient,
but other augmentation could be
Supports required for the worse cases of
SIL2 max obscuration. A form of spot
transmission augmentation is an
alternative, but in principle
should not be necessary. A
Class B receiver should be
capable of platform
discrimination
A locator with multipath
Depends mitigation is required. The use
on use of of track coordinates assists in
the
limiting the build-up of errors of
information the augmentation during
obscuration
A locator similar to that required
for precision stopping should
None
suffice. Accuracy is traded
against service coverage
See GE/GN8577
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Quality of Service Requirements
Application
Accuracy
(Note 1)
Coverage
(Note 2)
Integrity
(Note 3)
GNSS Solution
Supports
SIL4
The guaranteed accuracy of an
ETCS-like application need not
be high, as there is no precision
stopping control and the errors
can be accommodated in a
safety margin. The integrity
requirement could be as high as
5- to 7-sigma
Low
ETCS
Medium2
Dynamic
precision a
possibility
Table 7 Summary of some GNSS applications
Notes:
1.
Accuracy applies at the point in time and space where the information is required, otherwise there
is no demand on the locator. Positional accuracy at the stated integrity (if any) for the purposes
of this table is:
a)
Low accuracy is considered to be worse than 20 m
b)
Medium accuracy is considered to be in the range 3 m to 20 m
c)
High accuracy is considered to be better than 3 m, but excludes surveying applications.
2.
Coverage indicates the maximum duration of GNSS obscuration that the locator can sustain and
remain within specification.
3.
Integrity indicates the confidence that can be placed in the position, speed and time indicated.
2
Unless track identification is specified
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Definitions and Explanations
Accuracy
Accuracy can be defined as the degree of conformance between the measured position at
a given time and its true position at that time. For more explanation see 3.1.1b) and 3.4.5.
Almanac
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s, giving the
time of day, GPS week number and satellite health information (all transmitted in the first
part of the message); an ephemeris (transmitted in the second part of the message) and an
almanac (later part of the message).
The almanac consists of coarse orbit and status information for each satellite in the
constellation, an ionospheric model, and information to relate GPS-derived time to
Coordinated Universal Time (UTC). A new part of the almanac is received for the last
12 seconds in each 30-second frame. Each frame contains 1/25th of the almanac, so
12.5 minutes are required to receive the entire almanac from a single satellite.
The almanac serves several purposes. The first is to assist in the acquisition of satellites
at power-up by allowing the receiver to generate a list of visible satellites based on stored
position and time. The second purpose is for relating time derived from the GPS (called
GPS time) to the international time standard of UTC. Finally, the almanac allows a single
frequency receiver to correct for ionospheric error by using a global ionospheric model.
The corrections are not as accurate as augmentation systems such as WAAS or dual
frequency receivers.
Assisted GPS
Conventional GPS has difficulty in providing reliable positions in environments surrounded
by tall buildings − the so-called 'urban canyon' effect, as well as under heavy tree cover.
Under these conditions the GPS signal is often of very poor quality, making it hard for
receivers to obtain a position. In addition, when first turned on in these conditions, a
conventional GPS receiver may not be able to download the orbital information from the
GPS satellites, rendering it unable to function until it has a clear signal for around one
minute.
Assisted GPS (A-GPS) improves availability by reducing the Time To First Fix (following
complete signal loss), therefore reducing the duration of outages associated with reacquisition of the signal. This is achieved by providing GPS receivers with data that they
would ordinarily have to download from the GPS satellites. This data is obtained from
external sources, such as assistance servers and a reference network, and communicated
to the receiver via a communications network such as GSM.
Complementary systems
Radio navigation systems, such as eLORAN, which also provide a radio-based location
signal, complement the satellite navigation systems without being dependent on them.
Core systems
The main satellite navigation service providers foreseen for the future, formally GPS,
Galileo and GLONASS (see 3.5).
Data interface
The point of data transfer between two units, one transmitting data and the other receiving
data.
Dead reckoning
Dead reckoning improves availability, that is to say, reduces service gaps, due to poor
satellite visibility. It can use tachometry, speed sensors (such as Doppler) and directional
information (say, a compass), together with a form of map.
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Differential GPS (DGPS)
Differential techniques improve accuracy and integrity, but only where satellites are visible.
These also enable RAIM enhancement (see Appendix A, A.2.2.2). They can be spacebased or ground-based. The ground-based DGPS service is operational and approved for
coastal navigation up to 50 NM from the coast. This service is provided by the UK General
Lighthouse Authorities and is financed by ‘Light dues’. There is now an additional beacon
south of Birmingham to ensure complete land coverage in addition to the coastal service. It
is transmitted by a network of beacons operating at 280-320 kHz. Space-based DGPS may
be free-to-air (EGNOS, WAAS, MSAS) or subscription based (OmniSTAR, SkyFix,
StarFire).
Digitised Route Map (DRM)
In the context of the railway, a map provides a geo-coded description of the tracks in an
area, with the cost being influenced by the resolution. It requires maintenance to
accommodate changes and corrections (depending on the accuracy of the map). A DRM
is useful for map-matching (that is to say, click to map) as an additional sensor input to the
data fusion process.
EGNOS
The European Geostationary Navigation Overlay Service improves accuracy and integrity
(variable alarm limit) of the signal-in-space, but only where satellites are visible. It is
expected to be operational during 2008. EGNOS is provided via a satcom signal
(compatible with GPS) from a geostationary satellite, which makes it difficult to receive in
the railway environment, due to obscuration of the line-of-sight. There are plans to
disseminate the EGNOS signal to road / rail users via GSM / GPRS. Many COTS units
now come equipped as standard to receive and process EGNOS signals.
e-LORAN
The U.S. and Britain have decided to investigate this system as a national system to
complement GPS. It is a project for a commercial service, based on the LORAN-C
maritime radio navigation network. It is a low-frequency, terrestrial navigation system
operating at 100 kHz and synchronised to Co-ordinated Universal Time (UTC). It is
intended to meet the required navigation performance parameters for a range of transport
applications, including marine general navigation, and can be used on its own or as an
augmentation to GNSS. An advantage is that its availability is not affected by line-of-sight
considerations. It should be free-to-air and requires no subscription. It is of particular
interest to the maritime community.
Ephemeris
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the
time of day, GPS week number and satellite health information (all transmitted in the first
part of the message); an ephemeris (transmitted in the second part of the message) and an
almanac (later part of the message).
The ephemeris data provides the satellite's own precise orbit. The ephemeris is updated
every two hours and is generally valid for four hours, with provisions for updates every six
hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is
becoming a significant element of the delay to first position fix, because, as the hardware
becomes more capable, the time to lock onto the satellite signals shrinks, but the
ephemeris data requires 30 seconds (worst case) before it is received, due to the low data
transmission rate.
External augmentations
These are radio transmissions which provide corrections to the data received from the
signals-in-space, and can be broadcast from satellites. Free-to-air spaced-based services
are EGNOS, WAAS, and MSAS. Space-based subscription services are OmniSTAR,
SkyFix, and StarFire. There is also the ground-based differential service for the maritime
sector, known as DGPS.
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FMEA
Failure Mode and Effects Analysis (FMEA) is a method, first developed for systems
engineering, that examines potential failures in products or processes. It may be used to
evaluate risk management priorities for mitigating known threat-vulnerabilities. This in turn
helps select remedial actions that reduce cumulative impacts of life-cycle consequences
from a systems fault.
Galileo
The future European satellite navigation service, expected to become fully operational
before 2013.
GLONASS
The GLObal NAvigation Satellite System, operated by Russia. This is not currently fully
operational, but is expected to become so in the near future. Its signals are similar to, but
do have differences from, the GPS signals. There are receivers on the market that operate
with GPS and GLONASS signals.
GNSS
A generic term to describe the technology of navigation by satellite applicable to all such
systems, for example, GLONASS, GPS, and Galileo.
GPS
The Global Positioning System, originally deployed by the USA for military use but now
available for civil use. It covers the entire globe and is currently the only global satellite
navigation system available. Its resolution is good and is being improved.
Hybridisation
For the purposes of this document, a term used for augmentation of the GNSS information
by data from additional sensors on board the train that is processed within the locator unit
to improve one or more of service coverage, integrity and accuracy.
Integrity
Integrity is defined as the ability to provide users with warnings within a specified time and
at specified probability (risk) when the output from the locator unit should not be used, as
its accuracy falls outside a predefined threshold. For more explanation see 3.4.3 and
3.5.3.
Locations database
A database of information relevant to a given location (set of coordinates). The information
meets the requirements of a specified application. It could, for example, be a station name
and permissible stopping points along the platforms.
Locator
The navigation unit, which receives inputs from the core systems (see above) and perhaps
other sources, and outputs the processed location information.
Map-matching
A means of checking if an estimate of location determined by GNSS is reasonable by
comparing the estimate with a database of known positions of the track.
Multipath
GPS signals can also be affected, where the radio signals reflect off surrounding terrain:
buildings, canyon walls, hard ground, etc. Signals delayed in this way can cause
inaccuracy. A variety of techniques, most notably narrow correlator spacing, have been
developed to mitigate multipath errors.
For long delays (long delay multipath), the receiver itself can recognise the wayward signal
and discard it. To address shorter delays from the signal reflecting off the ground,
specialised antennas may be used to reduce the signal power as received by the antenna.
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Short delay reflections are harder to filter out because they interfere with the true signal,
causing effects almost indistinguishable from routine fluctuations in atmospheric delay.
Multipath effects are much less severe when the train is running at speed. When the GPS
antenna is moving, the false solutions using reflected signals quickly fail to converge and
only the direct signals result in stable solutions.
Navigating by the IMU
The locator continues to determine position and speed when the satellites are not visible
because of the use of hybridisation.
On-board augmentations
a) Processing within the receiver (RAIM)
b)
Integration with other on-board systems (for example, tachometer)
c)
Integration with inertial navigation systems
d)
Use of more complex processing, such as phase information
e)
Map-matching.
On-train networks
An on-train network is a network that supports communications between two end-points on
the same train set, where a train set is comprised of one of more carriages and power units
that are normally coupled together. Both wired and wireless networks can support on-train
communications.
Pseudorange
The pseudorange is a first-approximation measurement for the distance between a satellite
and a satellite receiver unit. To determine its position, a locator unit determines the ranges
to (at least) three satellites, as well as their positions at the time of transmission. Knowing
the satellites' orbital parameters, these positions can be calculated for any point in time.
The pseudoranges are the time the signal has taken from these positions to the receiver,
multiplied by the speed of light. The difference between the internal receiver time, and the
satellite time, needs to be known. This is achieved by introducing the receiver clock offset
∆t into the positional computation, which requires a fourth satellite signal. With four
signals, solutions for the receiver's position along the x, y, z and ∆t axes can be computed.
Service coverage
Service coverage refers to the proportion of the railway network for which a train’s position
can be reported at the required level of accuracy and confidence. It is the characterisation
of a lack of service coverage that determines whether an application can operate with
GNSS alone, or whether augmentation is required.
It is in the nature of the railway and GNSS processing that geographic limitations to service
coverage is present, and the acceptable loss of service depends upon the needs of the
application.
Train-to-shore networks
Train-to-shore networks provide wireless communications between a train set and a fixed
communications infrastructure. These can be either local-area or wide-area networks.
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Abbreviations and Acronyms
3G
Third generation mobile telephone technology
A-GPS
Assisted GPS
CCTV
Closed Circuit Television
CDMA
Code Division Multiple Access
COTS
Commercial Off-The-Shelf
CS
Commercial Service
DfT
Department for Transport
DGPS
Differential GPS
DOP
Dilution of Precision
DRM
Digitised Route Map
EC
European Commission
EGNOS
European Geostationary Navigation Overlay Service
eLORAN
Enhanced LORAN, a future development of LORAN-C augmentation
EMC
Electromagnetic Compatibility
EMI
Electromagnetic Interference
EMU
Electric Multiple Unit
ERTMS
European Rail Traffic Management System
ESA
European Space Agency
FD
Fault Detection
FDE
Fault Detection and Exclusion
FDI
Fault Detection and Isolation
FMEA
Failure Mode and Effects Analysis
FOC
Full Operational Capability
FTA
Fault Tree Analysis
Galileo
European global satellite navigation system (under development)
GLONASS GLObal NAvigation Satellite System
GNSS
Global Navigation Satellite System
GPRS
General Packet Radio Service
GPS
Global Positioning System (United States)
GSA
GNSS Supervisory Authority
GSM
Global System for Mobile communication
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Guidance on the Use of Satellite Navigation
ICAO
International Civil Aviation Organisation
ICD
Interface Control Document
IEC
International Electrotechnical Commission
IMU
Inertial Measurement Unit. Often used interchangeably with INS
INS
Inertial Navigation System
IOC
Initial Operational Capability
IPS
Internet Protocol Standard
ITU
International Telecommunications Union
LRK
Long Range Kinematic
MEMS
Micro-Electro-Mechanical System, a classification of gyroscope
MOPS
Minimum Operational Performance Standards
MSAS
Multi-functional Satellite Augmentation System, providing DGPS
NASA
National Aeronautics and Space Administration
NOBO
Notified Body (for approvals)
NR
Network Rail
NRN
National Radio Network
ORR
Office of the Rail Regulator
OS
Ordnance Survey
OTMR
On-Train Monitoring Recorder
PDA
Personal Digital Assistant
PIS
Passenger Information System
PLD
Passenger Load Determination
PPS
Precise Positioning Service
PRS
Public Regulated Service
RAIM
Receiver Autonomous Integrity Monitoring
RHCP
Right-Hand Circulary Polarised
ROGS
Railways and Other Guided Transport Systems (Safety) Regulations 2006
RTCA
Radio Technical Committee – Aeronautical
RTCM
Radio Technical Commission for Maritime Services
RTK
Real-Time Kinematic
SBAS
Space-Based Augmentation System
SC
Steering Committee
SDO
Selective Door Operation
RAIL SAFETY AND STANDARDS BOARD
GE/GN8578 Issue One December 2008
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Guidance on the Use of Satellite Navigation
SIS
Signal-In-Space
SISNeT
Signal-In-Space over the internet.
SoL
Safety-of-Life
SPS
Standard Positioning Service (GPS)
SRD
System Requirements Document
TCN
Topology Change Notification
TSI
Technical Standards for Interoperability
TSO
Technical Standards Order
TTF
Time To Fix
TTFF
Time To First Fix
UPS
Uninterruptible Power Supply
USNO
United States Naval Observatory
UTC
Coordinated Universal Time
VAIM
Vehicle Autonomous Integrity Monitoring
WAAS
Wide Area Augmentation System, provided for GPS by the FAA
WAD
Wide Area Differential
Wi-Fi
Wireless Fidelity
WiMAX
Worldwide Interoperability for Microwave Access
WLAN
Wireless Local Area Network
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RAIL SAFETY AND STANDARDS BOARD
GE/GN8578 Issue One December 2008
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Guidance on the Use of Satellite Navigation
References
The Catalogue of Railway Group Standards and the Railway Group Standards CD-ROM
give the current issue number and status of documents published by RSSB. This
information is also available from www.rgsonline.co.uk.
Documents referenced in the text
RGSC 01
The Railway Group Standards Code
Railway Group Standards
GE/RT8015
Electromagnetic Compatibility between Railway Infrastructure and
Trains
GE/RT8270
Assessment of Compatibility of Rolling-Stock and Infrastructure
GM/RT2100
Structural Requirements for Railway Vehicles
GM/RT2120
Requirements for the Control of Risks Arising from Fires on Railway
Vehicles
GM/RT2149
Requirements for Defining and Maintaining the Size of Railway
Vehicles
GM/RT2300
Warning Signs and Labels Fitted to Electrical Equipment on Rail
Mounted Vehicles
GM/RT2304
Equipotential Bonding of Rail Vehicles to Running Rail Potential
RSSB documents
GE/GN8573
Guidance on Gauging
GE/GN8577
Guidance on the Application of Selective Door Operation Systems
GE/GN8579
Guidance on Digital Wireless Technology for Train Operators
Other references
1. ICD-GPS-200C / IRN-200C-005R1 for civil L1 and L2 signals, IS-GPS-705 / IRN-705001 / IRN-705-002 for civil L5 signals
2.
RTCM 10402.3, RTCM Recommended Standards for Differential GNSS (Global
Navigation Satellite Systems) Service, Version 2.3
3.
RTCM 10403.1, Differential GNSS (Global Navigation Satellite Systems) Services,
Version 3
4.
ITU M.823: Technical characteristics of differential transmissions for global navigation
satellite systems from maritime radio beacons in the frequency band 283.5-315 kHz in
Region 1 and 285-325 kHz in Regions 2 and 3, version M.823-3, approved 03/2006
5.
EGNOS Interface Control Document (not yet published)
6.
NMEA 0183: interface standard, v3.01, 2002
7.
BS EN 50155:2007 – Railway applications – Electronic equipment used on rolling
stock
8.
BRB/RIA Specification No 12 (1984) – General Specification for Protection of Traction
& Rolling Stock Electronic Equipment & Surges in DC Control Systems
9.
‘Vulnerability Assessment of the transportation infrastructure relying on the Global
Positioning System’, John A Volpe, National Transportation Systems Center. August,
2001
10. ‘Recommendations towards a European Union Radionavigation Plan (ERNP)’, Helios
Technology Ltd, October 2004
11. ‘Civilian GPS systems and potential vulnerabilities’, P Benshoof, CGSIC, Prague,
March 2005
RAIL SAFETY AND STANDARDS BOARD
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To be superseded by GEGN8578 Iss 2 published on 03/09/2011
Guidance on the Use of Satellite Navigation
12. Network Time Protocol (NTP) Version 4 Reference and Implementation Guide, NTP
Working Group Technical Report 06-6-1 – David L. Mills, University of Delaware, June
2006
13. ‘The potential impact of GNSS/INS navigation integration on maritime navigation’,
T. Moore, C. Hill, A. Norris, C. Hide, D. Park, N. Ward, The Journal of Navigation
(2008), 61, pp221-237, published by the Royal Institute of Navigation
BS EN 60529:1992
Specification for degrees of protection provided by enclosures
(IP code)
BS EN 61508-1:2002 Functional safety of electrical / electronic / programmable electronic
safety-related systems
FAA TSO C129a
‘Airborne Supplemental Navigation Equipment Using the Global
Positioning System (GPS)’, 20/02/1996
FAA TSO C145
‘Airborne Navigation Sensors Using the Global Positioning System
(GPS) Augmented by the Wide Area Augmentation System
(WAAS)’, 15/05/1998
FAA TSO C145a
‘Airborne Navigation Sensors Using the Global Positioning System
(GPS) Augmented by the Wide Area Augmentation System
(WAAS)’, 19/09/2002
FAA TSO C146
‘Stand-Alone Airborne Navigation Equipment using the Global
Positioning System (GPS) Augmented by the Wide Area
Augmentation System (WAAS)’, 10/06/1999
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