Research
Brief
Obtaining data to assess the dependability of GNSS
information and accuracy of odometry
T510 - December 2009
Background
In recent years a number of research projects (such as APOLO,
LOCO, GADEROS, RUNE, ECORAIL and LOCOPROL) have
investigated various aspects of the application of global
navigation satellite system (GNSS) technology, such as GPS, to
the railway domain. These projects have confirmed that, when
suitably aided with other sensors, it is functionally possible to use
GNSS technology to determine train position and speed.
Whilst these projects have demonstrated that it is, indeed,
feasible to use GNSS technology for position and speed
determination on the railways, the dependability of the
information from these systems has not have been addressed. To
meaningfully support the introduction of GNSS technology to the
GB railway, the dependability of the information provided must be
assessed alongside the accuracy of train odometry systems.
This research provides the reassurance that GNSS information
and odometry accuracy is a reliable means of determining speed
and positioning of trains on the GB rail network.
Aims
The aims of this research were to enable the theoretical
understanding of the dependability of GNSS derived position and
speed to be verified consider the augmentation and use of
current odometry systems for the odometry function for ATP on
low density and rural lines.
Method
The research designed a GNSS data collection system for
installation on a train in normal revenue service. This equipment
collected GPS and odometry data over a 12 month period. The
collected data was processed and analysed and the dependability
of the position and velocity obtained was assessed.
A sub-set of this equipment was fitted to an additional train, with
different wheelslip/slide protection and odometry systems. This
train was used to collect odometry data during a three month
period in the leaf fall season. This data was analysed to assess
the performance of the odometry systems during their most
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Obtaining data to assess the dependability of GNSS information and accuracy of odometry
T510 - December 2009
challenging periods of operation using the same techniques as
used for the main data collection.
Two trains (A Class 323 operating in the Birmingham area with
London Midland and a Class 170 operating in East Anglia with
National Express East Anglia) were equipped with identical sets
of equipment containing a number of different GPS sensors, an
inertial measuring unit (IMU) and GSM modem together with
benign interfaces to the train's speed measurement system and
the cab occupied signal. Once the driver's key had been inserted
in any cab of the train formation the equipment started logging
information. Data collection on the Class 323 started in
November 2007 and in March 2008 on the Class 170. The
analysis of the data was performed between the start date and
December 2008 for both trains, although data collection
continued for longer than this.
The data was correlated against data obtained from the Ordnance
Survey's national network of GPS stations that continually record
raw GPS data. The data from the dual frequency, survey quality
GPS receiver on the train was processed to give accurate
locations of the train and also used for an independent
assessment of the quality and accuracy of the GPS system.
The GPS data collected on the two trains was then analysed in
terms of the ability of the GPS system to provide a positioning
solution, including analysis of the distribution of the duration of
any outages. The number of satellites tracked by the receiver and
also used by the receiver for the generation of the position
solution was also obtained and this was used to determine the
ability of the GPS system to provide not just a solution, but a
solution with integrity.
The raw GPS measurement data that was recorded was also
used to generate information regarding the variation of the
received strength of the satellite signal and the effects that this
could have on the positioning solution.
The investigation of the odometry provided by the two different
speedometer systems was performed using the stand alone GPS
velocity as the source. To analyse the distance function a simple
velocity integrator was used and the speed was integrated
between pairs of stations to obtain the distance between the start
and stop points. The analysis of this data provided information on
the suitability of the distance derived from the speedometer data
as a source for the odometry function for train positioning on rural
routes.
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Deliverables
The research has delivered:
 A final report, aimed at the specialist technical reader, who
needs to understand the project and its methodology, and
how satellite navigation performs in the railway
environment.
 A recommended specification for GPS equipment on trains,
aimed at the more general technical/managerial reader,
who may need to make decisions when specifying satellite
navigation equipment.
 A database of raw GPS and odometry information; collected
during this project, and which could be of value in other
research in the rail industry. It is available for other
investigations through application to RSSB.
Findings
The findings are inevitably complex in nature and the full report
should be consulted to understand the findings in their entirety. In
summary, this research provides the reassurance that GNSS
information and odometry accuracy is a reliable means of
determining speed and positioning of trains on the GB rail
network.
GPS dependability
The availability analysis showed that in the urban area around
Birmingham four satellites could be received for approximately
85% of the time, reducing to 75% of the time if six satellites were
required to provide autonomous satellite based techniques for the
integrity of the GPS system. In rural East Anglia, four and six
satellites are available for over 95% of the time.
The distribution of the length of the outages observed was that
over 85% of the outages in the Birmingham area lasted five
seconds or less and over 90% lasted ten seconds or less. The
effect of the environment around Birmingham New Street is seen
in that approximately 4% of the outages last for three minutes or
longer. The average duration of an outage computed for the Class
323 was found to be just under ten seconds. The Class 170
showed outage statistics that were significantly better than the
Class 323, because of the flat nature of East Anglia with very few
large obstructions. The number of outages that were five seconds
or less was over 98%, and around 99% for 10 seconds or less.
The average outage duration was found to be just over two
seconds in length. For the Class 170, out of a total of almost
56,000 recorded outages only 14 lasted longer than 50 seconds
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Obtaining data to assess the dependability of GNSS information and accuracy of odometry
T510 - December 2009
and only 64 lasted longer than 20 seconds. It is believed that
these occurred on the rare occasions that the equipped unit
worked in to London Liverpool Street.
The analysis of the stand alone GPS positional accuracy, for the
Birmingham area only, yielded statistics showing that the
performance of GPS in the urban environment was quite good,
with maximum positional errors in 95% of the "route bins" being
less than 105m based on data from the entire year.
From the analysis of the SNR information and the positional
accuracy information, there appears to be very little seasonal
effect on the accuracy of the position solution generated. Skyplots
clearly showed the obscuration caused by deep, steep sided
cuttings and reinforced the theoretical models.
Odometry
The odometry experimentation suggests that, after GPS
calibration, either speedometer system could be used to provide
accurate distance measurement, however these results are
based on data sampled at 1Hz over a limited time period. It is
possible that there are occasional undetected excessive errors
present in the system because of the low sampling rate or the
relatively short data collection period, which gives some
uncertainty as to the intended ATP application.
From the experimentation performed, the following results were
obtained for the Class 323 and Class 170 units.
Differences in distance between the GPS and speedometer
derived values for the class 323 were within ±10m for over 90%
of the time for the vast majority of the inter station distances
computed. There were however areas where the performance
was worse, but these tended to be in areas where there were
fewer journeys or the GPS coverage was poor. If a difference of
±50m between the two derived distances is considered the worst
percentage seen to lie within this range for the Class 323 was
approximately 92%. A difference of 50m on the average journey
length for the Class 323 of 1.74 miles is an error of about 1.8%.
The Class 170 odometry experimentation yielded similar statistics
to the Class 323, although the range of differences was larger
with around 90% of the values lying within ±50m for most of the
journeys. It is believed that this is due to the longer journeys for
the Class 170 in comparison to the Class 323 together with a
higher operating speed and potentially a more sinuous route
between the station stops. A difference of 50m on the average
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journey length for the Class 170 of 11.40 miles is an error of about
0.3%.
Next Steps
V/TC&C SIC has reviewed the recommendations and agreed that
the next steps resulting from the T510 project are that:
Based on the results obtained, Network Rail will derive a
statistical model describing the different GPS error distributions
seen and assess these derived models in terms of the different
applications for which the use of GPS has been considered. This
aspect is encompassed in the LiveTrain project.
The research results will be used as an input for the future review
of GE/GN8578 'Guidance on the Use of Satellite Navigation' to
parameterize performance in different railway environments.
Contact
For more information please contact:
Head of Engineering Research
R&D Programme
RSSB
research@rssb.co.uk
RSSB
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