AB
An Introduction to
Inertial Navigation Systems
International Marine
Contractors Association
www.imca-int.com
IMCA S 022
April 2015
AB
The International Marine Contractors Association (IMCA)
is the international trade association representing offshore,
marine and underwater engineering companies.
IMCA promotes improvements in quality, health, safety,
environmental and technical standards through the publication of
information notes, codes of practice and by other appropriate
means.
Members are self-regulating through the adoption of IMCA
guidelines as appropriate. They commit to act as responsible
members by following relevant guidelines and being willing to be
audited against compliance with them by their clients.
There are two core activities that relate to all members:
u
Competence & Training
u
Safety, Environment & Legislation
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covering a specific area of members’ interests: Diving, Marine,
Offshore Survey, Remote Systems & ROV.
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issues affecting members in their local geographic area – Asia
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India and South America.
IMCA S 022
This guidance was produced for IMCA, following initial work and
guidance by members of the Offshore Survey Division
Management Committee, by Gordon Johnston.
Front cover image supplied by Kongsberg.
www.imca-int.com/survey
The information contained herein is given for guidance only and endeavours to
reflect best industry practice. For the avoidance of doubt no legal liability shall
attach to any guidance and/or recommendation and/or statement herein contained.
© 2015 – International Marine Contractors Association
An Introduction to Inertial Navigation Systems
IMCA S 022 – April 2015
1
Glossary and Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
2
Background and Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.1
Inertial Navigation Systems and their Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
2.2
Applications of Inertial Navigation Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
3
General Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
4
INS Technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
4.1
Inertial Measurement Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
4.2
IMU Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
4.3
Inertial Navigation Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10
4.4
Aided INS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
4.4.1
4.5
Typical Aiding Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
4.6
Aiding Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
4.7
Quality Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
4.7.1
5
Field Verifications (Calibrations) ...........................................................................................................................13
4.7.2
System Level Errors.................................................................................................................................................14
4.7.3
Positioning Errors ....................................................................................................................................................14
4.7.4
Influences on Positioning Quality.........................................................................................................................14
Application of INS Technology for Positioning . . . . . . . . . . . . . . . . . . . . . . .17
5.1
5.2
5.3
GNSS Aided INS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
5.1.1
GNSS Aided INS for Availability ...........................................................................................................................17
5.1.2
GNSS Aided INS Solution for Accuracy .............................................................................................................17
USBL Aided INS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
5.2.1
USBL and DVL Aided Navigation .........................................................................................................................18
5.2.2
ROV Tracking and Positioning ...............................................................................................................................18
5.2.3
Pipeline Out of Straightness Multibeam Survey................................................................................................19
5.2.4
ROV Mid-Water Station Keeping.........................................................................................................................19
LBL Aided INS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19
5.3.1
6
The Kalman Filter ....................................................................................................................................................12
Vessel DP ...................................................................................................................................................................20
5.4
AUV/ROV Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
5.5
Swell Elimination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
5.6
Subsea Metrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
5.6.1
Simultaneous Localisation and Mapping (SLAM) Sparse LBL for Metrology .............................................21
5.6.2
Free Inertial Metrology...........................................................................................................................................21
INS Aiding Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
6.1
USBL Smoothing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
6.2
Sparse LBL Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
6.3
Long Range LBL Positioning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
6.4
Zero Velocity Aided Navigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
6.5
Image Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24
7
8
Current Limitations of INS Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
7.1
Sensor Reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
7.2
System Initialisation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
7.3
System Complexity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
7.4
Real Time versus Post-Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
7.5
Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
Future Developments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
8.1
Size Matters – Smaller and Cheaper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
8.2
Improved Capability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
Appendices
A
IMU Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
B
Export Licensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
C
References and Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
1
Glossary and Definitions
Accelerometer Device that measures acceleration; can be electromechanical or purely solid state
AECA
Arms Export Control Act (US)
AHRS
Attitude and heading reference sensor
AUV
Autonomous underwater vehicle
DDTC
Directorate of Defense Trade Controls (US)
DP
Dynamic positioning
DVL
Doppler velocity log – a DVL utilises the Doppler effect in sound travelling through water to
measure ocean currents, vehicle speed over ground, and height above seabed
FMEA
Failure modes and effects analysis
FOG
Fibre optic gyro – these consist of a coiled optical fibre in each of three axes. They use light
interference to measure rotation or angular velocity
GNSS
Global Navigation Satellite System
GUI
Graphical user interface
Gyroscope
Device that measures or maintains physical orientation in space. Traditional mechanical
gyroscopes with spinning rotors based on the principles of angular momentum are still used as
gyrocompasses in vessels. Aircraft and space vehicles use ring laser gyros or fibre optic gyros
which use changes in light frequency – the Sagnac Effect – to sense change in orientation
IMU
Inertial measurement unit – the main component of an inertial navigation system, comprising
three accelerometers and three gyroscope sensors. Each sensor is mounted orthogonally to
the others such that acceleration and angular rate can be measured in the X, Y and Z axis of
the IMU block. This enables measurement of rotation and acceleration in three dimensions
INS
Inertial navigation system – a computer, motion sensors (accelerometers) and rotation sensors
(gyroscopes) which continuously calculate via dead reckoning the position, orientation and
velocity (direction and speed of movement) of a moving object without the need for external
references
ITAR
International Traffic in Arms Regulations (US)
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LBL
Long baseline
MBES
Multibeam echosounder
MEMS
Microelectromechanical systems – miniaturised mechanical and electromechanical devices and
structures that are made using the techniques of micro-fabrication. A criterion of MEMS is that
there are at least some elements having some sort of mechanical functionality
MTBF
Mean time between failure
NM
Nautical mile
OGEL
Open General Export Licence (UK) – pre-published export, trade or transhipment licences in
the public domain, designed to licence controlled military or dual-use goods that are of a less
restricted nature or are being exported to non-sensitive destinations
OOS
Out of straightness
QC
Quality control
RLG
Ring laser gyro – these use a triangular arrangement of mirrors and lasers to measure the
interference between two opposing beams of laser light, from which rotation or angular
velocity can be derived
RMS
Root mean square
ROV
Remotely operated vehicle
SBL
Short baseline
SLAM
Simultaneous localisation and mapping
USBL
Ultra-short baseline
USML
United States Munitions List
ZUPT
Zero velocity update
2
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Background and Introduction
The increased use of inertial navigation systems (INS), particularly in support of offshore survey, installation and
inspection tasks, has emphasised the need for a general introduction and outline of the use of inertial navigation
technology used offshore. Within the industry there has been a gradual increase in the use of INS in areas where
there is a true benefit. For example, the adoption of aided INS in the dynamic positioning (DP) industry has been
driven by the need to improve the availability and stability of position reference systems where low update rates
or external interference would otherwise render them unusable, and autonomous underwater vehicle (AUV)
operations have required an INS solution to be able to successfully track the path of the AUV.
In general, INS technology is not used offshore as a positioning system in its own right, but rather in combination
with, or aided by, existing surface and subsurface positioning systems. Data from INS is blended with data from
other positioning systems using complex mathematical formulae. The reason for this is that the accuracy of an
INS is very good over a short period of time but will drift without external aiding.
Metrology and deep water positioning are areas in which inertial navigation is more commonly used, due to
potential time and equipment savings leading to a reduction in overall costs. INS should be deployed carefully to
ensure that quality is not compromised; poor aiding will result in a poor overall solution. The various aiding
techniques used in INS operations are constantly being reviewed so as to ensure the required high quality
positioning results.
This document provides a broad overview of INS technology currently in use in surface and subsurface
positioning solutions, giving a general overview of the technologies, the applications and a number of important
considerations in their use. It is a developing area and as ever proper planning, preparation and risk assessment
is critical to ensuring the successful use of these systems.
This document does not attempt to provide the reader with an in-depth understanding of the algorithms behind
inertial navigation systems, but covers the main components of such solutions and some of their strengths and
weaknesses.
2.1
Inertial Navigation Systems and their Benefits
A single accelerometer measures acceleration along a single axis. This output can be integrated once to obtain
velocity and twice to obtain change of position. By arranging accelerometers in three axes and adding a heading
reference the instant change of position can be derived. By introducing accelerometers in three axes, and a
heading reference too, so that the direction of travel is known, the current position can be derived.
This combination of accelerometers and heading reference sensors is referred to as the inertial measurement
unit (IMU). An INS comprises an IMU accompanied by a computer processor to calculate and process the data
to derive position. The use of an INS without additional aiding is termed ‘free inertial’ but suffers from scale and
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positional drift errors. In order to mitigate against unwanted positional drift, various other sensors are
introduced to create a more accurate solution with each sensor used to compensate for some of the biases.
Inertial navigation is a form of dead reckoning with the initial position known and the subsequent updates based
upon data from additional sensors and their error models, used to reduce any unwanted biases. When used
offshore, INS is typically ‘aided’ by frequent and regular updates of the position or other individual observations
by additional sensors and systems, such that the INS-based solution maintains its overall accuracy. This blending
or ‘aiding’ of the INS sensor with existing positioning systems provides a more robust and potentially more
accurate overall positioning solution than could be obtained from any of the individual component parts.
Figure 1 – Free inertial navigation without position aiding, compared with acoustic position
INS can complement other positioning methodologies such as long baseline (LBL) and ultra-short baseline
(USBL) very well. Various different combinations can offer better performance depending on what is required.
For example, LBL acoustic positioning can be expensive and complex to deploy, calibrate and use. It offers good
accuracy at the seafloor, even in very deep water, but can be limited in the area of coverage and may suffer from
interference or acoustic blanking. INS can improve this situation by interpolating between acoustic updates or
even drop out periods, and may also extend the capability of the positioning methodology to operate over a
larger area. However, a key benefit of the INS technology is the rapid update of position and very high resolution
and stability over short periods. Combining these attributes with the known benefits of acoustic positioning can
offer a better solution overall.
Whether used in a ‘free’ mode with only an IMU or even if aided by position, altitude, depth and Doppler velocity
log (DVL) sensors, inertial navigation systems are relatively immune from external interference such as unwanted
noise. They do not transmit or interfere with acoustic systems and can operate in harsh acoustic environments
without the need for any complex frequency planning. Although this independence appears initially to be an
advantage, realistically the use of INS is always reliant upon some form of external aiding to minimise the internal
biases and errors.
4
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By combining technologies the operator benefits from an increase in position data availability and stability, with
INS providing very high update rates and overcoming any drop-outs or low update rates of the traditional surface
or acoustic positioning. For certain operations INS contribute to the piloting and control of subsea vehicles and
enables a more stable platform for camera and sensor work.
In summary the key benefits of the combination of INS with other sensors include:
u
improved options for positioning solutions that may improve the redundancy of a solution by combining (and
so increasing) observations;
u
improved stability of positioning;
u
improved positioning update rates;
u
increased resilience from observation drop-outs;
u
improved real-time visualisation.
2.2
Applications of Inertial Navigation Systems
In the marine environment there are a growing number of applications of inertial navigation systems.
As deeper water operations mature and their demands are understood there is increasing emphasis on
operational effectiveness and efficiency. The introduction of INS to existing system configurations allows
operating limits to be extended beyond what is currently safe or possible. The general categories of use include
the following:
u
Global Navigation Satellite System (GNSS) surface positioning;
u
acoustic surface positioning;
u
sub-surface positioning;
u
metrology;
u
autonomous and robotic controls and guidance uses.
These will be described later in the document but it is useful to note that INS can potentially support and provide
a valuable input to almost any offshore survey activity.
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6
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General Discussion
The introduction, adoption and acceptance of new technology requires industry to develop, test and prove its
value and benefit over time. The benefits of INS have been often demonstrated in other areas and uses such as
in the aviation industry. Over the past twenty years or so, the use of INS in offshore surveying projects has
rapidly increased as the benefits have been recognised. In certain offshore survey operations, INS has become
the only practical solution.
A high specification INS relies upon the quality of internal sensors: accelerometers and gyroscopes are built to
very high specifications but residual biases and scale factor drifts still exist. In relative terms the accelerometers
are not the dominant error source as they account for only a few metres of drift per hour, perhaps less than
5-10 metres. By comparison the gyroscopes drift at a rate of at least a magnitude larger, thus causing errors of
up to 500m per hour. An indication of typical drift rates of an INS are as follows:
u
2 minutes – 3 metres;
u
5 minutes – 20 metres;
u
60 minutes – 750 metres.
To minimise these unwanted effects a number of aiding solutions have been developed to augment the internal
sensors and reduce their errors. Aiding is crucial to the successful use of INS on a project, but it should also be
noted that reliance upon a single source of aiding, such as the use of an altimeter or single acoustic range, can
mean that if that source should fail it would result in the unaided INS position drifting off very quickly. As an INS
is reliable and effective when supplemented with multiple sources of aiding this should be the approach taken
when considering its use. It is strongly recommended that the INS should use multiple sources of aiding. Also,
it should be noted that the use of a very high specification INS may not be the most cost effective approach if
the aiding sensors used essentially dominate the performance being attained.
However the INS is being utilised, it is important to bear in mind that the operator and the customer should
carefully plan and agree its use and the approach to obtaining appropriate positioning results. There may be very
limited options for inclusion of additional checks or for operating short survey lines in ideal directions. The many
influences on the positioning solution require that the user and customer should discuss, agree and properly plan
the use of INS beforehand.
Whatever INS configuration is used, it is likely to generate a huge amount of data. Thus it is worth distinguishing
between INS for general use (such as in remotely operated vehicle (ROV) work for construction support) and
INS use in surveying applications (such as out of straightness (OOS), seabed mapping with multibeam
echosounders (MBES) or pipeline inspection, etc.). This is because the types of INS project that collect data in
real-time for general use and use the position output immediately rely upon the real time performance and
graphical user interface (GUI) for visualisation and decision making.
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However those projects that collect and record the data in real-time but then post-process their positioning data
to derive the optimum results rely upon efficient data storage, processing and then visualisation and interface.
For survey applications that require the processed mode and use of INS data it is important for the project
surveyor, or supervisor, to establish how the data will be collected, managed and processed for the optimum
results. Additional processing, analysis and quality control (QC) tools will be required to gain a suitable outcome
and these are not always available from a single source. Users therefore need to have planned how to access
the tools and software that are required.
When planning a survey, or INS based activity, it is important to assess the possible merits of post-processing
and consider what actions may be necessary to achieve the project goals. As ever the data collection and its use
must be fit for purpose and appropriate for the task envisaged.
Further information on the management of survey data can be found in IMCA S 020 – Guidelines on the safe
management of survey and inspection data.
Figure 2 – Gimballed IMU (source www.hq.nasa.gov/alsj/lm_imu.gif)
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4
INS Technology
4.1
Inertial Measurement Unit
The central component of any INS is the inertial measurement unit (IMU). The IMU block is typically made up
of three accelerometers and three gyroscope sensors. Each sensor is mounted orthogonally to the others such
that acceleration and angular rate can be measured in the X, Y and Z axes of the IMU block. This enables
measurement of rotation and acceleration in three dimensions.
The typical output from an IMU would be:
u
rotation in X;
u
rotation in Y;
u
rotation in Z;
u
acceleration in X;
u
acceleration in Y;
u
acceleration in Z axis.
Figure 3 – Strap-down IMU (source unknown)
Historically, the main driver for development of inertial based guidance has been for rockets, guided missiles and
other military applications. Initially to maintain the correct orientation the (normally) three accelerometers were
suspended in a gimballed frame and gyro stabilised to maintain direction. The technology was developed during
the 1950s and 60s and overcame a number of complications to provide accurate and reliable position, velocity,
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9
attitude and heading. The complications included the Schuler effect, tilt errors and gyro and azimuth drifts. These
errors were in the main eliminated or at least mitigated, but the gimballed arrangement was mechanically
complex and expensive to maintain. Consequently, engineers developed the ‘strap-down’ method taking the
necessary benefits of improved processing, better accelerometers and, perhaps most significantly, ring laser gyros
(RLG).
Current IMU blocks used in survey applications contain very few or even no mechanical parts, and are strapped
to the navigation platform – hence ‘strap-down’ IMUs. For military navigation purposes (aircraft and submarines
etc.) where extremely low drift is required, gimballed IMUs are still the most common solution although modern
RLG based strap-down solutions can achieve a solution of comparable accuracy.
4.2
IMU Types
IMU sensors tend to be categorised by the components and methods used to measure the rotation on the
gyroscopes. The three basic technologies regularly used are ring laser gyro (RLG), fibre optic gyro (FOG) and
microelectromechanical systems (MEMS). More details on these techniques can be found in Appendix A. For the
purposes of this document, IMUs can be categorised thus:
u
navigation grade – having the capability to seek and find north, suitable for survey purposes;
u
tactical grade – lacking the capability to seek north, suitable for survey, high specification attitude.
An important factor in determining the overall accuracy or sensitivity of an IMU is its positioning performance
or drift. This is typically expressed as a distance over a time period – i.e. if an IMU is left stationary in any given
position, how much will the position calculated by the IMU have moved or drifted over a fixed period of time.
Navigation grade IMUs are characterised by a very low drift rate. This low drift rate can mean that these sensors
are classified as a ‘dual use sensor’ that could be used for military purposes. Drift rates for high specification
Navigation grade IMUs are typically less than one nautical mile (NM) per hour.
However in most survey applications the IMU is not being used unaided, so drift rate is used as a gauge of IMU
stability rather than an indication of the quality of overall positioning the system will provide.
Navigation grade IMUs have sufficient sensitivity to enable them to be able to measure accurately the rate of
rotation of the earth and thereby determine north (north seeking). This enables them to operate as heading
sensors typically with a heading accuracy of less than 0.5° secant latitude.
Tactical grade IMUs do not have the sensitivity suitable for accurate north seeking but can provide high quality
attitude data. Such IMUs form the main components of attitude sensors that provide pitch and roll observations.
The accuracy of pitch and roll from tactical grade IMUs will typically be 0.02° or better. This makes them
particularly suitable for motion compensation of USBL and MBES. These sensors may be used together with dual
antenna GNSS equipment to deliver an aided or blended solutions.
4.3
Inertial Navigation Systems
An inertial navigation system (INS) is an IMU combined with processors which can handle the navigation
equations to calculate acceleration and angular rate data, and thus arrive at a position, velocity and attitude.
An unaided INS solution has unlimited drift in position, velocity and attitude, due to errors inherent in the gyros
and accelerometers. Therefore in most marine and survey applications the INS will have other external inputs
to aid the solution and constrain this drift.
The INS will be processing observations at extremely high update rates, typically 100-200Hz. The IMU core may
be sampling the sensors at an order of magnitude faster than this. This extremely fast update rate, combined
with the low noise level, means that INS provides high positioning stability in the short term. Even low grade
IMU-based INS which would be characterised by a larger error due to drift can provide a high degree of stability
over a short period of time (a few seconds).
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Figure 4 – Example of the relative performance of INS and aided INS positioning. (source iXBlue)
When selecting the appropriate INS for a project, there are a number things to take into consideration. It will
not always be the case that the higher specification unit is required. A low specification IMU that has the benefit
of being well aided within the INS can provide results of a similar accuracy to that of a higher quality, more costly
system.
Category
Gyroscope
Technology
Typical Drift
Typical Heading
Accuracy
Typical Attitude
Accuracy
Export
Restrictions
Navigation grade
(high spec)
RLG
FOG
0.8 NM/hour
0.05° sec lat
0.01°
Y
Navigation grade
(low spec)
RLG
FOG
5 NM/hour
0.1-0.5° sec lat
0.03°
Y
Tactical grade
FOG
MEMS
No direct
capability
>0.02°
Y
MEMS grade
MEMS
No direct
capability
Few/limited export
restrictions
Table 1: Different grades of IMU
Realistically the requirement to integrate the INS with other sensors, for aiding purposes, as well as the main
survey computer will require the survey service provider to have already developed and tested the INS unit for
the selected purpose. Whilst the specifications of the units are important, so too are the initial set-up and proving
period and its performance in terms of reliability and accuracy within the overall system. The user defined
settings associated with the aiding sensors will often play a part in any biases or errors. The offsets and associated
settings for the performance of the sensors may require refinement and several iterations of the parameters to
determine the optimum set up.
A further consideration is that in some parts of the world it will not be possible to import a high specification
IMU due to import and export restrictions. Such restrictions and controls may hamper the choice of the
surveyor when selecting a technology or INS. See Appendix B for more details.
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4.4
Aided INS
In general most classical positioning sensors (USBL, LBL, GNSS and tracking systems producing lines of position
or actual co-ordinates) will provide a degree of short term instability, i.e. there will be noise within the positioning
systems in the short term. For example, USBL updates in 1000m water depth could have an update rate of
three seconds and a scatter of +/-10m between observations. However these positioning systems will provide
a good relatively long term stability, as positions generated by USBL will not drift with time.
Figure 5 – Aided INS block diagram (K Gade, Navlab)
To limit drift INS can be aided by other sensors to provide direct measurements for input to a Kalman filter,
allowing feedback to the navigation computation.
4.4.1
The Kalman Filter
The Kalman filter is a mathematical algorithm which estimates an optimal value of parameters (e.g.
position) and their errors from a set of observations (e.g. raw GNSS observables), using a predefined
model which explains the relationship between the two. Kalman filtering is a real-time process which
means that the new values of parameters are estimated (updated) every time new observations are
available. Kalman filters tend to be dynamically scalable such that additional sensor inputs can be added
to the solution or withdrawn from it without the requirement for the filter being reset.
For some applications, where additional accuracy is required, this can be obtained by post-processing
the INS data. In post-processing it is possible to use past and future data to compute a solution, i.e. for
any time (epoch) data will be available both before and after the epoch which can be introduced. This
is known as a Kalman smoother and generally provides improved solution accuracy. It is also possible
in post-processing to run the solution in reverse, this can provide an additional quality control check
and help identify any errors within a data set.
An INS solution (or Kalman filter) should not be seen as a tool that enables short cuts to be taken with
survey systems. If an aiding system is poorly calibrated or working sub-optimally this will feed through
into the resulting solution. Like a least squares solution, a Kalman filter is very good at removing random
or ‘white’ noise from data sets. Systematic or other system bias – ‘coloured noise’ – should either be
eliminated or minimised by good survey practice, or properly accounted for in the setup of the models
underlying the Kalman filter. The adage ‘garbage in, garbage out’ applies to a Kalman filter.
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4.5
Typical Aiding Sensors
The following table details sensors that are typically used in the aiding of INS solutions in the marine
environment. Other positioning sensors may also be used.
Sensor
Observation Types
GNSS
Position
USBL
Position
LBL
DVL
Position
3D velocity
Lines of position
Lines of position
Sound velocity sensor Sound velocity
Depth sensor
Depth
SBL
Position
Lines of position
Table 2: Sensors typically used in INS aiding
It is also possible to constrain the solution with a zero velocity update (ZUPT) – effectively, holding the IMU
stationary for a period of up to several minutes. This technique can be used to measure the residual bias in the
velocity and acceleration sensors.
4.6
Aiding Methodology
Kalman filters can be described as either loosely or tightly coupled depending on the methodology used for input
of the aiding observations.
A loosely coupled GNSS solution would accept a receiver computed position (and quality parameters) into the
Kalman filter. If the GNSS receiver was unable to compute a solution, due to a reduction in available satellites,
the aiding source would stop.
A tightly coupled GNSS solution would accept satellite ranges from the GNSS receiver and compute the solution
within the Kalman filter. Even if the receiver was unable to compute a solution due to a reduction in available
satellite ranges, these ranges could still contribute to the tightly coupled Kalman filter solution provided there
was enough information from other sensors. Analogous to this is the situation with acoustic positioning where
sparse LBL arrays, incapable of producing an acceptable position on their own, are still able to contribute ranges
to the tightly coupled Kalman filter.
4.7
Quality Control
The results of an INS solution should be subject to quality control using comparisons against standard positioning
methodologies. It should be possible to compare conventional least squares solutions and the Kalman filtered
solution.
In practical terms the operator will need to use computers to provide a means of identifying and potentially
compensating for or removing any sources of errors that the INS may be incorporating into a solution. However,
with many dynamic situations creating rapidly changing values and data outputs, this is more of a theoretical
rather than practical solution and the operator will be reliant upon comparison of associated solutions and
sensor parameters.
4.7.1
Field Verifications (Calibrations)
The potential for unwanted biases to degrade the position solution requires the operator to check and
verify the set up being used is appropriate for the project. A field verification, or field calibration is a
common practice where the system is checked and proven for its use on the project. These checks are
normally conducted along a survey line or a suitable figure or even perhaps a physical structure such as
a exposed pipeline. After the individual sensors are checked and confirmed, the system is moved along
a short distance, perhaps 200m. Depending upon the specific system in use some differences in the
checks are possible. It may be possible to operate the IMU using a coarse mode and then a fine
alignment mode and then to repeat the sequence to generate a multiple set of readings and data.
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In this way the surveyor can build up a series of datasets that will demonstrate that the system is
appropriate for use on a particular operation, such as an out of straightness survey of a pipeline.
It is important to recognise the types of errors and to identify if the INS is working properly. In general
terms there are two types or categories of errors – system errors and position errors. Most INS
manufacturers provide some software utility to configure the INS and to then monitor and report on
the status of the unit. Information to the user is often presented as statistical data that has been
generated directly from the INS.
4.7.2
System Level Errors
These errors may include reported faults with a sensor, data communication drop outs from sensors or
a sensor data overload, algorithm status or INS aiding status messages.
4.7.3
Positioning Errors
The INS will output position and orientation messages with associated quality metrics and accuracy
metrics. These should be familiar metrics to the users and indicate whether or not the unit is
performing to the required accuracy. The user should monitor these values regularly and frequently, and
be alert to any changes in them.
As the INS is using Kalman filters to estimate its position there is the possibility of unrealistic position
estimates, so the user should monitor the residuals. Relatively small biases in position or angle/
orientation may be detected by the INS, however, the user will need to identify any systematic problems.
The minimum positioning parameters to which the user should have access are:
4.7.4
u
position; position standard deviation, root mean square (RMS) and error ellipse;
u
orientation and heading standard deviation;
u
velocity;
u
bias estimates for position and sensors;
u
aiding residuals between the INS value and the sensor input.
Influences on Positioning Quality
In general, it is preferable – but not essential – to have dual sensor aiding. This may not be practical in
certain applications. The surveyor and system operators should be experienced in the use of their INS
so that appropriate advice and recommendations can be offered before the start of a survey using INS
technology.
The way the survey operations are carried out can also have an influence upon the overall positioning
performance. Often the use of an INS implies that the survey lines should be kept relatively short to
limit the effects of any unwanted drift. Adopting a consistent length and period of time, i.e. equal and
opposite directions, can help to keep unwanted residual biases to a within acceptable levels for the
survey. Whilst this is possible in clear and open areas, it may not be possible for a pipeline survey or
other elongated survey route. In such cases alternate strategies will be needed. In general it is possible
when using ROVs to bring them to a complete and absolute stop and thus enable a ‘zero velocity update’
to be performed. Alternatively the aiding sensors may have to be relied upon to provide the absolute
control and limit to the biases. For example, acoustic aiding may offer a means to limit the biases if no
GNSS or other external source of positioning is possible.
It is very important for INS operators and customers to plan an agreed approach to obtaining the
appropriate positioning performance. There may be limited time and resources for additional checks,
installation of extra sensors and optional aiding, or operating with optimal line lengths and directions.
INS operators will need flexibility to cover different possible scenarios. There are many influences on
the positioning solution and carefully considered applications may require specific, but quite acceptable,
alternative solutions.
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u
Aiding
Where there is a reliance upon the sensors aiding the INS, it is important to understand and
mitigate the impact of the loss of any sensor. The use of two or more aiding sensors will reduce
the probability of such failures. As an example if a depth sensor failed the INS might suffer from
biases in the vertical but if the aiding also included an altitude sensor then the vertical
component would remain unbiased. See Section 6 for more details.
An issue with depth aiding is that it introduces the concept of the sea surface as a reference
and consequently it may be important to eliminate any tidal effects in the data. It is not
essential to apply tides to depth aiding data in an INS but as with any source of depth data,
subsequent tidal correction may be necessary offline through normal processing paths. Both
real time modelling and measurement or by post-processing a correction in the vertical axis
will compensate for the possible changes in sea surface level.
The method of use and the aiding of the INS receive considerable attention but there are other
influences that should also be considered. Some of these are perhaps not fully appreciated in
terms of their influence on the quality of positioning solutions.
u
Installation and accurate determination of offsets and lever arms
An INS should be installed as carefully as possible but it is also necessary to establish the
relationship between the INS and the other related sensors, to as high an accuracy as possible.
All offsets should be very carefully measured so that any lever arms that derive from the
accelerations do not introduce any residual biases.
u
Alignment, verifications and the settings of the aiding sensors
The initial set-up and initialisation of the system is a critical process in obtaining successful INS
based positioning. The various peripheral sensors used, if aiding is available, should be fully
calibrated and aligned accurately with respect to their measurement axes and the IMU.
The IMU, as well as needing to be accurately aligned, may also allow the set-up of the Kalman
filter. The Kalman filter relies upon appropriate weights and settings for the sensors. These
may be established based upon the results of the initial set up and checks. This may be part of
a system’s operating set up process but not all systems require the operator to intervene as it
is an iterative process that can require some time. If it’s required as part of the set process
then time should be allocated for it to be completed properly. Once an initial set-up is
completed and the results reviewed, a fine alignment and set up of the filters, bias corrections
and scale factors can take place.
u
Operational scenarios
In some operations there may be some flexibility and choice in terms of how vehicles
manoeuvre and operate with their INS. This is of particular importance for pipeline as-laid
surveys and other route surveys where the longer duration of a survey line could impact on
the quality of the positioning (due to residual biases not being fully taken into account). Where
possible and where there is flexibility in the design of the survey lines, they should be planned
to be of a short enough duration to avoid any unwanted biases. For example lines may be
limited to no more than 40-60 minutes’ duration in any one direction or at least limited in
order to meet the general survey positioning specifications. This approach may be used for
surveys over an area (e.g. by AUV) where the line lengths can be managed to help minimise
drifts and biases.
Additionally, it is not possible to reduce the unwanted biases by moving the survey platform
more slowly. In fact the opposite scenario may occur where slower motion actually allows the
biases to build up, as it takes a longer time to complete a survey line. So the survey platform
should maintain an appropriate and steady speed for the sensors to provide useful aiding, and
in order avoid unwanted residuals in the positioning solution.
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u
Quality control
It is a common concern amongst the professional surveying community that INS positioning
solutions offer little associated quality indicators or QC parameters to enable the surveyor to
validate or verify the output. There is a perception that INS is a ‘black box’ operating with
various sensors, the inner workings of which remain unknowable. In this context, it is good
practice, where possible, to record as much of the associated raw data from the sensors as is
practical. For example, acoustic updates, depth and altitude sensor data, plus heading
information, may be available to record and compare for QC purposes. Additionally, a good
technique is to use an MBES in combination with an INS aided solution to analyse the
positioning data set, especially if an ROV survey is run along a section of pipeline allowing the
opportunity to spot any residual artefacts and biases due to the INS positioning.
u
Time stamping and latencies
Obviously, accurate time stamping is important. This is most commonly performed by
synchronising to a common GNSS clock, whereby possible latencies are being eliminated for
surface and subsea parts of the INS aided system.
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5
Application of INS
Technology for Positioning
5.1
GNSS Aided INS
Under normal conditions GNSS provides robust vessel positioning with an update rate of one position update
per second. INS solutions can be combined with GNSS in a number of methods to either provide improved
solution accuracy and/or improved solution availability.
5.1.1
GNSS Aided INS for Availability
When undertaking vessel work in areas with reduced sky visibility, GNSS aided INS can maintain a
positioning solution when there is a reduced number of, or even potentially no, GNSS satellites visible.
For example surveys undertaken very close to, or under, offshore installations, or bridges.
The inertial component will also enable the GNSS aided INS solution to be reacquired more rapidly
after leaving the area of poor visibility. It is possible to use as few as one GNSS satellite observation to
constrain the drift of the INS solution.
The combination or blending of GNSS and INS may also provide increased robustness during periods
of high solar activity. This can be an issue in equatorial regions and at high latitudes. For further details
see IMCA S 018 – Guidance on the selection of satellite positioning systems for offshore applications – and
IMCA S 015/OGP 373-19 – Guidelines for GNSS positioning in the oil & gas industry.
The deployment of multiple GNSS receivers can also be utilised when working in areas with
compromised sky view; this approach is sometimes known as ‘round the corner GNSS’. In some
circumstances, one (or more) receivers may not track enough satellites to compute an independent
solution. A good positioning solution can be obtained by combining satellite observations from a series
of two or more receivers at different locations on a vessel, each with different sky views, and aiding with
an INS.
5.1.2
GNSS Aided INS Solution for Accuracy
A GNSS aided INS solution can provide better (more precise and more accurate) positioning than a
standalone GNSS solution. This may be a relatively small improvement over only short periods,
essentially between GNSS position fix updates, which can be important for specialist operations with
high data rates, such as MBES or synthetic aperture sonar. The INS component can smooth out any
short term noise within the GNSS data, providing improved short-term accuracy, and it can offer a more
stable solution if there are changes to the geometry of a GNSS constellation.
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The INS solution introduces, by its own operation, increased redundancy, and may improve the overall
QC and detection of GNSS outlier observations. In fact the INS can become the solution aided by the
GNSS. This complementary arrangement is very useful but the individual inherent weaknesses remain.
INS biases may still influence and degrade the GNSS position if biases are not mitigated or their
influence removed. This can be a challenge resulting in the advantages of INS being seen as relatively
small compared to the effort required to ensure the maintenance of an accurate and reliable positioning
solution.
INS technology helps to mitigate the weaknesses of a single subsea positioning solution. However it can
also offer improved performance especially if high update rates are required, and if the traditional
acoustic positioning update is limited by acoustic range. The influence of the aiding sensors is important,
and for certain operations these peripheral sensors will dominate the overall performance of the
positioning. In such cases the accuracy of a solution may not be improved but its availability and
reliability may be. Furthermore, some positioning solutions will experience considerable improvement
in their short term performance but little improvement in their absolute results, due to the residual
systematic biases introduced in the positioning solution. Consequently the use of INS may not always
improve performance but rather simply reduce its sensitivity to dropouts and other gaps.
5.2
USBL Aided INS
In certain applications the lack of availability of GNSS observations or corrections may mean that alternative
vessel positioning solutions are required. In most cases this will be either USBL or LBL acoustic positioning.
USBL position calculations normally rely on a single measurement which makes it sensitive to interference. It also
requires that the acoustics transmitted from the surface vessel travel down to the mobile device and then return,
which means the update rate is relatively slow. This can be compensated for by using INS, which provides a
relatively high update rate, and integrating other sensors improves system tolerance to interference.
The following examples are provided to illustrate the potential benefits and advantages of using INS in the
scenarios provided. It is recognised that these scenarios may also benefit from other aiding, such as LBL
acoustics, but for the purposes of this document they are included here to emphasise the USBL element and how
INS contributes to the improved positioning.
5.2.1
USBL and DVL Aided Navigation
The use of USBL for tracking subsea vehicles and towed bodies has been extended into deeper waters
where some form of augmentation is necessary to provide a stable positioning solution. INS can offer
this by complementing the above-mentioned shortcomings of USBL. A USBL system offers an accurate
link to the real world and limits drift of an INS whilst the INS provides a stable output at a relatively
high update rate.
USBL combined with a Doppler velocity log (DVL) can be used to aid INS. Once an ROV or other unit
is close enough to the seabed the Doppler velocity log can track the seabed return and thus provide an
accurate speed over the ground. This is useful in bounding the drift of the INS. The resultant track of
the ROV is cleaner and generally smoother due to the more frequent updates.
A recent development has been the increasing use of the INS in shallow water environments where the
vertical motion of the ROV cannot be well tracked or modelled due to the dynamic motion of the ROV
caused by swell. INS offers a means to quantify and mitigate the worst effects of this vertical motion
on the sensor data and records.
5.2.2
ROV Tracking and Positioning
ROV tracking and positioning is an activity where the industry is seeing increasing use of INS to aid
USBL-based positioning solutions. As the industry moves into deeper water and more demanding
operations, the use of INS can provide much needed stability and high speed position outputs enabling
the USBL range to be effectively extended into deeper waters than was previously possible.
In addition the INS can provide relatively rapid and stable position updates to support ROV pilots. ROVs
operating from a surface vessel may encounter tasks that involve prolonged vehicle flights (or dives) and
require the pilot to maintain a stable and steady hovering position or to follow predetermined routes
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or assets, such as pipelines. Such ROV flights can be supported using INS to provide relatively rapid,
stable and accurate positioning, heights, attitude and flight tracks. The INS also provides accurate
heading, pitch and roll for the ROV, thus removing the need for a separate sensor.
The current trend is for ROVs to be fitted with INS based positioning solutions to upgrade and improve
their positioning and tracking capability.
Figure 6 – Showing corrected and uncorrected ROV tracks (image: Kongsberg)
5.2.3
Pipeline Out of Straightness Multibeam Survey
Here, INS installed on an ROV aided with USBL and DVL is used in conjunction with a multibeam system
to provide a digital terrain model for detection of pipeline out of straightness.
u
u
5.2.4
Advantages: Provides the very precise relative positioning needed for this application. If INS data
is post-processed, artefacts such as ocean swell effect of pressure depth sensors can effectively be
removed.
Disadvantages: Requires careful quality estimates for the USBL aiding to ensure the observations
do not degrade the relative DVL-INS positioning solution.
ROV Mid-Water Station Keeping
ROV mid-water station keeping is an example of a positioning scenario when there is no DVL lock, just
external aiding from USBL or LBL. The INS is installed on an ROV aided with USBL. The INS can provide
position and orientation to an ROV control system to aid the ROV operators to maintain mid-water
station keeping ability.
u
u
5.3
Advantages: Provides the very precise relative positioning needed for this application, USBL
acoustics alone being insufficiently precise. Reduces ROV pilot burden for mid-water operations,
e.g. riser inspection.
Disadvantages: Requires interface between INS/ROV control and vessel USBL system, typically
ROVs are operated/interfaced in isolation.
LBL Aided INS
As noted above, in certain applications the lack of availability of GNSS observations or corrections may mean
that alternative vessel positioning solutions are required. In most cases this is likely to be either USBL or LBL
acoustic positioning.
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5.3.1
Vessel DP
Integration of INS to ad hoc vessels and their DP systems may be problematic with recertification and
new failure modes and effects analysis (FMEA) trials required. As such, INS installations should be well
planned if their full benefit is to be gained.
In general, DP systems receive inputs from more than one positioning source. Current DP requirements
suggest that an INS feeding a DP system is only aided from one position source; LBL or GNSS or USBL,
so it would not be acceptable to have two or more position references interfaced.
In moderate water depths the positioning aids can include taut wire systems and USBL acoustics, and
in deeper water acoustics as well as GNSS. The use of a shipborne USBL for relative range and bearing
values is a practical solution for moderate water depths but since it is surface based it is also depth
limited. It does operate without the need for the deployment of a seabed (LBL) array. Therefore LBL
acoustics with an array of seabed beacons deployed provide a suitable number and geometry of acoustic
ranges to the transducers on the surface vessel independent of the water depth. The LBL solution can
be either a calibrated absolute position or one relative to seabed assets, and the beacons on the seabed
provide a reference for the surface vessel to track or hold station over.
u
u
5.4
Advantages:
– The INS fills in any delays or outages of the conventional positioning solutions. The INS offers
availability of a position solution during periods of interference or signal disturbances
– For a deep water operation (such as a drilling rig) remaining in a single location for a
considerable period of time, the investment in a seabed deployed LBL array is worth it
– A USBL offers a commonly adopted positioning aid that is consistent during operations that
may transit along long lines such as on a pipeline installation;
Disadvantages:
– The set-up of the LBL array can be costly and time consuming. The use of LBL and GNSS
without INS offers a sufficiently robust solution for most circumstances; the adoption of INS
may only be cost effective for critical operations
– The USBL system, being surface based, is limited in the depths it can provide accurate and
reasonably fast updates.
AUV/ROV Positioning
To some degree, inertial applications in the subsea environment have been driven by the development of deep
water AUVs. These vehicles are often working in very deep water with only occasional acoustic positioning
updates. The ability to dive and operate at such depths for extended periods is made possible by the use of INS
technology. AUVs are initialised on the surface and then dive, positioned with INS and some acoustic aiding (such
as USBL) to the seafloor where the peripheral sensors, such as the DVL and altitude sensors, will lock on to the
seabed return signal to aid the vertical component and the scale of the INS positioning. In deeper water, of over
1500 metres, the dive alone can take several hours to complete, before positioning can be refined at the seafloor.
After the dive the survey operation will begin with very limited aiding and in essentially a true autonomous mode.
On completion of survey operations the AUV will then depart the seafloor area and return to the surface for
recovery, again taking several hours to complete this activity. During these periods the INS positioning will lose
its traditional seafloor aiding although USBL may be again available as it nears the surface vessel.
INS technology is now increasingly used on ROVs operating in deep and shallow water. The core components
of such systems are:
u
IMU core – high or navigation grade IMU such that vehicle heading can be computed;
u
DVL – Doppler velocity log. This provides vehicle velocity information at better than 1Hz;
u
depth sensor – provides data for the vertical component of the position solutions;
u
external positioning – typically USBL or LBL.
Increasingly complex operational requirements, demanding better positioning, have stimulated the growth of such
integrated positioning solutions involving INS – for example, ROV mounted MBES work, which requires
consistent very high accuracy positioning. The performance and relative accuracies and characteristics can vary
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considerably on AUV and ROV survey and inspection operations. Therefore, it is important that there is
thorough planning and preparation to establish the performance and operational capability required. Range,
accuracy, payload and sensors may be flexible or adjusted for certain types of operation.
5.5
Swell Elimination
The impact of swell on a depth sensor in shallow water can affect the quality of data in the vertical component
of a vehicle’s position. The use of INS can isolate this and compensate for the effects of swell on the vehicle’s
movement and on positioning data. Other specific operations, particularly those involved in determining vertical
distances and heights, can benefit from the use of INS to again reduce the unwanted effects of swell on the
submerged vehicle.
5.6
Subsea Metrology
The use of INS for subsea metrology is relatively new to the offshore industry. Subsea metrology is the process
of acquiring accurate and traceable dimensional measurements for the design of subsea structures. The INS is
used to collect the position and orientation data, which is then used to determine exact measurements for the
subsea structure – such as a spool piece. Often the INS will travel from the start to the end and return with
possibly a repeat procedure to verify, check and collect extra data. Over relatively short distances the INS may
be operated in a free and unaided mode. As the distance increases the INS will be aided with acoustic ranges
and various peripheral sensors.
Further information on the use of INS for subsea metrology can be found in document IMCA S 019 – Guidance
on subsea metrology.
5.6.1
Simultaneous Localisation and Mapping (SLAM) Sparse LBL for Metrology
Sparse LBL involves the ‘tightly coupled’ INS solution that allows individual ranges and observations to
be integrated as an aid rather than purely as a position. With fewer signals required, the deployment of
LBL seabed transponders in a relatively sparse array enables greater coverage per unit, thus minimising
costs. If planned well, overall position accuracy can be maintained. Metrology is often required for
existing structures and in restricted areas where obtaining good acoustic positioning coverage may be
difficult. The reduced number of transponders allows greater flexibility when planning locations and
improves the likelihood of placing them in suitable locations where they can aid the INS solution.
5.6.2
Free Inertial Metrology
With no external aiding of INS this operation involves the repeated movement of the INS core from
one location to another. The movement and precise location of the unit is controlled by careful
assembly of brackets and mountings to ensure accurate alignment and a close fit. The actual movement
of the INS should be steady and smooth so as to minimise the introduction of residual drifts or biases.
This type of operation is often adopted where acoustics or line of sight measurement systems cannot
be used, and a clear path is available for the ROV to transfer the INS between two points. Because of
INS drift, the technique is inherently limited to situations where the distance between the objects is
relatively small. Taking account of the travel time between objects using an ROV, distances between
objects for free inertial metrology are normally less than 60 metres.
In order to ensure the validity of data collected, proper attention should be paid to deriving any offsets,
obtaining depth, tidal and speed of sound values for the operating period and applying these to reduce
any biases affecting the peripheral sensors. Each metrology operation is different and the performance
of INS positioning can only really be assessed with the specific information relating to a particular
metrology project. However, the industry is rapidly gaining experience and expertise in achieving
appropriate results with INS and INS aided solutions.
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6
INS Aiding Techniques
This section describes in general some of the available methods and techniques for aiding INS. It is important
when planning the use of INS to properly assess the possible methods and techniques used for positioning. Some
of the sensors more commonly used to aid INS are Doppler velocity logs (DVL), depth sensors and altitude
sensors. Some or all of these sensors are often integrated into the INS position solution. These are relatively
inexpensive peripheral sensors and each offers some potential to reduce drifts and biases. The other main aiding
is through the use of the acoustic positioning systems available.
u
u
u
Doppler velocity log: Limits drift and to provide accurate scaled and rate of movement across the seabed.
Is best used for relatively smooth and continuous movements and may not offer much aiding if hovering.
Alignment with the IMU is critical.
Depth sensor: To assist in bounding any drift in the vertical by providing a vertical measurement but may
need to be corrected for tides and is sensitive to any density changes of the water column.
Altitude sensor: A bottom sensing altitude sensor can also provide aiding in the vertical. This requires the
vehicle to remain within a limited height above the seabed to maintain signal lock of the sensor on the
seafloor.
Further information on these and other peripheral sensors can be found in IMCA S 021 – Guidelines for the
management of peripheral survey sensors.
6.1
USBL Smoothing
In water depths greater than 250m and when working close to the seabed, the use of USBL integrated with INS
and the above-mentioned peripheral sensors can provide very high precision and a very accurate position. Such
high precision and accuracy may be required for certain work, such as an MBES survey. For many other
positioning operations, e.g. the use of the ROV to assist in setting an accurately positioned drilling well or
template, the technique is valid for water depths greater than 750m.
u
Equipment: USBL+ DVL+ depth sensor + INS.
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23
6.2
Sparse LBL Positioning
Here the LBL provides ranges rather than actual position to the INS solution. As noted previously the INS may
be coupled with the aiding sensors in such a way that their range observations are included and not simply their
position solution output. Along long route corridors and over relatively large areas the use of sparse LBL ranges
can be an option. This approach reduces the overall number of seabed units deployed per area (compared to a
traditional LBL array) but does not offer the full acoustic position solution as an aid.
u
6.3
Equipment: LBL+ DVL+ depth sensor + INS
Long Range LBL Positioning
The advent of improved signal processing and broadcast acoustic arrays has enabled the introduction of very long
range acoustic positioning and sparse array solutions. These differ from the more traditional LBL acoustic arrays
as the update rate is faster and the ranges are over considerably longer distances, thus reducing the number of
units deployed etc.
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6.4
Equipment: LBL+ DVL+ depth sensor + INS
Zero Velocity Aided Navigation
Zero velocity aided navigation is a technique where static position fixes are acquired. During survey operations,
ROVs will often be used to take static fixes on structures. Zero velocity updates can be used to aid the INS
during the period the system is motionless as this allows the unit to determine the amount of residual drift and
compensate for this bias.
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6.5
No additional equipment is required however it does require that the platform on which the IMU is placed
can come to a complete stop;
Advantages: Determines very accurately the drift rate of the IMU components;
Disadvantages: Requires manual control and is limited to vehicles that can stop still. Can interrupt the
operations.
Image Matching
Image matching involves the use of images to process the spatial relationship between multiple passes of an
object. It uses proprietary algorithms and is limited to AUVs at present.
The actual steps and quality of the output can be of varied quality due to a number of factors such as the
resolution of the camera system, the geometry between the objects and the camera, and the nature of the
objects being used to control the imagery. Post-processing should improve the solution but may come at the
cost of a time-consuming extra step.
Although AUVs potentially benefit from this process it is possible that certain ROV operations could also be
supported. If a suitable structure is being monitored, or if the ROV is working in close proximity to such a
structure, it may be possible for the ROV to maintain a form of ‘image lock’ to support its movement and
hovering on station. It is anticipated that use of image matching to aid the control of the motion of AUVs and
ROVS will develop and increase in the future.
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Current Limitations of INS Solutions
7.1
Sensor Reliability
Some of the technologies used in INS are now also commonly found in everyday commercial applications.
This vastly increased volume of production and mass market use has led to the introduction of lower grade units
but has also made it necessary for professional-level INS equipment to differentiate itself through performance
and reliability. In the aviation industry the mean time between failure (MTBF) of a ring laser gyro (RLG) is
expected to be 5000-10000 hours. It is the related electronics that are expected to fail first.
7.2
System Initialisation
All INS require a period of time to initialise and achieve optimum positioning accuracy. This period could also
include time taken for the IMU to undertake a series of calibration movements. This procedure could require
up to one hour of ROV time before starting a survey.
7.3
System Complexity
INS is still a relatively specialist solution, and the number of specialist operators of such equipment may be
limited. In some cases, specialist personnel from the manufacturer may need to be present for the initial
mobilisation. The system manufacturer may need to provide appropriate sensor weighting to tune the filter to
the specific installation.
7.4
Real Time versus Post-Processing
Many survey operations require that the INS delivers positioning in real time, with little or no need for a postprocessed solution. However it is recognised that the filtering and modelling can often be improved by some
form of post-processing of the data. This requires that the appropriate INS and sensor data are recorded and
that there is post-processing functionality. Not all INS solutions offer such a capability; users should consider
beforehand what sort of INS is selected for a survey operation, in order to ensure that all the necessary various
outputs and options are available.
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7.5
Cost
The IMU sensor at the core of the INS solution is typically relatively expensive, particularly in the case of higher
end navigation grade sensors, which can cost in excess of £100,000 (2014 prices). In selecting the IMU sensor
the perceived benefit should be weighed against the cost. However, this calculation may not be as straightforward
as it may seem. New devices are being produced, often at lower costs, and the benefits of an INS, whether a
costly unit or a relatively low cost one, may be significant. For example with continuous aiding it may not be
necessary to invest in a ‘high end’ IMU component at all. There are a number of reputable equipment hire
companies that can offer appropriate devices on a relatively short term basis, thus avoiding the need for capital
expenditure on what may be considered a specialist device. Appropriate advice should be sought, and case
studies and examples reviewed, to see what the performance gains may be for a particular project and operation.
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Future Developments
This section identifies a number of possible trends or developments that may influence how INS is used and
adopted in the future. Some of these can already be identified in other uses and applications of INS.
8.1
Size Matters – Smaller and Cheaper
Smaller and ultimately cheaper units will have a bearing on their use as will the continued trend to upgrade and
improve the performance and operational capability of expensive subsea assets and vehicles.
Advances in MEMS technology are likely to lead to smaller and cheaper INS equipment, based upon the increasing
use of mass market products such as smart phones. INS applications are likely to grow, and the use of IMUs will
perhaps become much more commonplace even for relatively straightforward offshore operations. This will
open up the possibility of greater variety and therefore redundancy for surface positioning and underwater
positioning solutions.
The trend for small units may mean their power consumption will also reduce and this will open up their use in
systems of limited power such as seabed transponders, AUVs and towed fish and arrays. Their installation will
also therefore be possible in difficult locations and beside the MBES and other sensors.
8.2
Improved Capability
The trend currently is for Class III (work class) ROVs and an increasing number of smaller Class I (inspection
and monitoring) ROVs to be fitted with INS based positioning solutions and to upgrade and improve their
positioning and tracking capability.
Something similar is taking place with AUVs where their initial payload for position and guidance is now being
replaced and improved with many benefiting from higher performance sensors, techniques and solutions.
The improvements are not simply a means to an end but of significance in advancing the autonomous methods
and techniques that could have a sustained and significant impact on offshore activities.
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Appendix A
IMU Types
There are essentially three types of technology used for IMUs, these being RLG, FOG and MEMS.
Ring laser gyro (RLG): These use a triangular arrangement of mirrors and lasers to measure the interference
between two opposing beams of laser light, from which rotation or angular velocity can be derived based upon
the Sagnac effect. They were considered to be the most accurate gyro technology currently available although
now FOG technology is comparable. This high performance comes with the disadvantages of a relatively high
price and the possibility of export and movement restrictions since the technology involved is potentially
applicable to weapons and military use. RLGs initially entered a market dominated by spinning mass gyros (such
as rate gyros, single-degree-of-freedom integrating gyros) because as a technique it is ideal for strap-down
navigation. The RLG was thus an enabling technology for high dynamic environmental military applications.
Fibre optic gyros (FOG): These use the same physical principle as RLGs with a light comparison using the Sagnac
effect. They consist of a coiled optical fibre in each of three axes. They use light interference to measure the
angular velocity. They are considerably more accurate than MEMS and have a well-regarded lifespan. FOGs were
developed primarily as a lower-cost alternative to RLGs but are now matching RLGs in terms of performance,
being widely adopted in many military and commercial applications, due to their lack of moving parts and
improved light processing techniques.
Figure 7 – Ring laser gyro (a) and fibre optic gyro (b) (Grewal, Weill, Andres 1997)
Both FOG and RLG technologies are based on the same physical principle discovered at the beginning of the
twentieth century and called the Sagnac effect. This effect shows that the propagation time of light along a closedloop path depends on its rotation rate. There are engineering differences between the two technologies, one
important one being that at low rotation rates (typically under 100°/h), the two counter-propagating waves of an
RLG can experience a lock-in effect that prevents direct measurement around zero. To overcome this dead-zone
effect, an RLG uses a mechanical dither or jitter mechanism, which introduces a number of moving parts with
consequent possible issues of reliability.
Microelectromechanical systems (MEMS): These are the least expensive, least accurate and most commonly
found type of IMU, largely due to their use in mass market applications in smart phones. They are typically small,
rugged and consume little power. They are generally inexpensive to produce, are reliable and have great future
potential.
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Figure 8 – Vibrating structure gyroscope (after Titterton and Weston 1997)
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Appendix B
Export Licensing
In general, it should be noted that a number of countries impose licence conditions on the import and export
of military and paramilitary goods and dual-use technologies, and members need to be aware of their
responsibilities in this regard. Many inertial measurement units now being used for offshore survey and
positioning operations are considered to be potential military equipment and, as such, they are subject to the
export controls in certain countries.
Some IMUs, INS and attitude and heading reference sensors (AHRS) are included under Part VIII of the
International Traffic in Arms Regulations (ITAR). They may not be exported from the USA without the prior
written approval of the US Department of State.
Further, it should also be noted that the controls prohibit the ‘retransfer’ (also called ‘re-export’) of items on the
United States Munitions List (USML) by foreign persons unless the retransfer is specifically authorised under the
relevant export authorisation.
The International Traffic in Arms Regulations (ITAR) and the Arms Export Control Act (AECA) are the governing
laws that are overseen through the Directorate of Defense Trade Controls (DDTC) within the US Department
of State. See www.pmddtc.state.gov/.
Advice and Consequences
Large contractors may have logistics departments which are well able to handle these restrictions and the
administrative burden imposed thereby. Smaller organisations may find it a challenge to support and keep track
of the moving controls and restrictions being applied. It should be noted that individuals are responsible and
liable if something goes wrong.
Companies should appoint a suitable manager or director to liaise with their appropriate government export
controls department or division and act as a single point of contact. That person should be kept informed of any
movements of the units with sensitive technology.
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Appendix C
References and Further Reading
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IMCA S 013 – Deep water acoustic positioning
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IMCA S 015/OGP 373-19 – Guidelines for GNSS positioning in the oil & gas industry
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IMCA S 018 – Guidance on the selection of satellite positioning systems for offshore applications
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IMCA S 019 – Guidance on subsea metrology
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IMCA S 020 – Guidelines on the safe management of survey and inspection data
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IMCA S 021 – Guidelines for the management of peripheral survey sensors
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Global Positioning Systems, Inertial Navigation, and Integration, Mohinder S Grewal, Lawrence R Weill, Angus P
Andrews
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Strapdown Inertial Navigation Technology, David Titterton, John L Weston
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Fiber-Optic Gyroscopes Key Technological Advantages, Fabien Napolitano, An iXSea company document
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International Traffic in Arms Regulations – https://www.pmddtc.state.gov/regulations_laws/itar.html
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OGEL – Open General Export Licence (UK) – https://www.gov.uk/military-goods-ogels
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