Subsea Positioning by Merging Inertial

and Acoustic Technologies

Eric Willemenot(1), Pierre-Yves Morvan

(1),

Hubert Pelletier(1), Arne Hoof

(2)

IXSEA Main place: IXSEA, 55 Av. Auguste Renoir, 78160 Marly-Le-Roi, France

Authors: (1) IXSEA, Rue Rivoalon, 29200 Brest, France

(2) IXSEA, Asternstr. 32, 30167 Hannover, Germany

Abstract

Inertial sensors have high data rate and smooth drifts, while

acoustic positioning systems have low data rate and bounded

errors. Merging the two technologies requires that the user can

access (i) the knowledge of all error sources of both technologies,

(ii) the detailed interfaces of the products, (iii) the technical keys

and the funding for the calibration of the coupling. By doing this

once for all, IXSEA provides innovative fused solutions "off the

shelf" for the benefit of the users. The fully portable high

performance USBL "GAPS" has an embedded inertial system,

with pre-calibration of misalignments at the factory. It offers new

and unprecedented applications for ships of opportunity, buoys,

shallow and very shallow water... in addition of a usual high end

USBL. The technology of GAPS will be presented together with

detailed applications where it makes the difference: shallow and

very shallow water operations such as pipe survey, noisy

environments like ploughing operations, positioning from a barge

where traditional USBL cannot be calibrated, etc. Since the

GAPS includes all the necessary technologies with pre-calibration

of the system (USBL, INS inside for the attitude and the position

of the antenna, GPS, embedded computation of data), it makes it

a fully plug&play solution. The performance is the following:

4000m range, 0.2% of range for the position accuracy, acoustic

head aperture of 200 degrees.

For applications where smoothness, accuracy, and ease of use are

even more stringent, a new concept called RAMSES will be

presented. RAMSES is a Range Navigation System with

positioning data fusion engine. It can be coupled to an Inertial

Navigation System. It is able to stabilize the drift of the INS just

by dropping a single transponder on the seafloor at unknown

position, with no calibration of any kind. The coupling

technology comes from robotics, and is called SLAM

(Simultaneous Localization and Mapping). This technique has

been adapted to subsea positioning. The main applications will be

presented: AUV survey (traditional or very large one), pipe route

survey, stringent ROV positioning and navigation. A new way of

thinking is now possible in reducing transponder deployment.

RAMSES is tolerant to acoustic shadows; there is no calibration

of the transponder array. In the case of an AUV, the users

generally have an INS and a DVL. Just by plugging a RAMSES

to the positioning system, it allows to perform a straight line

survey of hundreds of kilometres with just one beacon every 10

kilometres, keeping a positioning accuracy of around 10m. For

this application, the positions of the beacons just have to be

known thanks to GPS when deployed from helicopter or from

ship. The INS drift is then automatically reset when the AUV

comes into the range of a beacon (4000m range).

I. IXSEA KEYS FOR ACOUSTIC AND INERTIAL FUSION

IXSEA develops its acoustic and inertial technologies

internally. This gives technical teams full intimate knowledge

of all details of the products and allows us to develop

innovative concepts and systems while keeping repair costs

low and turnaround times short.

Acoustics: IXSEA has more than 30 years experience in

Acoustics from conventional beacon transducers to chirp sub

bottom profilers and Synthetic Aperture Sonar imaging

technologies.

Inertial: Company founders were pioneers in Fiber Optic

Gyroscope (FOG) technology in the early 1980’s. From this

starting point, IXSEA has grown to produce renowned FOG

based motion sensors and Inertial Navigation Systems

including IMO certified systems such as the OCTANS and

PHINS, MARINS - High Accuracy Miltary INS, Space

Qualified IMU’s for Satellites.

Fusion of technologies: inertial sensors have high data rate

(>100Hz to 1000Hz) and smooth drifts, while acoustic

positioning systems of any kind have low data rate (<1Hz to

0,03Hz) and bounded errors. Merging the two technologies

accurately and efficiently requires that the user can access

- the knowledge of all error sources of both technologies,

- the detailed interfaces of products,

- the technical keys for calibration of the system,

- the funding for such non recurrent tasks.

By doing this once for all, IXSEA provides innovative fused

solutions such as:

PHINS DVL READY: PHINS is an Inertial Navigation

System in a 6000 m housing coupled to a DVL and pre-

calibrated at the factory.

GAPS: a USBL antenna with INS and GPS integrated and

pre-calibrated at the factory. The high grade INS makes it

really 100% pre-calibrated, even for the heading, which means

that the user has nothing else to do than plugging and start

tracking. The main interest for the users are: unprecedented

and reliable performance thanks to definitive mechanical

mounting of INS with USBL, portability for use on ship of

opportunity, operational in very shallow water and deep water

(unique 3D antenna), exceptional ease of use.

1-4244-2523-5/09/$20.00 ©2009 IEEE

SHADOWS: the deep integration of INS/DVL, USBL,

synthetic aperture antenna, and gap filler, makes Shadows a

complete subsea mapping tool providing real time mosaic in

GeoTiff format generated from the tow fish’s highly accurate

position.

FUNS - Fused Underwater Navigation System. This

modular positioning and navigation system provides ultimate

robustness and redundancy by combining GAPS, PHINS DVL

READY, RAMSES, and DELPH INS post-processing

software, in any custom configuration depending on the

application and the architecture of the subsea asset of the user.

In this paper presents the main bricks for subsea positioning

by merging inertial and acoustics technologies:

- GAPS concepts and benefits (USBL+INS+GPS),

- RAMSES acoustic system and PHINS INS coupling, and

- Examples of more complex architectures for custom needs.

II. GAPS CONCEPT AND BENEFITS (USBL+INS+GPS)

GAPS was introduced on the market in 2003, and improved

to achieve full maturity in 2007 with impressive track record

for all kind of application : Scientific, Offshore, Defense…

INSINS

Fig 1: GAPS internal architecture with INS and acoustic antenna (4 legs).

GAPS internal architecture (see Fig 1) is composed of a

rigid USBL antenna made of one transducer for interrogation

and 4 transducers mounted on 4 “legs” for receiving acoustic

signal from the beacon. A high grade INS is mounted in the

housing of the antenna. It allows a rigid mechanical coupling

between the USBL and the INS, key for the definitive

calibration at the factory. Another key point is the

performance grade of this INS. With a true heading, roll and

pitch at 0,01° accuracy, all degrees of freedom are pre-

calibrated, contrary to any other USBL system. Last but not

least, the smooth and very accurate antenna position is

determined thanks to this high grade INS coupled to GPS,

giving a smooth absolute reference which improves subsea

asset positioning, robust to GPS dropout of several minutes.

As a summary, the GAPS is an electronically stabilized

USBL antenna with full pre-calibration in the factory, with

smooth and high rate (100Hz) absolute positioning reference

thanks to high grade INS coupled to GPS.

The principle of operation of the antenna itself is simply the

combination of (i) interferometry of the acoustic signal

coming from the beacon, for angle computation, and (ii) time

of flight of the interrogation+response signals, for distance

computation (Fig 2).

Fig 2: principle of interferometry between the 4 legs of the USBL antenna.

The particular so called “3D antenna” allows a wide

coverage of 200° (Fig 3). It’s very useful for tracking any

asset at low depth.

… up to 200°!… up to 200°!

Fig 3: the 3D antenna allows a 200° wide coverage

But it’s not enough to have a wide coverage angle. One

must be robust to multipath from surface echoes, and/or sea

bottom echoes if shallow water, and a very good signal

detection in order to separate the signal from all the noise the

antenna can hear in this wide angle of sensitivity.

Scientists from Ixsea have developed unprecedented

detection and rejection algorithms that makes GAPS the only

USBL able to track at long distance in shallow or very shallow

water, and in very noisy environment.

As an example of GAPS performance and versatility, the

US Navy installed GAPS hanging on a cable 5 feet below

surface, with 15m water depth, at 1 nautic mile from the

beach. They tracked a beacon at 5 feet below surface. Then the

beacon was moved toward the beach and was traked correctly

till the surf zone (Fig 4).

Fig 4: Tracking a beacon with GAPS in extreme shallow water (surf zone).

The 3D antenna and the detection algorithm of GAPS are

operational and fully field proven. Being pre-calibrated in the

factory, even for the heading, GAPS offers many other new

opportunities for the users. Recent feedback from users have

shown GAPS on a Catamaran (Fig 5), and on a simple buoy

very easy to mobilise (Fig 6).

Fig 5: GAPS on a Catamaran

Fig 6: GAPS on a buoy, very easy to mobilize

By merging inertial and acoustic technologies, and adding a

touch of innovation like 3D antenna and modern detection

algorithms, GAPS not only offers a high end USBL. We can

see that it offers the user really new applications, new

opportunities. GAPS is very often part of the solution for the

user.

III. RAMSES (ACOUSTIC SYSTEM) AND PHINS (INERTIAL NAVIGATION SYSTEM) COUPLING

RAMSES is the most recent building block in the IXSEA

underwater positioning product range. It is an all in one

integrated underwater acoustic positioning system with

internal positioning data fusion engine. In its preferred

configuration, RAMSES is used with PHINS inertial

navigation system as a revolutionary package providing much

improved performance and features over pure inertial or pure

acoustics techniques; e.g. many less transponders to be

deployed, redundancy, and robustness to acoustic shadow

which allow easy deployment in construction area for

instance.

RAMSES operates with RT9 new releasable transponders

(see fig 7).

Fig 7: RAMSES Range Navigation System, and RT9 releasable transponder.

The data fusion engine uses Kalman filtering and SLAM

techniques (Simultaneous Localization and Mapping). As a

first example, this allows smart coupling of ranges and INS

data, working with only one beacon at unknown position (no

calibration). This configuration allows RAMSES to stabilize

the INS drift, and it is tolerant to acoustic shadows. This

preferred configuration is presented hereafter in more detail,

and examples of applications are described.

Architecture of INS + RAMSES navigation system

The heart of the system is an INS (Inertial Navigation

System, PHINS) and a RNS (Range Navigation System,

RAMSES).

Fig 8: Architecture of PHINS+DVL+RAMSES, for example as on board a

ROV/AUV.

PHINS, DVL and RAMSES are installed on the subsea

vehicle that requires position and navigation. The releasable

transponder is deployed on the seabed at an unknown position.

The system is able to estimate both the transponder position

and the vehicle position, in order to navigate from the INS

with no inertial drift, even during dropouts due to acoustic

shadows. The complexity of such a tightly coupled system has

been kept inside the products, so that the user can just “plug

and play” with no required expertise in the field of Kalman

filtering, inertial navigation, SLAM, or acoustics.

Algorithms and their architecture

Kalman filters (RAMSES + PHINS configuration)

Two individual Kalman filters run simultaneously, one

inside the PHINS, the other inside the RAMSES. This

architecture has been chosen to be able to use PHINS without

RAMSES but also RAMSES without PHINS. The figure 9

presents the two Kalman filters and their interfaces.

Transponder position is estimated inside the RAMSES. The

PHINS sends its data to RAMSES (position and speed), and

the RAMSES sends back the ranges relative to estimated

position of the transponder. Final vehicle position is provided

by the PHINS’ Kalman filter.

Fig 9: Kalman filters

SLAM techniques

Simultaneous Localization And Mapping is a well know

technique developed in the 80’s for robots. They can position

themselves while building up a map of their environment,

taking into account particular points in video images and/or

ultrasonic data.

Inside RAMSES, this technique has been adapted to our

field: underwater positioning and navigation.

The particular points in the environment are simply

transponders at unknown positions.

As a consequence, when the vehicle is equipped with

RAMSES or PHINS+RAMSES, it is able to navigate as soon

as it is in the water. There is no need to do an initial survey or

calibration of the seafloor transponders. The system

automatically analyses the data available, navigates, and

progressively improves the accuracy of its outputs (standard

deviations).

At the very beginning of the navigation, the system does not

know the position of the beacon(s). The range(s) are sent to

the PHINS, but are not very useful at this stage. The SLAM

estimates the position of the beacon(s), and improves this as

navigation and successive interrogations of the beacons

continue. Automatically, the range(s) sent by the RAMSES to

the PHINS are more and more useful for the PHINS, its

navigation data rapidly stabilizes, and the position of the

beacons become very precisely known (up to 10cm accuracy).

Initialisation and rejection algorithm

From RAMSES side, a rejection and initialisation algorithm

is essential to have a robust system even in unknown initial

conditions and in case of bad measurements (difficulties in

acoustic transmission). This greatly contributes to the ease of

use since everything is automatic, and the user doesn’t need to

be an expert in acoustic positioning, i.e. doesn’t have to

choose which beacon is valid or not, at what time etc. The

merge of inertial and acoustic data allows this algorithm to be

very robust.

Typical applications

Generally speaking, the applications of RAMSES are the

one that needs high accuracy in positioning, the one that need

to stabilize the drift of an INS, the one where image distortion

must be reduced, or georeferencing must be accurate…

Typical AUV survey

During a typical AUV survey, the INS drift is of the order

of a few meters per hour (DVL aided). If the survey lasts 24h,

the drift can reach around 100m or more. Such a drift can be

unacceptable for georeferencing the data depending on the

requirements of the survey.

With only one beacon deployed on the seabed, and no

calibration of any kind, the INS drift is reset every time

RAMSES is in range of the seabed transponder, and the

positioning performance is bounded to a fixed value. This

value roughly depends on the INS pure inertial drift during the

AUV descent until the transponder is in the range of the

RAMSES (4000m), e.g. 10m or less. If the beacon is already

in the range of the RAMSES during the descent, drift is

limited to just the time required for RAMSES to converge on

the mathematical solution for the position of the beacon and

its own position. This can be just a few minutes, providing a

few meters of bounded error for the whole AUV survey.

Fig 10: typical AUV survey. Periodic position reset thanks to RAMSES aiding

the INS.

Large AUV survey

In a case where the AUV survey is a very large area, the

fixed beacon may be outside the approximate 4000m range of

the RAMSES system for a while. In this case, the INS+DVL

system will drift again when outside the beacon range. But

each time the AUV comes back near a known beacon, the drift

will be cancelled, because the RAMSES knows the beacon

position.

Another extreme AUV survey is the case of a straight line

(e.g. pipe or cable route survey…). In this particular case, one

can drop beacons every 10km or more, with rough positions

known from GPS when deployed from ship or helicopter. The

INS+DVL (or INS alone) can have its drift reset by the

RAMSES automatically.

ROV positioning and navigation

High accuracy ROV positioning often requires the

deployment of a LBL array, with continual contact of at least

3 beacons to achieve the required accuracy within the

navigation area. Such configuration is costly in equipment

(numerous transponders to deploy) and in time: preliminary

study of the array design, preliminary calibration of the array.

Acoustic propagation in difficult environment (shallow water

or proximity with subsea structures) is not reliable and the

operator can face shadow areas where high accuracy will not

be possible.

RAMSES coupled with PHINS/DVL is an easy, calibration

free and robust solution to provide performance, even when

acoustics are less than optimal or even completely out. The

drift accumulating in the PHINS/DVL while acoustics are not

available is easily compensated from as soon as the ROV

comes into acoustic contact of a single RAMSES transponder

previously deployed. This configuration is highly beneficial

for operation near platforms or other noisy, acoustically

challenged environments.

Pipe / cable route Survey

Pipe, cable route, or geophysical surveys require accurate

positioning information for georeferencing collected data.

USBL systems usually provide 0.2% x range (GAPS)

accuracy, which may not be sufficient in case of long range or

if LBL-grade positioning is expected.

PHINS/DVL + RAMSES here again is an easy to deploy

and accurate solution. PHINS/DVL provides short/medium

term accuracy; the drift of it versus time will be cancelled any

time RAMSES is in range with a transponder on the survey

route. For a same job the solution divides the number of

transponders to be deployed on the sea bed by a factor of

2,3,10…, since interval between transponders is a function of

the maximum acceptable drift accumulated by PHINS/DVL

between each of them. And there is no need for two

transponders in a ‘gate’ configuration, as LBL requires, for

each acoustic contact! Only one (with increased intervals) is

required with RAMSES technology. From this point of view, a

new way of thinking is now possible in simplifying

transponder deployment.

Fig 11: example of performance achieved with a very flexible configuration for AUV survey. Tolerance to acoustic shadows of the PHINS+RAMSES

solution allows rapid deployment and automatic operations.

Fig 12: Thanks to its “sparse array” capability, PHINS + RAMSES can

drastically reduce the number of beacons for pipe survey operation (e.g. factor 2, 3, 5 or more).

Example of performance during sea trials

SLAM of one beacon from unknown position

Following figures present the results obtained in real time,

during sea trials in the bay of La Ciotat (France). A fixed

transponder has been deployed on the seabed at unknown

position. PHINS, DVL and RAMSES are installed onboard

the surface vessel (black trajectory). The surface vessel also

logs a GPS RTK (accuracy < 10 cm) as ground truth.

Fig 13: deployment of a transponder

Initially, the transponder position is completely unknown

and navigation starts with performance close to PHINS + DVL

performance (i.e. 3 m / hour @ 3 knots). As time goes by,

RAMSES’ Kalman Filter estimates transponder location with

measured ranges and PHINS data (by use of SLAM

algorithm). The global performance of the positioning system

improves as the navigation and subsequent interrogations of

the transponder continue. When the positional accuracy has

reached its optimum, the SLAM is automatically switched off

and the transponder position is considered as known. From

this time, ranges relative to transponder position will be sent to

the PHINS and will aid navigation by controlling inertial drift.

Fig 15: mobile (red) and transponder (green) positions estimated by the

SLAM. Starting with an unknown position (arrow), the system assumes that the transponder is at the same position as the RAMSES acoustic head unit. As navigation continues, RAMSES rapidly estimates the transponder position

(green curve).

Case of a linear trajectory from the beacon

Range test shows how a single transponder solution can

update PHINS+DVL in one particular axis. In this case the

error ellipse is very large. If positioning accuracy is needed on

a larger zone, a second transponder can be deployed in order

that 2 large ellipses cross perpendicularly.

Fig 16: range test

Position repeatability

To perform a repeatability test, an initial SLAM positioning

of the transponder was performed and the final calculated

position given by RAMSES noted. The ship then returned to

perform a second circumnavigation of the transponder and the

operator started a new SLAM positioning sequence. Figure 17

presents the results of this trial. After a few interrogations,

and without completing a full circle, the final calculated

position of the transponder from the second SLAM

localization is within 13 cm of the first position.

15,5 cm

Position from

the first trialFinal position

from the second

Position during navigation

of the second trial

12 cm

Nav of RAMSES

on board the ship

Fig 17: Positioning Repeatability

The beacon position is estimated a second time by the

SLAM algorithm. One can see the estimated position starting

from the previous one and converging to the second one which

is very close to the first one. This demonstrates the very good

repeatability of RAMSES+PHINS.

RAMSES and PHINS coupling summary

and extended modes

The positioning system based on PHINS+RAMSES, or

PHINS+DVL+RAMSES, is an innovative coupling of

acoustics and inertial technologies. Together with the SLAM

technique, it is the first time that all the necessary automatic

features are embedded in a single package that allows plug &

play operation by non specialists and users requiring high

accuracy and robustness of positioning.

This paper is focused on applications where the user can

gain great navigational advantage with little effort: Starting

with a single transponder on the seabed, at unknown position,

with no calibration of any kind, the drift of the INS (or

INS+DVL) can be reset and stabilized.

AUV survey, ROV positioning and navigation, are the main

applications that can benefit by saving time from RAMSES

simple deployment and operation.

Beside this simple configuration, the RAMSES system is

designed to be operated with more than one transponder,

allowing pipe survey operations or other extreme surveys

where the area covered is much more than the transponder

range of 4000m. Here, the ability of “sparse array”, the

tolerance to acoustic shadows, gives significant improvement

over any other positioning solution.

RAMSES is also designed to fuse the data from many other

positioning devices such as USBL (updated through the

umbilical of the ROV, or acoustic modem), pressure sensor,

DVL, etc.

The users will ultimately themselves discover additional

ways RAMSES can increase their productivity.

Fig 18: flexibility of the coupling configuration of PHINS+RAMSES gives great opportunities for many kind of positioning and navigation use.

IV. CONCLUSION

Merging inertial and acoustic technologies is much more

than putting a Kalman to get mathematical optimal solution.

The GAPS positioning solution takes advantage of rigid

mounting of INS and USBL, careful synchronisation of data,

an innovative 3D antenna, modern and powerful acoustic

detection algorithm… and thus creates new opportunities for

the user, new markets.

The coupling of RAMSES and PHINS takes advantage not

only of ranges and inertial sensors, but also SLAM techniques

adapted to subsea positioning needs, and thus new schemes for

navigation, providing more productivity for the users.

Coupling of inertial and acoustic technologies, from Ixsea

point of vue, is composed of 4 key points:

- mastery of sensors technologies in order to optimise their

use in filters and quality estimates of data

- fusion of sensors data (of course)

- innovative concepts added to the two first points

- user feedback, in order to develop evolutions focused on

the new needs which are created.

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F. Napolitano, T. Gaiffe, Y. Cottreau, T. Loret, PHINS: the First High

Performance Inertial Navigation System Beased on Fiber Optic Gyroscopes,

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R.Siegwart, I. Nourbakhsh, Introduction to Autonomous Mobile Robots,

MIT Press, 2004 S. Lacroix, Lessons of the “college de polytechnique“, reconstruction of

environment of mobile robots, June 08, private communication.

J.J. Leonard, H.F. Durrant-Whyte, NEC Res. Inst., Princeton, NJ; Simultaneous map building and localization for an autonomous mobile robot;

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