Acoustic Positioning Techniques

LBL (Long BaseLine)

The Long BaseLine (LBL) acoustic method provides accurate positioning over a wide area by measuring ranges to three or more transponders deployed at known locations on the seabed or on a subsea structure. 

LBL is designed to position multiple subsea targets and structures with the highest attainable levels of accuracy independent of water depth.  The technique offers the highest degree of positioning repeatability available and with range redundancy, an estimation of the position quality can also be made. 

The system can be configured to support simple tracking tasks through to the most complex deepwater construction project. Typical operations might involve positioning multiple subsea vehicles working in close proximity to each other as well as streaming sensor data from gyros, digiquartz depth sensors, inclinometers and the like. 

An LBL system has two main elements. The first element comprises a number of acoustic transponders moored in fixed locations on the seabed. The positions of the transponders are described in a co-ordinate frame fixed to the seabed. The distances between them form the 'baselines' used by the system.

The second segment comprises an acoustic transceiver which is normally installed on the vessel or on a towfish. The distance from the transducer to a transponder can be measured by commanding the transceiver to transmit a short acoustic signal which the transponder detects and causes it to transmit an acoustic signal in response. The time from the transmission of the first signal to the reception of the second is measured. As sound travels through the water at a known speed, the distance (range) between the transducer and the transponder can be estimated. The process is repeated for the remaining transponders and the position of the vessel relative to the array of transponders is then calculated or estimated.

LBL Positioning Technique

The positioning technique employed in the Sonardyne LBL solution is trilateration, a method of determining the relative positions of objects using the geometry of triangles in a similar fashion to triangulation. Unlike triangulation, which uses angle measurements (together with at least one known distance) to calculate the subject's location, trilateration uses the known locations of two or more reference points, and the measured distance between the subject and each reference point. To accurately and uniquely determine the relative location of a point using trilateration alone, generally at least three reference points are needed.  In LBL the depths of the array transponders and mobile unit are also required.

As trilateration is a range-only solution geometry has a significant effect on the quality of a position fix.  The optimal angle subtended by the intersection of lines of position (LOP) formed by circles of constant range from two reference points is 90°.  The uncertainty (error) associated with each LOP means that the possible position of the ‘fix’ lies within an ‘error diamond’ the dimensions of which will increase as the angle of cut becomes more acute.  For this reason the angle of cut between LOPs should be limited to between 45° and 135°.

As all observations contain errors a positioning system should be designed to minimise the effect of those errors and to eliminate gross error where possible.  However, it is not possible to eliminate an observation if only 3 reference points are used as this is the minimum number required to compute a unique solution.  The introduction of a ‘redundant’ fourth observation enables the detection of discrepancies and inconsistencies in observed values.  This, in turn, makes possible the practice of adjustment computations for obtaining the most probable values based on the measured quantities.  Redundancy is therefore a key requirement in applications for which the reliability of the position solution is important.

 

USBL (Ultra-Short BaseLine)

Ultra-Short BaseLine systems calculate the position of a subsea target by measuring the range (distance) and bearing from a vessel mounted transceiver to an acoustic transponder fitted to the target. This is combined with vessel attitude, heading and GPS sensor information. 

The technique is widely used by the offshore and oceanographic industries as it offers high accuracy performance combined with ease of operation. Typical applications for the USBL technique include tracking ROVs, AUVs and towfish or as a position reference input for vessels equipped with a Dynamic Positioning (DP) system. One of the main advantages of the USBL technique is that no other in-water acoustic equipment has to be deployed before underwater operations can commence. Only the targets being tracked need to be equipped with a transponder.

USBL Positioning Technique

With USBL systems there are a number of sources of error and care needs to be taken to ensure system design, configuration and user understanding mitigate as many of these as possible.

1. Precision, bias and accuracy
It is very important to understand these words. USBL systems can be highly precise but inaccurate due to systematic biases or they can be accurate but imprecise due
to random errors.

2. Signal to Noise
Positioning quality is hugely effected by the level of signal from the target being tracked compared with the background noise level seen by the USBL transceiver (SNR) below the vessel. Decreasing SNR causes increasing random error. The noise from thrusters and machinery onboard varies tremendously from vessel-to vessel to and tends to increase in heavier weather conditions as thrusters are more active.

Choosing the right transponder is critical too. The higher the output power the greater the SNR. This either improves the system performance or increases the operating range. Increasing the output power reduces battery life, and so choosing a transponder that has sufficient output power to ensure low random errors, while providing sufficient battery life is important.

3. Elevation
When a transponder is under the vessel, the estimation of depth is extremely good, provided the average sound velocity (SV) through the water column is accurate, as mainly derived from the range measurement. When the transponder is at higher elevations, the azimuth or bearing estimation remains very good but the elevation estimation degrades. However this can easily be mitigated by using depth aiding, for example, by using transponders fitted with depth sensors.

4. Refraction
USBL systems measure both the range and direction to a transponder.

The range measurement can be affected by changes in the effective SV throughout the water column. Refraction causes the signal path to bend as the sound speed changes, which means that the measured distance will have a bias error. This is a particular issue when tracking a transponder at a high elevation angle and over long range, as the time that the signal spends in each sound speed layer will vary according to the angle that it meets that layer.

When tracking a transponder mainly below the vessel, refraction is minimal so an average sound speed can be used with little problem. Refraction does not affect the estimate of horizontal direction, however, it does cause the estimated elevation of the transponder to be in error, since the USBL system is measuring the direction of arrival at the transceiver and not the true direction to the transponder. When operating at high elevations and long ranges, for example when tracking a towfish at a long layback, depth aiding will reduce both the random and systematic errors that arise.

In addition, if an SV profile is available, then the USBL system can correct for the systematic refraction errors. It is important, however, when using SV profiles to be aware that profiles can change within the operating area. SV profiles can change significantly at different stages of the tide and between day and night. Using an inaccurate SV profile can cause greater errors than just using an average sound speed figure.

5. Reference sensors
USBL systems need to remove the huge effects of vessel motion. To do this they use heading, pitch and roll motion sensors on the vessel. These come in a variety of types, ages and cost. The quality of these can dramatically limit positioning quality by introducing random error and bias. The better the sensor and installation, the better the performance of the total system.

Older sensors tend to be of poorer quality and can cause errors due to data latency. Analogue output sensors can introduce scaling and sign convention error, and so it is important to make sure that these are set-up correctly.

Users need to make sure the sensors are rigidly mounted and fitted as near to the roll/pitch centre of the vessel as practical and that they are compensated by GPS velocity and heading inputs if appropriate. Watch out for filter settings in the sensors which can over filter data and introduce latency. If the latency is known, Sonardyne USBL systems can be setup to compensate.

If a separate pitch and roll sensor (VRU) and heading sensor are used (Gyro) then it is important that the two reference frames are aligned within a degree or so or errors can result. There are significant advantages in using integrated heading and attitude sensors.

6. USBL transceiver deployment

The deployment of the USBL transceiver is critical. It should ideally be rigidly mounted to the vessel well below the keel away from any weather or vessel induced aeration.

The motion compensation sensor should be mounted at the roll/pitch centre of the vessel where it is subject to the least motion induced acceleration. Sonardyne supply through-hull deployment systems that are extremely rigid and ideal for high accuracy USBL positioning in deep water.

It is not always practical, however, to install such a permanent deployment system and other alternatives are required. Many operational installations have shown that if care is taken then moon pool and over-the-side deployment systems can be very accurate.

In these cases, fitting an integrated heading and attitude sensor to the top of a moon pool pole or over-the-side deployed pole can compensate for movement relative to the vessel, such as when lowering and raising the pole. If the pole flexes substantially, then, although more exposed, putting the sensor at the bottom of the pole can improve performance by compensating for bending of the pole.

7. Verification of system accuracy

USBL system performance varies so it is important for users to know the actual performance achievable on a particular vessel. Sonardyne’s CASIUS software and procedures verify to the client the actual accuracy of the system achieved and simultaneously calibrates the whole system to remove systematic biases.

A transponder should be deployed in a suitable depth of water. The USBL software then guides the operator through the data collection process where range and GPS observations are acquired during a series of vessel manoeuvres. Simultaneously, USBL and motion sensor data is logged. CASIUS software provides an accuracy verification report for the user or client containing the actual USBL system accuracy. In addition it computes the GPS antenna offsets from the acoustic transceiver, the pitch, roll and heading corrections and sound speed through the water column to be used by the system.

8. Update rate
To some extent, random error can be reduced by collecting more observations. If there is time and the target is stationary, a more precise fix can be achieved by averaging over a number of observations. Increasing the update rate and using responder mode can achieve this in a shorter period of time.

Sonardyne systems can use simultaneous interrogation modes where all transponders reply to a single interrogation signal every positioning cycle. This is not always possible when using transponders from other vendors which require individual interrogation signals for each transponder. Another benefit of simultaneous interrogation is that it reduces the number of signals in use, leaving more of the frequency band available for use by other vessels.

The use of Sonardyne’s unique ‘ping stacking’ acoustic interrogation technique also provides an advantage when working in deep water by enabling USBL system users to transmit acoustic interrogations to subsea transponders before the last reply was received. This can substantially improve dynamic tracking performance, producing a smoother track.

9. Wideband
Sonardyne Wideband acoustic signals improve USBL performance. They enable more transponders to be used without interference issues. They improve the SNR, as discussed earlier, reducing the random errors seen, particularly on noisy vessels or you can reduce the transponders output power level and so increase the battery life. The signals are more robust so improving detection, particularly in harsh environments, at high elevations or where there is substantial multi-path.

10. Summary
Choose the most appropriate USBL transceiver for a particular vessel, the right transponder for the application and install a good quality integrated heading and attitude sensor in the right place. Ensure the USBL deployment is rigid or compensate appropriately where necessary. Verify the actual accuracy achieved and calibrate the system using Sonardyne’s new CASIUS software. Use depth aiding and SV profiles when required and choose the best interrogation method. With good planning and configuration your USBL system performance can be exceptional.

LUSBL (Long and Ultra-Short BaseLine)

The Long and Ultra-Short BaseLine (LUSBL) positioning technique is particularly suited to deep water applications as it combines the repeatability derived from Long BaseLine (LBL) subsea positioning, where accuracy is virtually independent of water depth, with the operational convenience of Ultra Short BaseLine (USBL) positioning.

LUSBL Positioning Technique

The high level of range redundancy ensures that in poor acoustic conditions in which deep water often DP vessels often operate, or when recovering transponders for maintenance, many range measurements can be corrupted before the computed vessel position degrades.

Sonardyne’s LUSBL design circumvents the delays of calibrating the LBL arrays by using a ‘Top Down’ calibration technique as the transponders are deployed. By not having to take baseline measurements, arrays can be calibrated quickly and reliably. An important advantage of this approach is the uninterrupted supply of acoustic information to the vessel’s DP system. This can be from arrival on station and the deployment of the first beacon, through the array calibration process and during the long term maintenance of the array.

Wideband LUSBL systems can also be set-up as a conventional dual redundant system where two systems are cross-linked in a ‘master and slave‘ configuration. This allows positioning to continue in the event of a single point of equipment failure.

System Performance
Repeatability, the most important aspect when related to dynamically positioned rigs, is typically 0.5% of slant range.  This can be significantly improved through the application of Sonardyne’s CASIUS calibration program together with the use of high resolution VRUs and gyro’s. CASIUS is used to determine the inherent errors or offsets that are typically found within the vessel’s various attitude sensors. Compensating for these errors and offsets can improve the repeatability to as little as 0.2% of slant range.

Wideband LUSBL Upgrade
Users of existing Sonardyne LUSBL systems are now able to take advantage of the latest Sonardyne Wideband signal technology with an easy and cost-effective upgrade. As the Wideband LUSBL upgrade is installed on existing hardware, operators can take advantage of the new technology whilst the vessel remains in service. The use of familiar hardware and software on a proven topside platform eliminates the need for re-training and enables users to work with confidence from the outset. This low risk approach ensures operators can make a smooth transition to wideband whilst maintaining compatibility with their previous generation acoustic hardware.