Subsea positioning typically relies on two main acoustic methods: Long Baseline (LBL) and Ultra-Short Baseline (USBL), with Sparse LBL providing a hybrid solution to bridge the limitations of both.
Comparison of Sparse LBL and traditional LBL accuracy
Sparse LBL (range-aiding) achieves positioning performance near full LBL levels by tightly coupling acoustic range data with an Inertial Navigation System (INS).
- Traditional LBL accuracy: Full LBL positioning can achieve up to 3 cm accuracy.
- Sparse LBL accuracy: Sparse LBL aided by SPRINT INS can also achieve positioning accuracy of up to 3 cm. Fusion 2, the software platform used to manage these operations, enables centimetric subsea positioning in all water depths.
In practical field comparisons, Sparse LBL shows close agreement with traditional methods:
- During simultaneous tracking operations, the difference between the Sparse LBL INS solution and the full acoustic LBL solution was shown to be less than 50 cm (with baseline lengths between 300 and 500 m).
- In one specific case using a single Compatt 6+ transponder for ranging, the horizontal difference between the Sparse LBL INS solution and the full LBL solution was just 12 cm.
- Comparisons of survey results showed that SLAM-calibrated sparse LBL arrays achieved centimetric agreement when compared against full LBL sections.
- Experimental figures show that using ranges from one single transponder yields about 50 cm accuracy, while using two transponders yields about 25 cm accuracy. Using three or more transponders yields positioning accuracy around 10 cm, approaching full LBL performance.
| Method | Advantages (pros) | Disadvantages (cons) |
| Classic Long Baseline (LBL) | Provides exceptional accuracy, delivering up to 3 cm LBL positioning accuracy. The precision is independent of water depth. It is used when the highest level of subsea positioning accuracy is called for. | Requires dense arrays of transponders (typically five or more, often six or more). Deployment and calibration of these dense arrays demand significant vessel time and cost. Requires a minimum of four ranges (ideally five or more) to detect and reject erroneous readings and compute a high integrity position. |
| Ultra-Short Baseline (USBL) | Enables rapid mobilisation. Provides speed and flexibility. | Accuracy decreases with depth and is calculated as a percentage of slant range. It struggles to deliver required accuracy for critical tasks in depths exceeding 1,500 m. Performance can be affected by vessel noise and acoustic conditions. Short-term behaviour can be erratic. |
| Sparse LBL (range-aiding) | Combines USBL’s speed with LBL’s integrity. Uses a minimal number of transponders (as few as one or two). Significantly reduces vessel time and cost. Faster calibration can be performed using real-time SLAM. Provides enhanced integrity, particularly when augmenting USBL. | Requires an Inertial Navigation System (INS) such as SPRINT or SPRINT-Nav. Has less acoustic range redundancy than full LBL. Requires careful planning and management of array geometry to achieve optimum performance. If only two transponders are used, the precision degrades significantly when moving directly between them, as geometry is lost. |
What about Sparse LBL navigation during signal loss (masking)?
Sparse LBL is specifically designed to maintain navigation even if a transponder signal is momentarily lost due to masking, interference or acoustic dropouts.
This capability is enabled by the tight integration of the acoustic system with the inertial navigation system (INS), such as SPRINT or SPRINT-Nav.
- Acoustic dropout ride-through: The INS provides high-rate, stable relative positioning. If the acoustic link is lost temporarily (an “acoustic dropout”), the INS can ‘ride-through’ the loss, continuing to output a position using its internal sensors and potentially a Doppler Velocity Log (DVL). This prevents significant degrading of performance over short periods of time.
- Masking near structures: The INS reduces the line-of-sight dependency to LBL transponders. If the ROV dips behind a subsea structure, the INS will hold the position for that short period.
- Kalman filter integrity: The core of the system, the Kalman filter running within the INS, handles the range aiding. If a range observation is not received for a few seconds or even a few minutes, the INS and DVL navigation will still run. The system will continue to provide a very good positioning estimate until the acoustic line-of-sight is retrieved, at which point the Kalman filter updates with the new range information.