Sparse LBL (Long Baseline) is an advanced, hybrid acoustic positioning technique that offers high precision positioning with minimal seabed hardware.

It is more accurately described as range-aiding, as it functions by tightly integrating acoustic range measurements from a small number of transponders (as few as one, but typically two or three) with an inertial navigation system (INS), such as SPRINT or SPRINT-Nav.

This combination uses the high-rate, stable relative positioning of the INS to calculate a robust position, using the acoustic ranges to correct the INS drift, thereby achieving high integrity without the large, dense arrays required by traditional LBL.

Why is Sparse array planning important?

Planning a Sparse LBL array is crucial to achieving optimal system performance and maximizing the operational and cost benefits of the technique.

Careful planning is fundamental for Sparse LBL because the system relies on a minimal number of transponders compared to classic LBL, meaning it has less acoustic range redundancy.

A well-executed plan helps to avoid common pitfalls, such as acoustic interference, and allows teams to configure and check acoustic and INS projects onshore before vessel mobilization, streamlining procedures and de-risking operations

Planning ensures that the array design maintains high accuracy and efficiency throughout the operation:

• Geometry management: Planning is necessary to ensure that ranges are collected at a sufficient angle of cut (geometry) to constrain the positioning error ellipse. The precision of position is maintained when ranges are received at angles that best equalize the error ellipse.
• Cost and time reduction: Strategic placement of transponders, for example, placing Compatts at the top of mounds on the seabed, can lead to fewer Compatts being needed. This, in turn, saves vessel time and reduces costs.
• Systematic error detection: Sparse LBL has less redundancy than full LBL, which means systematic errors, such as a Sound Velocity (SV) error, can be difficult to detect without careful quality control planning. Accurate sound velocity measurement is a critical element to consider.
• Preventing acoustic gaps: The array plan includes an SV analysis to define the expected range of the transponders (e.g., Compatt units) at different depths. This analysis is crucial to prevent situations where a needed Compatt is out of acoustic range.
• Avoiding obstacles: Planners must identify and avoid placing transponders in areas such as overage curves, anchor scour locations, and deployment corridors.

How to plan a sparse LBL array to avoid gaps and maximise navigable area

Planning focuses on managed geometry—positioning transponders relative to the anticipated vehicle path (trajectory) to ensure optimal angles of cut across the whole area.

Trajectory dependence: The sparsity of the array depends heavily on the intended vehicle path. For instance, if an ROV is navigating along a pipeline, the transponders can all be placed on only one side of the pipeline while maintaining sufficient geometry by combining the acoustic ranges with the INS velocity and movement data.

Avoiding in-line placement: When using only two transponders, positioning precision degrades significantly when the vehicle moves directly between them because the necessary geometry is lost. This must be accounted for in trajectory planning.

Calibration trajectories: The planning process must define the calibration method:

• 2D SLAM calibration: This defines the horizontal position and requires the ROV to run a straight-line trajectory that covers an angle of cut greater than 60 degrees relative to the transponder. To reduce the effect of any depth error on the horizontal position estimate, the ROV must maintain a minimum offset (e.g., 150 meters) from the transponder.
• 3D SLAM calibration: If the transponder’s depth estimate also needs to be refined, a 3D SLAM trajectory is used, often involving an offset circle. This trajectory must be planned to include ranges taken “away from the transponder and over the top” to achieve the big vertical change observable by the Kalman filter.
• DTM and infrastructure: Planning should incorporate a Digital Terrain Model (DTM) of the area and a drawing (e.g., a .dxf drawing) of relevant subsea infrastructure.

What’s the minimum number of transponders for Sparse LBL to get 10 cm accuracy?

The number of transponders (e.g., Compatt 6+ units) required depends on the desired accuracy and integrity.

Minimum for aiding: Sparse LBL can be achieved by using ranges from as little as one single transponder.

Approaching full LBL accuracy: To achieve positioning accuracy of 10 centimetres (near pure LBL performance), a Sparse LBL array requires three or more transponders.

Intermediate accuracy benchmarks:

• ◦ 50 cm Accuracy can be achieved with a ** single transponder** on the seabed.
• ◦ 20–30 cm accuracy can be achieved by navigating with two transponders.

Does Sparse LBL need specialist planning software, or can it be done manually?

Specialist planning software and services are necessary to effectively plan and model a Sparse LBL array due to the critical nature of managing geometry and predicting performance.

Specialist planning tools: Specialist software is arguably necessary for array planning. Planning software allows users to model the navigation performance before going to sea, inputting bathymetry, sound velocity profiles, transponder placements, and vehicle trajectory to predict the positioning error.

Fusion 2 software: The Fusion 2 software suite controls LBL, Sparse LBL and SPRINT INS projects from a single interface and is optimised for this workflow. It enables the use of Real-Time SLAM calibration.

Software licensing: To use the range-aiding/Sparse LBL and SLAM capability within Fusion 2, the user must have the Fusion 2 INS dongle which has been programmed with the SPRINT LBL RANGE AIDING SPARSE UPGRADE.

In-house/remote services: Sonardyne offers an in-house array planning service and plans to offer a cloud-based planning portal. Additionally, our Remote Operations Access Module (ROAM) enables Sonardyne surveyors to remotely support operations and SLAM calibration onshore.