Sparse LBL, also known as range-aiding, is a hybrid positioning technique that leverages the combination of minimal acoustic ranges from seabed transponders with an inertial navigation system (INS), such as SPRINT or SPRINT-Nav.

This technique was developed to combine the positional integrity of LBL with the efficiency of requiring far fewer transponders. Fewer transponders are required because you are no longer calculating position with ranges, you are constraining INS drift.

This makes it suitable for a wide array of subsea operations, particularly those with high demands for speed and accuracy in deep water.

If you want to know the basic principles of LBL, read them here.

Primary applications and project types of Sparse LBL or range-aiding

Range-aiding is primarily used in the energy/oil and gas sector but also has applications in ocean science and defence.

Specific high-precision tasks include:

Subsea structure installation and construction – Sparse LBL is used for subsea structure installation where an ROV can be rigidly docked on to a structure. This approach is used in major offshore field development operations.

• Metrology – It is applied for high-precision measurement tasks, such as metrology (precise measurement of distances between structures). A comparison against conventional metrology showed that using Sparse LBL with SLAM resulted in differences as low as 8 mm in horizontal distance and 16 mm in depth difference.
• Pipeline work – The technique is crucial for operations involving pipelines, including pipeline positioning and monitoring. Sparse LBL has been successfully used to reduce the number of Compatts required for pipeline installation arrays by 50–66%. It can support survey-grade tasks such as determining pipeline out of straightness.
• Survey and IMR – It is adopted in inspection, maintenance and repair (IMR) operations and light construction. It is also used for environmental surveys, site survey and characterisation.

Vehicle navigation and hybrid functions

Sparse LBL significantly enhances the performance and efficiency of vehicle navigation by reducing the dependency on dense acoustic arrays:

• ROV and AUV navigation – It is used for ROV navigation and AUV (autonomous underwater vehicle) navigation. For AUVs, Sparse LBL is particularly adapted because it enables them to cover large areas with a minimum of deployed acoustic hardware. The robust positioning and high update rates provided by the integrated INS/acoustic solution are typically what an ROV pilot requires.
• Dynamic positioning (DP) reference – Sparse arrays can provide stable, depth-independent position data to the vessel’s DP system. This serves as a valuable alternative or back-up to GNSS, especially when operating near infrastructure that might compromise satellite signals.
• Overcoming line-of-sight dependency – The inertial aiding allows the ROV to maintain precise navigation even when acoustic ranges are momentarily lost or interrupted (acoustic dropouts), which happens frequently when operating near subsea structures

What are the benefits of using Sparse LBL for underwater positioning and navigation?

The use of Sparse LBL is driven by efficiency and cost reduction:

• Reduced costs: It reduces the number of transponders needed (by 30% or even 50-66% for certain pipeline arrays). This, in turn, reduces hardware requirements and costs, and streamlines logistics.
• Vessel time savings: It leads to significant vessel time savings. Since fewer transponders are required, the time spent deploying them and performing calibration routines is greatly diminished.
• Real-time calibration (SLAM): Sparse LBL operations can be calibrated using simultaneous localisation and mapping (SLAM), especially with Fusion 2 software. Real-time SLAM calibration can often be performed concurrently with other ROV survey operations (such as pre-lay surveys), effectively removing the time-consuming traditional baseline calibration workflow from the project schedule.
• Remote operations support: Using Fusion 2 and our Remote Operations Access Module (ROAM), Sparse LBL operations, including real-time SLAM calibration, can be supported or operated remotely by onshore surveyors, mitigating logistical risks and travel restrictions.