Sparse LBL, often referred to as range-aiding, is a hybrid approach developed to achieve high integrity positioning using a minimal number of transponders (e.g., just one or two).

The technique was pioneered to combine the integrity of LBL with greater operational efficiency. It significantly reduces vessel time and cost by cutting the number of transponders needed (by 30% or even 50–66% for certain pipeline arrays) and reducing deployment and calibration time.

A Sparse LBL or range-aiding set up

A Sparse LBL (Long Baseline) setup, often more accurately referred to as range-aiding, is a highly efficient underwater positioning system defined by its integration of an Inertial Navigation System (INS) with a minimal number of seabed acoustic transponders.

This setup uses fewer transponders than traditional LBL, relying on the INS to maintain precision and bridge acoustic gaps.

Core components

A Sparse LBL setup relies on three main groups of hardware and a central software system:

An underwater vehicle

Typically, an ROV or AUV, which must be equipped with systems that tightly couple acoustic ranging and inertial sensing:

• Inertial navigation system (INS): This is the primary technological enabler, providing stable, high-rate relative positioning. Systems such as SPRINT or SPRINT-Nav are used.
• Acoustic transceiver: The vehicle needs a device to interrogate the seabed beacons and measure the acoustic slant ranges (distances). This is often the ROVNav 6+ acoustic transceiver (or mini ROVNav 6+).
• Hybrid integration: High-performance solutions like SPRINT-Nav integrate the SPRINT INS, Syrinx Doppler velocity log (DVL), and a high-accuracy pressure sensor into a single housing. The DVL allows the INS to continue navigation during loss of acoustic aiding (acoustic dropouts).
• Time stamping: The ROVNav 6+ transceiver is connected directly to the INS, which acts as a multiplexer and provides the necessary accurate time stamping for the acoustic range data.

Seabed array (acoustic reference)

Sparse LBL minimizes the hardware deployed on the seabed, thereby reducing mobilization time and cost.

This is primarily an array of intelligent transponders, typically Compatt 6+ units.

While classic LBL requires five or more transponders for redundancy, Sparse LBL can operate with ranges from as little as one single transponder.

• Optimized Arrangement: To maintain positional integrity, transponders are usually arranged to ensure managed geometry. A commonly adopted setup uses a simple triangle of seabed transponders.
• Redundancy: Though one range is possible, at least two transponders are recommended as a minimum for improved quality control (QC) and redundancy.

Positioning software

The entire workflow, including system control and calibration, is typically unified under a single software platform.

Our Fusion 2 software controls LBL, Sparse LBL and SPRINT INS projects from one interface. It unlocks the potential of the latest 6G+ and Wideband 3 instruments.

Fusion 2 eliminates the prior complexity of using two independent software and hardware systems (Fusion 6G for acoustics and a separate package for SPRINT INS).

Operational principles of range aiding

The key distinction of the Sparse LBL setup is how the range data is used:

• INS calculation: The INS algorithm continuously estimates the vehicle’s position based on its sensed movement (dead reckoning).
• Range observation: The ROV transceiver measures the range to the deployed seabed transponders.
• Kalman filter aiding: This range information (not a full LBL position solution) is fed directly into the Kalman filter running within the INS.
• Correction: The Kalman filter takes the acoustic range observations and combines them with velocity data (from the DVL) and internal position estimates to fine-tune and correct the INS position. This prevents the exponential position drift inherent in standalone INS navigation.
• Robustness: The high update rate of the INS provides a robust position, allowing the vehicle to “ride through” short periods of acoustic loss or interference that would otherwise compromise a positioning fix.

Deployment and calibration

Since the array is sparse, the calibration process is fundamentally different and more efficient than traditional LBL calibration (which relies on baselines and box-ins):

• Real-time SLAM: Sparse LBL operations are typically calibrated using Simultaneous Localisation and Mapping (SLAM), which is available in real-time in Fusion 2.
• No baseline measurement: SLAM eliminates the need for time-consuming traditional baseline calibration workflows.
• Trajectory-based fixing: Calibration is achieved by flying the ROV through the array along a specific trajectory (such as a pipeline route). The movement and resulting acoustic range observations define the transponder’s position.
• Depth and position: A 3D SLAM trajectory refines both the horizontal position and the depth estimate of the transponder by ensuring a sufficient vertical change is observable to the Kalman filter.
• Efficiency: Because SLAM calibration can be done concurrently with other ROV survey operations, it significantly saves vessel time.