LBL (Long BaseLine) underwater positioning is an acoustic positioning technique that provides high precision underwater positioning and navigation.

It involves the use of acoustic transponders placed on the seabed, typically hundreds or thousands of meters apart (hence “Long Baseline”).

Core principles of LBL

Ranging and positioning: LBL systems determine the relative position of a target, such as a remotely operated vehicle (ROV), by sending and receiving acoustic signals to transponders with known positions (relative to each other) on the seabed. The system calculates the distance to each transponder by measuring how long the signals take to reach them. Using these distances, the system computes the target’s location, much like a GPS system.

Note: The final positioning accuracy is dependent on several factors that begin with the GPS signals received at the vessel’s antennae. Antennae must be located so that there are minimal obstructions from vessel masts and superstructure, which will provide a clear line of sight to the GNSS satellites. Accurate offsets from the antennae to USBL transceiver and on the ROV are then required so that the best possible “real world” coordinates of the ROV’s common reference point (CRP) can be determined.

Is LBL depth independent?

A key benefit of LBL is that its precision (how consistently the system can reproduce the same position measurement) is independent of water depth.

This is because the LBL array is in a fixed position at the seabed. Distance measurements relative to these fixed points are unaffected by water depth variations.

It is used when the highest level of subsea positioning accuracy (how close the measured position is to the targets true position) is required.

How many ranges are required for LBL positioning?

Theoretically, LBL requires a minimum of three ranges plus knowledge of depth to calculate a unique position. However, in real-world applications, at least four, and ideally five or more ranges, are typically necessary to ensure redundancy, which helps to detect and reject erroneous readings (outliers) and compute a high integrity position.

• Acoustic operation: The system interrogates the transponders acoustically, they respond, and the time of travel is measured. This is then used to calculate the distance and then relative position.
• Speed of sound (SV): The most critical part of this calculation is the speed of sound, or sound velocity (SV). Unlike radio signals, where the speed of light is constant, the speed of sound in water is variable.

Sound speed in water varies primarily due to changes in temperature, salinity and pressure (depth). Warmer temperature and higher salinity increase sound speed, while increasing pressure with depth also raises the speed. These factors vary spatially and with depth, causing sound speed to be variable in water.

If the SV used in the calculation is incorrect, all measurements will be wrong. Sound velocity sensors (like those found in Compatt transponders) are used to measure the SV directly from the environment.

What equipment is involved in an LBL operation?

The hardware:

LBL systems use robust acoustic hardware like our Compatt 6+ transponders, which serve as a seabed reference (once calibrated as an array), and a ROVNav 6+ acoustic transceiver, which is typically installed on the ROV.

While classic (full) LBL operations typically require a dense seabed array, often consisting of six or more transponders, Sparse LBL, often referred to as range-aiding, is a hybrid approach developed to achieve high integrity positioning using a minimal number of transponders.

This reduction in transponders is primarily enabled by inertial aiding, which involves combining acoustic ranging data with an inertial navigation system (INS), such as our SPRINT or SPRINT-Nav. Sparse LBL relies heavily on the INS to provide precision positioning while using minimal acoustic range data.

The software:

Sonardyne’s LBL and Sparse LBL operations are managed using Fusion 2, which provides a single software suite to control LBL, Sparse LBL and SPRINT INS projects, streamlining workflows and maximizing efficiency. Fusion 2 also unlocks the potential of Wideband 3 signal technology, which is optimized for 6G+ instruments.

LBL and its variations are used for numerous applications in the energy, ocean science, and defence sectors:

• Subsea structure installation
• Metrology (highly precise measurement of distances between structures)
• Pipeline positioning and monitoring
• ROV, AUV and towfish navigation
• Deepwater nodal positioning

If you want to know more about Sparse LBL, click here.