Long BaseLine (LBL) refers to a positioning technique that uses an array (group) of transponders that are in a known formation on the seabed. The term long baseline refers to the fact that the distance between these transponders is typically hundreds of meters, much longer than other subsea positioning methods. The position of a target within the array can then be determined by measuring the distance between itself and each transponder in the array.
An LBL system has two main elements. The first element comprises a number of acoustic transponders moored in fixed locations on the seabed. The positions of the transponders are described in a co-ordinate frame fixed to the seabed. This co-ordinate frame can be absolute (real world co-ordinates) or relative. The distances between them form the ‘baselines’ used by the system.
To understand why LBL is used, it is essential to appreciate the benefits. The advantages of LBL over other subsea positioning systems are the achievable accuracies, and the fact that a LBL solution is independent of water depth. The system allows for substantial QC, which might be critical for certain types of offshore operations. A disadvantage of the system however, is its time consuming installation and calibration during offshore operations, which has a commercial impact when deciding on suitable positioning systems.