Example of an Long Base Line (LBL) Acoustic Positioning System; Photo Courtesy of Sonarydyne

Long Baseline Acoustic Positioning System

The Long Base Line (LBL) acoustic method provides accurate positioning over a wide area by measuring ranges to three or more transponders deployed at known locations on the seabed or on a subsea structure.

LBL is designed to position multiple subsea targets and structures with the highest attainable levels of accuracy independent of water depth. The technique offers the highest degree of positioning repeatability available and with range redundancy, an estimation of the position quality can also be made.

The system can be configured to support simple tracking tasks through to the most complex deepwater construction project. Typical operations might involve positioning multiple subsea vehicles working in close proximity to each other as well as streaming sensor data from gyros, digiquartz depth sensors, inclinometers and the like.

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. The distances between them form the 'baselines' used by the system.

The second element comprises an acoustic transceiver which is normally installed on the vessel or on a towfish. The distance from the transducer to a transponder can be measured by commanding the transceiver to transmit a short acoustic signal which the transponder detects and causes it to transmit an acoustic signal in response. The time from the transmission of the first signal to the reception of the second is measured. As sound travels through the water at a known speed, the distance (range) between the transducer and the transponder can be estimated. The process is repeated for the remaining transponders and the position of the vessel relative to the array of transponders is then calculated or estimated.

Contact our Marine Construction Division managers.

V. P. of Marine and Construction Surveys - Ralph Coleman
Marine Operations Manager - Eric Granger
Survey Advisor - George Buhler