| system for tracking the position of underwater objects and vehicles by use of acoustic pulses transmitted between transponders|
Note 1 to entry: Underwater acoustic positioning systems operate by means of acoustic distance measurements (using time of flight) combined with positional triangulation from multiple transponders, and/or directional discrimination (using phase differences across a receive array).
Note 2 to entry: Long-baseline (LBL) systems use a sea-floor baseline network of multiple transponders, using positional triangulation to fix the object position, and operate without an acoustic path to the sea surface.
Note 3 to entry: Short-baseline (SBL) systems use a baseline consisting of three or more individual transducers connected by wire to a central control unit. SBL systems are often mounted on a surface vessel.
Note 4 to entry: Ultra-short-baseline (USBL) systems rely on a small transducer array that is typically mounted on a rigid pole, which is installed through the bottom of a surface vessel. Unlike LBL and SBL systems (which determine position by measuring multiple distances and triangulation), the USBL system determines the transponder distance from the transducer array by using both time of flight and direction of arrival.
Note 5 to entry: Underwater acoustic positioning systems are commonly used in a wide variety of offshore engineering work, including oil and gas exploration, ocean sciences, salvage operations, marine archaeology, law enforcement and military activities.