USBL (ultra-short baseline, also sometimes known as SSBL for super short base line is a method of underwater acoustic positioning. A complete USBL system consists of a transceiver, which is mounted on a pole under a ship, and a transponder or responder on the seafloor, a towfish, or on an ROV. A computer, or "topside unit", is used to calculate a position from the ranges and bearings measured by the transceiver.
An acoustic pulse is transmitted by the transceiver and detected by the subsea transponder, which replies with its own acoustic pulse. This return pulse is detected by the shipboard transceiver. The time from the transmission of the initial acoustic pulse until the reply is detected is measured by the USBL system and is converted into a range.
To calculate a subsea position, the USBL calculates both a range and an angle from the transceiver to the subsea beacon. Angles are measured by the transceiver, which contains an array of transducers. The transceiver head normally contains three or more transducers separated by a baseline of 10 cm or less. A method called “phase-differencing” within this transducer array is used to calculate the angle to the subsea transponder. For accurate results, the speed of sound profile in the water column must be determined and taken into account.  Ordinary differential equations can then model the followed acoustic rays resulting in an improved phase-differenced signal. This is typically used in elliptical multibeam underwater positioning. Solutions without a measure of the speed of sound of the water column also exist; quadratic functions are hereby used to model range distances and phase incoherences. These solutions, however, tend to be much more inaccurate and therefore only used in side scan positioning.
USBLs have also begun to find use in "inverted" (iUSBL) configurations, with the transceiver mounted on an autonomous underwater vehicle, and the transponder on the target. In this case, the "topside" processing happens inside the vehicle to allow it to locate the transponder for applications such as automatic docking and target tracking.
- Hubert, Fay (01/12/1989). Dynamic Positioning Systems: Principles, Design and Applications. Editions Technip. pp. 43–56. ISBN 978-2710805809. Check date values in:
- Marco, Morgado (01/07/2011). Advanced Ultra-Short Baseline Inertial Navigation Systems. Instituto Superior Técnico, Universidade Técnica de Lisboa. Check date values in:
- Fanlin, Yang (2011-05-16). "Precise Positioning of Underwater Static Objects without Sound Speed Profile". Marine Geodesy 34:2: 138–151.
- Product Survey USBL Systems Hydro International, May 2008.
- Sonardyne Ranger 2 and Scout USBLs
- IXBLUE - GAPS pre-calibrated Ultra-Short BaseLine -
- IXBLUE - POSIDONIA Ultra-deep & long range Ultra-Short Baseline
- Nautronix USBL
- EASYTRAK USBL Applied Acoustics
- TrackLink USBL - LinkQuest
- Tritech International Ltd
- Kongsberg Acoustic Positioning
- EvoLogics USBL Modems