USBL (ultra-short baseline, also known as SSBL for super short base line) is a method of underwater acoustic positioning. A USBL system consists of a

transceiver In radio communication, a transceiver is an electronic device which is a combination of a radio ''trans''mitter and a re''ceiver'', hence the name. It can both transmit and receive radio waves using an antenna, for communication purposes. Thes ...

, which is mounted on a pole under a ship, and a transponder or responder on the seafloor, on a

towfish A towed array sonar is a system of hydrophones towed behind a submarine or a surface ship on a cable. Trailing the hydrophones behind the vessel, on a cable that can be kilometers long, keeps the array's sensors away from the ship's own noise sou ...

, or on an ROV. A computer, or "topside unit", is used to calculate a position from the ranges and bearings measured by the transceiver.

Mechanism

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 Subsea technology involves fully submerged ocean equipment, operations, or applications, especially when some distance offshore, in deep ocean waters, or on the seabed. The term ''subsea'' is frequently used in connection with oceanography, marin ...

beacon. Angles are measured by the transceiver, which contains an array of

transducers A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and control ...

. 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 direction to the subsea transponder. The presence of environmental noise reduces USBL positioning accuracy. Combining

Kalman filter For statistics and control theory, Kalman filtering, also known as linear quadratic estimation (LQE), is an algorithm that uses a series of measurements observed over time, including statistical noise and other inaccuracies, and produces estima ...

ing with an element array has been used to filter the signals and improve accuracy, using the minimum mean-square error rule.

Applications

USBLs are used in "inverted" (iUSBL) configurations, with the transceiver mounted on an

autonomous underwater vehicle An autonomous underwater vehicle (AUV) is a robot that travels underwater without requiring input from an operator. AUVs constitute part of a larger group of undersea systems known as unmanned underwater vehicles, a classification that includ ...

, and the transponder on the ship/shore that launches it. 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 Target may refer to: Physical items * Shooting target, used in marksmanship training and various shooting sports ** Bullseye (target), the goal one for which one aims in many of these sports ** Aiming point, in field artillery, fi ...

References

External links

Product Survey USBL Systems
Hydro International, May 2008 * * {{hydroacoustics Navigation Surveying