What is a USBL system?
Definition of a USBL system
USBL (ultra-short baseline, also sometimes known as SSBL for “Super Short Base Line”) is a method of underwater acoustic positioning. It is used to track subsea targets such as ROVs/AUVs or divers. It can also be used as a mean of position referencing for surface vessels Dynamic Positioning (DP).
USBL positioning is used from shallow to deep waters (down to 10,000m and more) and its accuracy is proportional to the distance, typical from 1-2% of the slant distance for basic equipment and up to 0.06% for the ultimate USBL systems.
USBL positioning is suitable for a wide range of applications, including subsea asset tracking, subsea structure placement, LBL array calibration, UXO survey, IMR (Inspection, Maintenance and Repair)…For metrology and sub-decimetric deep water applications LBL solutions will be more suitable.
How does a USBL system work?
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 then detected by the transceiver.
The time between the transmission of the initial acoustic pulse and the reply is then measured by the USBL system and is converted into a range.
A USBL head contains an array of transducers separated by a baseline from a centimetre to decimetres. When the subsea transponder reply pulse is detected by the different transducers, a phase-differencing method within the transducer array is applied to estimate the direction to the subsea transponder, also called “bearing”.
Knowing the range and the bearing, the relative position of the subsea transponder is then calculated.
It is worth noting that positioning precision is mainly sensitive to bearing, especially for long range applications. The bearing precision mostly depends on the array geometry. A larger antenna will provide a higher precision and transducer vertical diversity will provide a wider acoustic coverage (horizontal tracking). Bearing precision also depends on signal frequency and the higher the signal frequency is, the more accurate the system will be.
Finally to calculate the subsea position in an absolute reference (WGS84, UTM…), the USBL system must be coupled to a GNSS system, and a Gyro compass that will compensate the orientation of the ship at the detection instant reply. A preliminary calibration is mandatory to accurately estimate the installation misalignment.
Pre-calibrated USBL systems integrating an acoustic transceiver and gyrocompass in the same housing also exist, such as iXblue Gaps Series. This allows more rapid deployment and offers optimal performance without all the constraints linked to calibration.