Ultra Short BaseLine (USBL), sometimes called Super Short Baseline (SSBL), is an underwater positioning system that uses a vessel mounted transceiver to detect the range and bearing to a target using acoustic signals. This range and bearing technique is based on two principles.
The first is that an accurate range can be determined by knowing precisely the time taken for an acoustic signal to travel between the target and the transceiver and the speed at which the signal travelled (sound speed).
The second is that the bearing can be determined by knowing the discreet difference in phase between the reception of the signal at the multiple transducers present in the transceiver. This allows the USBL system to determine a time-phase difference for each transducer and therefore calculate the angle of the arriving signal.
In addition to an acoustic transceiver and in-water transponders, a USBL system is also comprised of several other components, which includes attitude sensors for the accurate determination of vessel pitch, roll and heading (e.g. VRU and Gyrocompass), and for calibration purposes, accurate surface positioning systems, which would typically be a DGPS. The USBL system also features the vessel mounted hardware and software, from which the USBL is controlled, along with the associated cabling and all important deployment pole upon which the transceiver is mounted.
USBL can be used in water depths from less than 10 metres up to several thousands of metres, without the requirement to alter any of the components. As most USBL systems operate in and around the MF frequency spectrum (~18-36KHz), they provide the potential to achieve good ranges to targets, whilst also maintaining a reasonable degree of range resolution, which can be improved when utilising Sonardyne Wideband technology. Once commissioned and calibrated, a USBL system can be simple to operate, requiring little or no operator training.