USBL systems also knows as Super Short Baseline systems SSBL, refers to the very short distance between the transducer elements built into one transceiver assembly. The transceiver assembly also known as the hydrophone mounted under e.g. a vessel makes it possible to track and position submerged targets like towed objects, ROV's etc from a dynamic platform. Where with LBL systems you are limited to a calibrated beacon array

deployed on the seabed, the use of a USBL system is not limited to such an area.

Position calculations are made by measure vertical and horizontal angles by signal phase measurements in conjunction with measured ranges to the transponder(s) providing a 3D position of the target(s) relative to the hydrophone. Since the Hydrophone (and herewith the transceiver assembly) may be mounted to a dynamic platform, the measurements to transponder beacons attached to the target(s) need to be externally or internally corrected for heading, roll and pitch very accurately. Except for the ORE Trackpoint 3 series all systems specify 0.1 deg or better. Sonardyne systems specify even 0.01 deg

for pitch and roll. The IXSEA GAPS contains the USBL but also integrated INS as well as GPS providing an all in one calibration free system. Since the introduction of USBL systems some decades ago were the maximum range was limited to a few hundred metres, nowadays range capabilities of 1000m (LinkQuest 1500) or even up to 11000m ( LinkQuest 10000) are possible. Longer range capabilities require online compensation of acoustic ray bending effects and very accurate angle measurements. Hence we have asked the manufacturers to particularly provide detailed information on the angular

accuracies of their systems. A drawback of some USBL systems is the inability to obtain redundant observations other than by obtaining multiple readings.

Hydro international is much indebted to the manufacturers who have contributed to this product survey.