At-sea calibration of a USBL underwater vehicle positioning system

In this paper the at-sea calibration procedure for IFREMER's ultrashort baseline acoustic positioning system (USBL) POSIDONIA for deep-sea underwater vehicles (UV) is presented. The USBL system comprises an acoustic vessel-born localization system and sensors measuring vessel position, heading and attitude. The latter are used to transform the 'acoustic' UV position into Earth related coordinates. Misalignments between the different sensors lead to important positioning errors. Quick calibration of the USBL system at sea without additional hardware is therefore an important operational feature. The procedure presented here uses position measurements obtained with the uncalibrated system for a fixed-point moored transponder beacon. The absolute beacon coordinates are not known initially; they are identified, together with angular misalignments of the USBL sensors, by an iterative adaptive estimator. The vessel track during the recording of measurements is arbitrary and calibration is accomplished in less than one hour. Examples are shown to illustrate the USBL performances and the amelioration achieved with the calibrated system.