Acoustic tracking system for AUVs exploiting DoA sensors -calibration and performance characterisation

This work describes the calibration procedure of a tracking system for Autonomous Underwater Vehicles (AUVs) based on Direction of Arrival (DoA) acoustic sensors. The final aim is to track the AUV along a significant set of imposed underwater trajectories and to exploit the collected dataset for identification of vehicle dynamic parameters. In the framework of this activity, the DoA sensors are Ultra-Short Baseline (USBL) devices moored in fixed positions on the sea bottom of the operational area. Measurements obtained by these devices are referred to a local frame. In order to express them in a global frame, an accurate estimation of devices pose (position and attitude) is necessary. Direct measurement of the pose is usually not easy as the USBL devices are typically deployed by hand from a Rigid Hull Inflatable Boat (RHIB). Attitude may be estimated by an integrated Attitude and Heading Reference System (AHRS) but may be affected by unpredictable magnetic disturbances. The calibration procedure aims at determining the pose of the involved USBL devices exploiting acoustic measurements collected when the AUV is on the surface and the GPS signal is available. It has a twofold goal: on one hand, the global-frame measurement errors due to devices pose error estimation are minimised; on the other hand statistics on the expected tracking error, for the particular configuration, are derived to characterise the future identification process.