The feasibility of a very-long baseline acoustic positioning system for AUVs

Positioning an AUV over a wide operational zone in deep water may be addressed through a very long baseline (VLB) acoustic positioning system. Ranges of 50 to 100 km can be obtained using modulated signals around 2-3 kHz, providing a sufficient signal-to-noise ratio and a good accuracy in time measurements for most environmental conditions. Nevertheless two major problems have to be solved if a good positioning accuracy (within 50 m) has to be achieved in real-time. The first issue arises from using the measured transmission delays for the computation of the AUV position. Because of the presence of multipaths, and of the nonlinear relation between time and range due to a non-constant sound-speed profile, classical long-baseline positioning algorithms based on a constant sound speed value are not usable within the expected accuracy. A global method using the whole multipaths arrivals structure, appears then to be theoretically effective; its interest is demonstrated on numerical simulations. Unfortunately it requires a good preliminary knowledge of the explored zone topography and hydrology, and moreover it does not seem to be practically usable by an AUV, due to the large required processing load. The second problem is the positioning precision of the acoustic sources; estimates of mooring motion are made, and the influence on the overall system performance is discussed. A solution is proposed, using GPS tracking of the emitters. Although being technologically satisfactory, the resultant system is not really compatible with the operational and economical requirements for the AUV´s missions