On field experience on underwater acoustic localization through USBL modems

Ultra-Short BaseLine (USBL) positioning systems born as a technology for underwater target tracking from a vessel. Despite they are commonly and efficiently used for this purpose, the recent literature about navigation of Autonomous Underwater Vehicles (AUVs) demonstrates that they can represent a valuable support for these systems. Measurements of the AUV position can be exploited as observations within the onboard navigation filter to limit the drift of the state estimation error, unavoidable drawback in case of dead reckoning based approaches. This way, the AUVs can indeed be involved in longterm missions without the necessity of periodic surfacing to get GPS signal for estimation error reset. The paper presents some deductions derived by a considerable set of acoustic position measurements obtained through an EvoLogics S2CR 18/34 USBL modem used in two different configurations (fixed to a tripod on the bottom and floating on the surface). Different results were obtained in the two cases. The proposed analysis of the results will be a solid base for the optimization of the configuration of an USBL-aided navigation system for AUVs that the authors plan to implement and test in the coming future. Part of the experimental data are obtained exploiting the Littoral Ocean Observatory Network (LOON). The LOON consists of four EvoLogics S2CR 18/34 modems deployed by the NATO Science and Technology Organization Centre for Maritime Research and Experimentation (STO-CMRE) in the sea in front of La Spezia, Italy.