Joint positioning and navigation aiding system for underwater robots

This paper proposes a new joint positioning and navigation aiding system for underwater robots. In the scenario adopted, a submersed target carries a pinger that emits acoustic signals periodically, as determined by a low precision clock. The target is tracked from the surface by a set of buoys equipped with acoustic hydrophones/projectors, GPS receivers, and electronic circuitry that measures the times of arrival of the acoustic signals emitted by the pinger. Once an estimate for the underwater target position is computed, the buoys transmit this information back to the target indirectly, without resorting to an acoustic modem. This is done by encoding the information in the relative times of emission of acoustic pulses, as if they were emitted by a set of buoys remaining in fixed positions agreed upon in advance with the target. The times of arrival of these pulses are used by the target as aiding data to be fused with other motion data available onboard, in a local navigation system. The scheme proposed allows for the compensation of the: (i) displacement experienced by the surface buoys, which are not required to be moored, (ii) clock bias due to the low accuracy clock onboard the target, and (iii) motion of the target during the time interval it takes to complete a tracking-positioning cycle, as predicted by a dynamic target model implemented in the local positioning system. Simulation results are presented to assess the performance of both the positioning tracker and an onboard simplified integrated navigation system.