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Positioning Small AUVs for Deeper Water Surveys Using Inverted USBL

Tom Hiller, Arnar Steingrimsson, Robert Melvin

Small autonomous underwater vehicles (AUVs) can collect high resolution side scan and bathymetric data suitable for engineering inspection and geophysics applications.  Such surveys require very accurate vehicle positioning; this is possible in shallow water (less than 40m) using an integrated inertial navigation system and doppler velocity log (INS/DVL), combined with regular surfacing for GPS position updates.  In missions launched in deeper water there may be an extended period at the start of a dive where the DVL does not have a lock on the seabed, resulting in INS position drift, and in deep missions resurfacing for GPS fixes is not possible.  This paper presents the implementation of inverted ultrashort baseline (iUSBL) techniques to improve small AUV positioning in deep water surveys, using a DAT USBL (Teledyne Benthos) mounted on a Gavia Surveyor AUV (Teledyne Gavia Ehf), a man-portable low-logistics AUV capable of surveying at 1000m depth.  The iUSBL configuration measures the range and bearing from the AUV and allows the use of the highly accurate INS and depth sensors mounted on the stable Gavia AUV platform.  The USBL fixes can be corrected in real time based on information gathered by the AUV sensors, including using the vehicle’s sound velocity sensor information for sound speed and ray tracing corrections.  The positional information obtained by the USBL system can then be fed into the AUV’s INS with the appropriate weighting to maintain navigation accuracy during extended duration surveying in deeper water.