Contribution of Direction-of-Arrival Observations for Geodetic Seafloor Positioning Using an Unmanned Surface Vehicle

Precise underwater geodetic positioning remains a challenging operation. Measurements combining surface positioning (GNSS) with underwater acoustic positioning are usually performed from research vessels. We present an alternative approach using a small Unmanned Surface Vehicle (USV) equipped with a compact GNSS/Acoustic experimental configuration, which is more cost-effective and easier to deploy. The positioning system included a GNSS receiver mounted above an Ultra Short Baseline (USBL) module integrated with an inertial system (INS) to correct the USV movements. The experiment conducted in the shallow waters (40 m) of the Bay of Brest, France, provided a data set to derive the coordinates of individual transponders from two-way-travel times and direction of arrival (DOA) of acoustic rays from the transponders to the USV. We tested different acquisition protocols (box-in circles around transponders and two static positions of the USV). Using a least-squares inversion, we show that DOAs improve single transponder positioning both in box-in and static acquisitions. From a series of short positioning sessions (20 min) over two days, we achieved repeatability of ~5 cm in the locations of the transponders. Post-processing of the GNSS data also significantly improved the two-way-travel times' residuals compared to the real-time solution.