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

Abstract

Precise underwater geodetic positioning remains a challenge. Measurements combining surface positioning (GNSS) with underwater acoustic positioning are generally performed from research vessels. Here we tested an alternative approach using a small Unmanned Surface Vehicle (USV) with a compact GNSS/Acoustic experimental set-up, easier to deploy, and more cost-effective. The positioning system included a GNSS receiver directly mounted above an Ultra Short Baseline (USBL) module integrated with an inertial system (INS) to correct for the USV motions. Different acquisition protocols, including box-in circles around transponders and two static positions of the USV, were tested. 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. 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 2 days, we achieved a 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.

Shallow Water Seafloor Geodesy With Wave Glider‐Based GNSS‐Acoustic Surveying of a Single Transponder

“…If acoustic rays curve at low degrees and the sea surface vessel's instantaneous positions can be measured accurately, a simple triangulation using multiple slant ranges can be used to locate the seafloor transponder (e.g., Chen et al., 2020; Gagnon, 2007; Yamada et al., 2002; Sakic et al., 2021). The observation equation becomes: $$$c\cdot t=\sqrt{{\left({N}_{T}-N\right)}^{2}+{\left({E}_{T}-E\right)}^{2}+{\left({U}_{T}-U\right)}^{2}}+\sqrt{{\left({N}_{R}-N\right)}^{2}+{\left({E}_{R}-E\right)}^{2}+{\left({U}_{R}-U\right)}^{2}}$$$ where ( N , E , U ) are the transponder's topocentric coordinates to solve for, ( N T , E T , U T ) and ( N R , E R , U R ) are the transducer's topocentric coordinates at the times of transmitting and receiving of the interrogations, c is the harmonic mean sound speed, and t represents the two‐way travel time (TWTT) between the transducer and transponder.…”

“…If acoustic rays curve at low degrees and the sea surface vessel's instantaneous positions can be measured accurately, a simple triangulation using multiple slant ranges can be used to locate the seafloor transponder (e.g., Chen et al, 2020;Gagnon, 2007;Yamada et al, 2002;Sakic et al, 2021). The observation equation becomes:…”

“…Therefore, previous GNSS‐A projects mainly targeted places with the highest research priorities, where the water depth is usually large (>500 m), for example, near the subduction zone trenches (Bürgmann & Chadwell, 2014; Evans et al., 2022; Gagnon et al., 2005; Tomita et al., 2017). With recent developments of using unmanned autonomous surface vessels (e.g., Wave Gliders) for GNSS‐A surveys, the cost of operating a sea surface vessel has been greatly reduced (e.g., linuma et al., 2021; Sakic et al., 2021; Evans et al., 2022). Compared with a conventional research vessel, a Wave Glider has a smaller and simpler hull, hence the GNSS and acoustic units are better coupled in the body frame, and the coordinate transformation error is reduced.…”

Experimental verification of seafloor crustal deformation observations by UAV-based GNSS-A

“…The current GNSS-A technique thus has limitations in terms of observation frequency and speed of observation (Table 1). Experiments have been conducted in which GNSS-A equipment was mounted on mooring and self-propelled buoys and also a wave glider [17][18][19][20][21] . Buoy-based GNSS-A, described in Table 1, has poor sea surface control…”

“…Experiments have been conducted in which GNSS-A equipment was mounted on mooring and self-propelled buoys and also a wave glider [17][18][19][20][21] . Buoy-based GNSS-A, described in Table 1, has poor sea surface control…”