GNSS/Acoustic positioning of acoustic beacons on the seafloor using an autonomous surface vehicle. Example from the FOCUS experiment offshore Sicily (Italy)

The FOCUS project funded by the European Research Council aims at monitoring deformation across an active submarine fault with an optical fiber using laser reflectometry. To calibrate the measured strains in an absolute reference frame, such as the International Terrestrial Reference Frame (ITRF), a network of eight seafloor geodetic stations was deployed on both sides of the cable and fault. The fault (North Alfeo) is located at the foot of Mount Etna collapsing slope, offshore Sicily, and shows evidence of right-lateral strike-slip in the order of 2 cm per year.

To locate the acoustic beacons relative to the ITRF, we use a GNSS/Acoustic positioning method. Its principle is to jointly acquire positions of a surface platform relative to the GNSS and, acoustically, relative to the beacons on the seafloor. Positioning a set of beacons over the years should yield their absolute displacement. The optical cable and geodetic stations were deployed in October 2020 at a depth of ~1850m. The first set of GNSS/A data was acquired in August 2021. The next set will be collected in July 2022.

GNSS/A positioning of acoustic beacons on the seafloor within 1 cm is a challenge. The lever arm between the GNSS and acoustic antennas on the surface platform must be precisely known; the motion of the platform (i.e. antennas) must be precisely monitored. Then, in addition to reducing the uncertainties in GNSS positioning, an acquisition strategy must be designed to minimize the uncertainties in the acoustic ranging data, due to the unknown sound-speed field in the water column and its variability during the ranging sessions (5-6 hours).

To address these challenges, we used an Autonomous Surface Vehicle (ASV) equipped with a GNSS antenna, an ultra-short acoustic baseline (USBL) transponder coupled with an inertial system (INS). The ASV (3m x 1.60m) has the advantage of being very maneuvrable, acoustically silent (electric power), and compact (reduced lever-arm between antennas). Instead of positioning a single beacon (e.g. boxin), we positioned the ASV relative to several beacons at once and tested different trajectories: quasi-static stations of the ASV (within few meters) at the barycenter of 3 beacons, or series of straight profiles equidistant to pairs of beacons. In addition, while the ASV was acquiring GNSS/A data, a series of vertical temperature/pressure/salinity (CTD) profiles was acquired from the support vessel (R/V Tethys II) to monitor changes in the sound-speed.

Here we discuss the first results in processing these data and the ensuing uncertainty on the positioning. The GNSS data are reprocessed using Precise Point Positioning (PPP) with Ambiguity Resolution (AR). The improved navigation is then reprocessed with the INS data to obtain a precise position of the USBL center of mass. Then the acoustic ranging data can be merged with the sound-speed information to locate the beacon barycenter, using a least-squares inversion.