Wave Glider-Based GNSS-Acoustic Surveying of A Single Transponder in Shallow Water

GNSS-Acoustic ranging is an important technique to measure seafloor motion. To reduce seafloor positioning error related to imperfect knowledge of the sound speed in ocean, a commonly practiced method is using multiple transponders to form an array and differentiating the observations. This method, though proved effective, requires at least three transponders to form one array, limiting the array's reliability and number of observation sites. The a priori coordinates used to estimate array center's displacement are usually determined through horizontally symmetric surveys of the individual transponders and are required to be accurate at decimeter level. In shallow water, due to the shorter ranges in GNSS-A, seafloor positioning error propagated from sound speed error should be lower compared to deep water. Using wave gliders as sea surface platforms, we tested GNSS-Acoustic surveying of a single transponder in shallow water (~55 m depth), aiming at obtaining centimeter level seafloor positioning repeatability with carefully designed survey trajectory. Our analysis shows that the repeatability for horizontal components is a few centimeters, whereas the vertical component cannot be well constrained without accurate sound speed information.