Mitigating the Impact of Ocean Sound Speed Variations on Wave Glider-based GNSS-Acoustic Surveys

• Applicant: Professor Dr. James Foster
• Subject Area: Geodesy, Photogrammetry, Remote Sensing, Geoinformatics, Cartography
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 562074179

The possibility of making GNSS-Acoustic measurements using Wave Glider unmanned surface vehicles is revolutionizing seafloor geodetic research. The slow underway speed of these vehicles, however, means that ocean sound speed variations – already the limiting factor for the accuracy of GNSS-A measurements - will be even more problematic. Taking full advantage of this technological advance for seafloor geodesy requires that the issue of sound speed variations be comprehensively addressed. This project will determine the most effective mitigation techniques to minimize the impacts of processes that perturb ocean sound speed on the accuracy of GNSS-Acoustic positioning solutions. We will focus on the particular case of GNSS-A surveys performed by Wave Glider unmanned surface vehicles where the slow underway speed constrains our ability to capture geometrically well-distributed sets of observations within the typical times periods of some of the processes that most impact the sound speeds. Our approach will include a) using numerical ocean models to provide statistical constraints on sound speed variations; b) determining optimal sea surface survey paths to resolve both transponder positions as well as lateral sound speed gradients; c) testing statistical techniques for extracting information on sound speed variations from our acoustic travel time observations; d) defining the impacts of all identified processes on the formal errors for our seafloor position solutions. Our working hypotheses will be: H1: Long-period processes will best be mitigated through optimized survey paths and improved characterization of the sound speed variations in the inversion solution, constrained by statistical characteristics derived through numerical ocean models. H2: Shorter period effects will best be identified and mitigated through their characteristic perturbations in the travel times of the modeled observations, constrained by the physics of ocean tides and propagating internal waves. The products will be: 1) guidelines for Wave Glider GNSS-A survey protocols that match seafloor depth and ocean conditions, as well as any practical mission constraints (such as available time for the survey, or positioning accuracy needs), and 2) procedures for providing representative error estimates for the position solutions. Also generated will be 3) updated position solutions and error estimates for the GNSS-A datasets examined in our analysis, and 4) an online tool that extracts sound speed statistics from the numerical ocean model to be used for a) planning GNSS-A missions and, b) processing the data collected.

• DFG Programme: Research Grants
• International Connection: USA
• Co-Investigator: Professorin Dr. Laura Wallace
• Cooperation Partner: Professor Dr. Benjamin A. Brooks