Impact of Ocean Sound Speed Horizontal Gradient on Global Navigation Satellite System–Acoustic Precise Seafloor Positioning

Global Navigation Satellite System–Acoustic ranging (GNSS-A) technology can achieve centimeter-level seafloor positioning. However, the horizontal gradient of ocean sound speed limits the seafloor positioning accuracy of GNSS-A. This paper evaluates the impact of ocean sound speed horizontal gradients on GNSS-A seafloor positioning utilizing Bayesian estimation. Publicly available GNSS-A datasets from 2012 to 2021 were processed using strategies with and without estimating sound speed horizontal gradients. The comparison of results demonstrates that the ocean sound speed horizontal gradient has a significant impact on horizontal positioning but a smaller impact on vertical positioning. The mean root mean square (RMS) of horizontal positioning differences for both strategies is 0.12 m, with a maximum of 0.19 m. The mean RMS of vertical positioning differences is 0.014 m, with a maximum of 0.021 m. The mean RMS of station velocity differences is 0.004 m/a and 0.008 m/a in the east and the north components, respectively, with a maximum RMS of 0.01 m/a in the horizontal component. The vertical station velocity differences for both strategies are relatively small, with a mean RMS of 0.002 m/a and a maximum RMS of 0.003 m/a. The mean RMS difference in sound speed correction for both strategies is 0.01 m/s. The sound speed horizontal gradient is larger in the shallow portion than in the deep portion. In the shallow portion, the mean RMS is 0.052 m/s/km and 0.072 m/s/km in the east and north component, respectively. In the deep portion, the mean RMS is 0.023 m/s/km and 0.024 m/s/km in the east and north components, respectively. The sound speed horizontal gradient varies significantly at different locations due to the marine environment discrepancies, which require refined GNSS-A processing to improve seafloor positioning accuracy.