Temporal Sound Speed Error Correction Model for Underwater Acoustic Positioning Based on In Situ Measurements

The error source related to sound speed variations in the ocean is the critical factor to limit the underwater ultra-short baseline (USBL) acoustic positioning system. Conventional positioning methods generally employ the ray-tracing approach based on a fixed sound speed profile (SSP), which is actually advantageous for considering vertical variations of sound speed. However, SSP is actually time-varying, and ignoring this temporal variation will lead to a worse estimation of vehicles' coordinates. To address this issue, we proposed a correction model supported by the in situ sound speed measurements obtained by conductivity-temperature-depth (CTD) sensor installed on the vehicle. In this paper, we first evaluate the impact of the temporal variation of sound speed profile on the acoustic traveling time based on the high-resolution sound speed series data obtained from a moving vessel profiler (MVP). Subsequently, the correlations between effective sound speed (ESS) and in situ sound speed measurements are analyzed. Next, the sound speed correction model based on the correlation relationship is established. Finally, the proposed method is verified by semiphysical simulation and sea trial data sets in the South China Sea. The results demonstrate that the two-dimensional (2D) RMS values of positioning errors with the proposed method are improved by 55% and 32% when compared with the fixed sound speed value (SSV) method and fixed SSP method, respectively, while the proposed method improves positioning accuracy by 20% compared to the fixed SSP method in the vertical direction.