Various error factors in GNSS-A seafloor geodetic observation

Japan Coast Guard (JCG) has been developing seafloor geodetic observation with positioning accuracy at the centimeter level using the Global Navigation Satellite System – Acoustic ranging combination technique (GNSS-A). JCG has deployed a seafloor geodetic observation network called SGO-A along the Japan Trench and the Nankai Trough, where megathrust earthquakes have repeatedly occurred. Up to the present, observations at the SGO-A sites showed the crustal deformation caused by the 2011 Tohoku earthquake (Sato et al., 2011), the subsequent viscoelastic relaxation (Watanabe et al., 2021), the interplate coupling of the Nankai Trough (Yokota et al., 2016) and shallow slow slip events along the Nankai Trough (Yokota & Ishikawa, 2020). Currently, JCG publishes the position time series of the SGO-A sites analyzed by the open-source software GARPOS (Watanabe et al., 2020), based on the empirical Bayesian method.

Recent efforts to improve of the accuracy of GNSS-A seafloor geodetic observation have revealed that there is a wide variety of GNSS-A observation error factors. So far, some error factors have been pointed out, such as the disturbances in the underwater sound speed structure (Yokota et al., 2019) and the influence of the observation plan (Nakamura et al., 2021). In addition, data reanalysis and water tank experiments have clarified that waveform reading errors in acoustic ranging, device characteristics and angle dependence of the acoustic sonar have a great influence on seafloor station positioning analysis. In this presentation, we discuss the error factors in such GNSS-A observations while comparing with the error factors in GNSS observations. For some of the error factors, we introduce our efforts to improve the observation accuracy.