Internal tides are crucial in ocean dynamics and are a source of error in marine acoustic measurements. In Global Navigation Satellite System-Acoustic combination technique (GNSS-A) seafloor crustal deformation observations, which contribute to earthquake and plate subduction sciences, the effect of internal tides has never been estimated, even though it has been predicted. In this study, we used data from the GNSS-A observation network around Japan to quantitatively identify the effect of internal tides on GNSS-A observations for the first time and reveal the mechanism of the effects of the horizontal wavelength, mode, and direction of internal tides. Long-term sound speed disturbances during GNSS-A observations were generally explained by a semidiurnal internal tide in modes 1 to 3. In future, these results are expected to contribute to progress in multidisciplinary research, such as the construction of advanced physical models and analytical methods.

Underwater disturbances are the largest error source in GNSS-A seafloor geodetic observation. In particular, the gradient of sound speed structure directly affects the horizontal accuracy and needs to be examined. Previous studies have not investigated its temporal change component. In this paper, we verified the assumption that the underwater gradient structure does not change significantly during GNSS-A observation for several hours through applying a modified version of an analysis software called GARPOS to actual data of SGO-A (provided by Japan Coast Guard). Obtained results suggested that this assumption holds at many observation data, and the positioning accuracy becomes better. Some non-improved observation epochs were speculated to be accompanied by structure changes for which this assumption was not valid. It is suggested that the sound speed structure change during observation will be an important research topic in GNSS-A.