Global Navigation Satellite System—Acoustic combination (GNSS-A) observation is a seafloor geodetic observation method that combines GNSS observation and underwater acoustic ranging on an ocean surface platform to determine the position of the seafloor. After being proposed in the United States in the 1980s, many practical studies were carried out by several Japanese and US institutions in the 2000s, and it has now grown into an important technology that is indispensable for marine earthquake disaster prevention in Japan. In recent years, many observations have begun in the Pacific Rim, including Alaska, New Zealand, Mexico, and Chile, and it plays an important role in marine earthquake disaster prevention internationally as well. GNSS-A acoustic ranging requires centimeter-level positioning accuracy at distances of 1000 m or more. The signal frequency is around 10 kHz, and many analytical techniques, analysis techniques, and experimental investigations have been conducted to achieve an accuracy shorter than a wavelength. In this presentation, we explain our technological developments that have contributed to improving the accuracy of GNSS-A observations in the past decade. Work supported by the Earthquake Research Institute, University of Tokyo, ERI JURP 2024-Y-KOBO12, the Secom Science and Technology Foundation, and JSPS KAKENHI Grant-in-Aid for Scientific Research Academic Transformation A “Science of Slow-to-Fast Earthquakes” JP21H05200.
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