To improve the acoustic positioning accuracy by reducing the number of acoustic positionings required for calibration of inertial navigation position and energy consumption of an underwater vessel.

The underwater vessel 6 is stopped without relative position variation with respect to the seabed 21 by fixed point holding or arriving at the bottom. In this state, the acoustic positioning from a support ship 7 side is performed a plurality of times, the average of the latitude direction of measurement results and the average of the longitude direction of the measurement results are determined, and an acoustic positioning position having statistically correct latitude and longitude of the underwater vessel 6. Positioning based on inertial navigation by the underwater vessel 6 itself is performed, the latitude and longitude are compared with the latitude and longitude of the acoustic positioning position, the deviation between the latitude direction and the longitude direction is determined, and the data of latitude and longitude based on the inertial navigation by the underwater vehicle 6 self is calibrated by the determined deviation between the latitude direction and the longitude direction. The acoustic positioning accuracy is improved by holding the support ship 7 near the vertical line of the underwater vessel 6 stopped near the seabed during the acoustic positioning.

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