PURPOSE: To detect bias errors of water speed sensors by rotating the water speed sensors, estimating the position of a ship from their outputs, and comparing the estimated position with the position from a Loran-C receiver or NNSS position sensor receiver.

CONSTITUTION: The water speed sensors 11 and 18 (in X and Y directions respectively are fitted to the ship rotatably around axes perpendicular to their detection speed directions and those sensors 11 and 18 are rotated at optional constant rates in the progress of the ship during calibration to decompose their output signals into the north and east directions by the output of a gyrocompass 13. Those decomposed components are integrated respectively to obtain position data, which are compared with the position data of the Loran-C receiver 16 or that of the NNSS position sensor receiver 15 by a position comparison calculation part 19. The biases of the sensors and a tidal speed (bias) separately by the comparison signal to calibrate the water speed sensors.