PURPOSE: To attain the reduction in the power consumption and wt. of space searching equipment and to achieve the enhancement in the reliability thereof while facilitating the orbit determination and working of an artificial satellite, by detecting the Doppler displacement frequency of the frequency spectrum of an electromagnetic wave from a celestial body and the azimuth angle of the celestial body.

CONSTITUTION: The frequency spectrum of the electromagnetic wave emitted from a celestial body 6 is detected by the spectrometer 7 mounted to an artificial satellite 1 while this detected output and the reference spectrum signal from a reference spectrum generator 8 are subjected to differential detection by a Doppler detector 9 to detect Doppler displacement frequency. The generator 8 generates the dummy signal of the frequency spectrum of an electromagnetic wave, which is observed when the relative velocity of the artificial satellite 1 and the celestial body 6 is zero, as a reference signal. On the other hand, the azimuth of the celestial body 6 is detected by an azimuth angle detector 10 and the detected azimuth angle is imparted to an operator 11. The detected Doppler displacement frequency and the azimuth angle of the celestial body are subjected to integration processing by the operator to calculate the velocity vector of the artificial satellite 1.