PURPOSE: To make possible a correct untrue oscillating source elimination even if an observation machine is moving by calculating a frequency bias except for the position of the observation machine to find two solutions of a true frequency bias and an untrue frequency bias.

CONSTITUTION: An electric wave received by a receiver 11 is frequency-converted by a frequency changer 13 by the use of a local oscillator 12 to transmit toward a ground station 16 by a transmission antenna 14 through a transmitter 15. This is received by a receiving antenna 17 and a receiver 18 in the ground station 16 to measure a received frequency by a frequency measuring instrument 19. Further, data contained in a received electric wave and collected by an observation sensor 6 are obtained. Finally, the position of an observation machine is presumed by a computer 20 by the use of the position and the speed of an earth observation artificial satellite 9 obtained from a Doppler shift found from the received frequency and a tracking control station and the like of the artificial satellite. A frequency bias except for the position of the observation machine 5 is calculated to find two solutions of a true frequency bias and an untrue frequency bias and the position corresponding to the true frequency bias value is made the true one.