PURPOSE: To obtain a high quality picture by inputting a calibration signal to change the input level of a receiving part to the receiving part, measuring the saturating level and the linearity of the receiving part, and using the calibration data at processing the picture on the ground.

CONSTITUTION: A duplexer 12 sends a transmitting signal, which is the output signal of a electric power amplifier 14, to an antenna part 11, radiates onto the ground, receives a reflecting signal from the ground and sends it to the duplexer 12. The duplexer 12 sends the receiving signal from the antenna part 11 and the calibration signal which is the output signal of a digital attenuator 18, to receiving part 13. The receiving signal is the observing data of the synthetic aperture radar obtained actually, and the calibration signal is the data including the characteristics of the radar device, for example, the information of the linearity, saturated level, etc. of the receiving part 13. By using the calibration signal for the picture processing on the ground, the high quality picture can be obtained.