Spaceborne Sar Antenna Pattern In-orbit Testing

There are many error resources for spaceborme SAR system during the earth-observation process and the antenna pattern uncertainty is a main factor of them. Inflight measurement of antenna pattern is a key technology of spaceborne SAR calibration. Above all, some methods of inflight antenna pattern measurements for spaceborne SAR are discussed in this paper. We described the principle of each measurement method briefly and compared the function, the precision, the cost and the applicability with each other. Then we focused on the technique of inflight antenna pattern measurement for spaceborne SAR using ground-based transmitters. In order to get more reliable results, we developed a computer simulation system for the inflight antenna measurement using transmitters. Based on the satellite orbit theory, a Satellite-Earth Geometrical relationship model is made and the arithmetic is derived.The key technique for SAR receiving antenna pattern measurement using transmitters is that the signal recorded by the SAR receiver coming from different transmitters could be separated correctly. We research two decoupling technique: one is separating signal by the difference of frequency, the other is separating data by the orthogonal characteristic of modulation code. By theoretic analysis and computer simulation, we researched some factors, such as the satellite orbit, the satellite attitude, the number of ground-based transmitters, the uncertainty of the transmit power, the noise due to the digital processing of SAR receiver, the amplitude and phase error of the SAR system, the background echo signal, the offset of transmit frequency from the center of the SAR bandwidth, the estimate error of Doppler parameter, and so on, which affect the precision of the antenna pattern inflight measurement. At the end, the performance of each separating algorithms is analysed.