Range ambiguity suppression by azimuth phase coding in multichannel SAR systems

Among the techniques for range ambiguity suppression, the azimuth phase coding (APC) stands out for its effectiveness for point and distributed targets with low implementation complexity. After modulation to the transmitted signal and the corresponding demodulation to the received signal, the residual modulation phase of the ambiguity signal shifts its Doppler spectrum, so that the spectrum of the useful signal and that of the ambiguity are not exactly superimposed. This enables an azimuth filter to eliminate a part of the ambiguity power. If it is desired to get an ambiguity-free SAR image, the pulse reputation frequency (PRF) should be several times larger than the Doppler bandwidth of the desired Doppler spectrum, which is hard to be satisfied in high-resolution wide-swath spaceborne SAR systems. This paper introduces a method for range ambiguity suppression based on the APC technology in multichannel SAR systems when the PRF is only a bit larger than the Doppler bandwidth. After applying the APC technique, the space-time spectrum of the ambiguity swath is separated from that of the useful signal. In multichannel SAR systems, the desired 2-D spectrum and the spectrum of the ambiguity swath can be obtained simultaneously by using the position information of antenna phase centres. Conventional imaging algorithms can then be applied to both spectrums to obtain SAR images of the imaging area and the ambiguity swaths. Simulation results confirm the effectiveness of this method.