MIMO SAR Digital Beamforming For Waveform Separation In Elevation

Synthetic Aperture Radar enables high-resolution observations throughout the day and around the clock,and is popular in civil and military applications.However,the traditional synthetic aperture radar is subject to the constraint of the minimum antenna area,and it is hard to simultaneously realize high-resolution wide-swath observation.The emergence of Multiple-Input Multiple-Output Synthetic Aperture Radar(MIMO SAR)solves the above contradiction and it can obtain more information while achieving multi-mode and multi-task coordination.This paper stuties MIMO SAR waveform separation with Short-Term Shift-Orthogonal(STSO)waveform and Orthogonal Frequency-Division Modulation Chirp(OFDM Chirp)waveform in combination with digital beamforming(DBF)in elevation.The second chapter introduces the basic principles of MIMO SAR and the key technologies for implementing MIMO SAR echo separation,including multidimensional waveform encoding(MDWE)and digital beamforming techniques.The establishment of MIMO SAR echo model based on planar digital array antenna and digital array-fed reflector antennas explain the origin of range ambiguities.The third chapter studies the MIMO SAR waveform separation method in elevation based on STSO waveform,and the echo separation model of linear-shift and cyclic-shift are derived.A range Doppler domain DBF waveform separation method is proposed for the application of the reflector antenna in MIMO SAR.This echo separation method effectively solves the problem that the reflector antenna pattern is hard to be decoupled by two-dimensional,which leads to the mismatch of the steering vector.For the characteristics that the STSO waveform is sensitive to the terrain,the steering vector error correction is realized combined with the robust DBF and the direction of arrival estimation,which well suppresses the main beam gain loss under elevation mismatch.In the fourth chapter,based on OFDM Chirp waveform,the MIMO SAR waveform separation method is studied.The generation and separation process of MIMO SAR OFDM Chirp waveform is deduced,and is simulated in application with planar digital array antenna together with array-fed reflector antenna.The impact of non-ideal spatial filter is analyzed,pointing out that the most serious interference to imaging quality comes from the second zero-frequency point of the transmitted waveform.The side lobe of the spatial filter is reduced by adaptive antenna static pattern control,and the effect of interference on imaging quality is weakened.