New insights into robust MIMO-SAR echo separation for irregular topographies

Multi-Input Multi-Output (MIMO) Synthetic Aperture Radar (SAR) offers the potential of preferable imaging performance in comparison with other forms of radar systems. However, to do so, one has to extract the signals corresponding to each transmit channel without suffering interferences from the other transmit channels, which constitutes a considerable challenge. One attractive alternative here is the use of orthogonal-waveform beamforming schemes which are becoming increasingly popular for addressing the echo separation issue involved in MIMO-SAR systems. When using such schemes, the Digital Beamforming (DBF) on reception in elevation should be implemented for wide-swath imaging. Generally, most current echo separation methods perform the DBF processing before azimuth focusing, causing the DBF performance to be very sensitive to Direction of Arrival (DOA) mismatch caused by topography variations. To alleviate such issues, we here propose an image post-processing echo separation strategy wherein the DBF is implemented after the Two-Dimensional (2-D) focusing. As a result, azimuth pulse extension effects are avoided and thereby reducing the sensitivity to the DOA errors caused by irregular topography variations. In the proposed technique, the well-known MUSIC estimator is exploited to acquire an estimate of the DOAs of the signal segments, with the number of the signal sources being determined using the characteristics of the eigenvalues. Using these estimates, refined signal steering vectors are used to form a Least-Square (LS) beamformer that has a distortionless signal response and deep nulls for the unwanted interference. Numerical simulations illustrate the robustness of the proposed technique in the presence of topographical variations, exemplifying the feasibility and potential in practical applications.