Abstract-This paper proposes an imaging method of multi-direction swath and digital beamforming (DBF) in elevation for spaceborne Hybrid Phased-MIMO SAR that combines traditional phased-array radar with a new technique for multiple-input multiple-output (MIMO) radar to achieve multifunctional synthetic aperture radar (SAR). At first, we build a signal model and derive a virtual control matrix of the Hybrid Phased-MIMO SAR. Furthermore, considering the image overlap and range ambiguity caused by multiple direction imaging, we present adaptive Digital Beamforming based on Linearly Constrained Minimum Variance (LCMV). In this approach, the first constraint is dedicated to make the overall beamformer response equal the quiescent response in the desired signal region so that the signal is not cancelled when it is present, and additional constraints are included to assure proper reception of the desired signal and form nulls in the direction of interference at the same time. The diagonal loading method is combined with this method to reduce small eigenvalue interference for its eigenvector, which improves the convergence speed in sidelobe. The substantial improvements offered by the proposed adaptive Digital Beamforming technique as compared to previous techniques are demonstrated analytically and by simulations through analysis of the corresponding range compression results and achievable output performance of interference suppression. Simulation results validate the effectiveness of the adaptive DBF.

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This paper proposes a hybrid multichannel processing method for spaceborne Hybrid Phased-MIMO SAR (HPMSAR) that can achieve different applications of multi-direction swath imaging on the same platform. The method is optimal because it is a combination of two-dimension (2-D) advanced digital beamforming (DBF) technology and multichannel pre-filter technology for highresolution wide-swath SAR signal processing. Multichannel signal processing technology for future spaceborne SAR will no longer be single and this combination may be the best choice. The proposed method could avoid spectrum aliasing caused by low pulse repetition frequency (PRF), separate the overlapped echoes caused by different subpulses corresponding to multi-direction swathes and remove the range ambiguity and azimuth ambiguity deeply. At first, we build the signal model of HPMSAR system. Furthermore, the pre-filter design is presented by using matrix inversion method. Then, we address different methods applied to 2-D DBF and propose the advanced linearly constrained minimum variance (LCMV) method. Image results on simulated distributed targets validate the proposed hybrid multichannel processing method.