Digital Beamforming on Receive in Elevation for Spaceborne Hybrid Phased-MIMO SAR

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.