First Study on the Processing Approach of DBF for Squint Spaceborne SAR Imaging

Digital beamforming (DBF) is an effective approach for accessing high-resolution wide-swath (HRWS) imaging in spaceborne synthetic aperture radar (SAR). By utilizing DBF, the performance of imaging, including signal to noise ratio (SNR) and system sensitivity, can be greatly improved. In actual missions, DBF for SAR imaging can be combined with various operating modes like squint-strip SAR, spotlight SAR and TOPSAR. In squint-looking scenarios, the receiving beam of antenna scans the scene not only along the vertical direction (elevation) but also along the horizontal direction (azimuth). For the first time, this paper proposes the application of DBF for squint SAR imaging, establishes a geometric model of beam scanning and derives the analytic expression of signal model. The performance of both the conventional DBF scheme and the digital scalloped beamforming (DSBF) scheme are analyzed in squint-looking scenarios. In order to further reduce the loss of gain when processing received signals with broadband widths and long pulse durations in the conventional DBF schemes, as well as to minimize the digital resource consumption in the DSBF scheme, where the resource usage is directly proportional to the number of sub-beams, a novel multi-beam digital beamforming scheme is proposed. Theoretical analysis and simulation results validate the effectiveness of the proposed scheme, making it as a more realistic technology for future spaceborne SAR.