Performance Investigation on Elevation Cascaded Digital Beamformingfor Multidimensional Waveform Encoding SAR Imaging

An important issue in a Synthetic Aperture Radar (SAR) system employing Multidimensional Waveform Encoding (MWE) is the fulfillments of Digital BeamForming (DBF) on receive in elevation for a reliable separation of the mutually overlapped echoes from multiple transmit waveforms. In this paper, the performance of a separation approach employing hybrid DBF in elevation by combining the onboard real-time beam-steering and a posteriori null-steering DBF on the ground is elaborately investigated. As a cascaded structure which comprises two subsequent DBF networks, the onboard part effectuates the steering of the mainlobes within multiple partitioned groups of antenna elements to ensure sufficient signal receive gain over the whole swath; the a posteriori adaptive DBF network on the ground mainly performs the task of placing nulls to cancel the range interference from other transmit waveforms, which enables adaptive beamforming to avoid the topographic height variation problem. Two type of onboard realtime beamformers are investigated, depending on the utilization of the transmit waveform structure information or not. The performance of the hybrid DBF approach is theoretically analyzed and evaluated in simulation experiment. It is shown that the hybrid DBF approach can provide additional dimensions of the trade-space to optimize the performance on range ambiguity suppression and signal-to-noise ratio improvement, as well as the onboard data volume reduction. In comparison with the a posteriori DBF on the ground, employing the hybrid DBF networks can get satisfactory performance while remarkably reducing the output data volume, in the presented example, the corresponding output channel number is decreased from 10 to 6.