On Spaceborne DBF-SAR Adopting the Degree of Freedom With NLFM Waveform: Optimization Framework and Simulation

Digital beamforming (DBF) is a fundamental technique for synthetic aperture radar (SAR) to get high-resolution wide-swath (HRWS) images, which significantly increases the signal-to-noise ratio (SNR) of the system and improves range ambiguity performance. Moreover, the performance of the DBF-SAR system can be improved by using the nonlinear frequency modulation (NLFM) waveform, which can provide a matched filtering output with lower sidelobes without the loss of SNR compared to the linear frequency modulation (LFM) waveform. Combining the DBF technique and the NLFM waveform will enhance the system performance of DBF-SAR from an additional degree of freedom, which has essential engineering significance for reducing the transmit power of the system. However, the previous system architecture and processing method of DBF-SAR are generally based on the LFM waveform and are not practicable in DBF-SAR adopting the NLFM waveform. This article demonstrates the potential of adopting the NLFM waveform in DBF-SAR and analyzes the problems of compensating pulse extension loss (PEL) and frequency dispersion loss (FDL) in the new system. Then, an optimized DBF framework that combines subdigital beamforming and a bank of unequal-width bandpass filters to suppress PEL and FDL in DBF-SAR adopting NLFM waveform is proposed. Simulations demonstrate that the proposed framework shows greater efficiency and stability in suppressing the severe PEL and FDL in the NLFM and LFM systems than previous methods. This article brings an additional degree of freedom to the next-generation spaceborne DBF-SAR and provides sufficient technical support for high-performance DBF-SAR when the LFM waveform is not adopted.