A Novel FDA-MIMO SAR Mainlobe Jamming Suppression Method Utilizing Modified STSO-Waveform Digital Beamforming

Frequency diversity array multi-input multi-output (FDA-MIMO) radar has emerged as an effective solution for combating mainlobe jamming, which conventional suppression methods fail to address. However, in distributed target application scenarios for synthetic aperture radar (SAR), the accumulation of the cross-correlation energy from non-ideal orthogonal waveforms leads to a degradation in both image quality and performance of jamming suppression. Short-term shift-orthogonal (STSO)-waveform digital beamforming (DBF) is an effective method of achieving robust echo separation in the MIMO SAR system. Nevertheless, the performance of echo separation and jamming suppression utilizing STSO-waveform DBF in an FDA-MIMO radar system has not been sufficiently studied. Therefore, we establish the precise signal model of echoes obtained by transmitting STSO waveforms. By analyzing the signal model, we demonstrate the effect of carrier frequency increment on the time shift of different waveforms in an FDA-MIMO radar. Based on the analysis, we reformulate the traditional weight vector calculation to ensure accurate alignment of the beampattern nulls with the unwanted signal components, thereby modifying the traditional STSO-waveform DBF. The modified beamforming method achieves superior separation performance compared with both orthogonal waveforms and traditional DBF, providing signals with a lower level of unwanted components for the subsequent filtering process. Building upon the modified DBF method, the spatial filtering is performed in the range-Doppler domain without requiring any prior knowledge of the jammers. Simulation results demonstrate that the jamming suppression ratio (JSR) of the proposed method is significantly higher than that achieved utilizing traditional methods.