An Improved Ultrahigh-Resolution Stepped-Frequency Spaceborne SAR Imaging Algorithm

Frequency stepping is a widely used technique for ultrahigh-resolution synthetic aperture radar (SAR). Although reducing the burden of hardware, this technique increases the complexity of imaging algorithms due to the intersubband time offsets and intersubband errors of delay, amplitude, and phase. To address the above problems, an improved ultrahigh-resolution stepped-frequency spaceborne SAR imaging algorithm is proposed in this article. By generating subband images individually, performing intersubband error estimation based on primary points, and then synthesizing the subband images in the imaging domain, the proposed algorithm effectively avoids the problem of time offsets and significantly improves intersubband error compensation accuracy benefiting from the high SNR in the imaging domain. Besides, considering the characteristics of nonideal factors in frequency-stepped SAR, a series of error compensation methods aiming at stop-and-go approximation, ionospheric error, and tropospheric delay are integrated to the proposed algorithm. The effectiveness of the proposed algorithms is verified via computer simulations, and real data experiments are also conducted based on both an X-band spaceborne SAR system, Taijing 4-01, and a Ka-band spaceborne SAR system, Luojia 2-01.