A Modified 2-D Autofocus Approach for 2-D Frequency-Dependent Residual Motion Error in Ultrahigh-Resolution Airborne SAR

Autofocus serves to mitigate the residual motion error (RME) in the airborne synthetic aperture radar (SAR). However, the 2-D frequency-dependent RME remains unresolved for ultrahigh-resolution (UHR) imaging up to the centimeter-scale resolution. To address this issue, a modified 2-D autofocus approach is proposed in this article. Regarding the range frequency-dependent error (RFDE), an efficient azimuth resampling strategy combined with the Chirp-z transform (CZT) is devised, which avoids the approximation imposed by the existing subband division method. With respect to the azimuth frequency-dependent error (AFDE), it is first reported in this article, and removed efficiently by a special filter on the support region. The simulation results show that the deviation of the azimuth phase error (APE) can be reduced from $\mathbf {\pm 5}$ to $\mathbf {\pm 0.5}$ rads after the RFDE removal and $\mathbf {\pm 0.05}$ rads after the AFDE removal. The real data from an airborne UHR microwave photonic (MWP) SAR is utilized for validation, and four existing autofocus methods are reproduced for comparison. In the cases without sensor-based motion compensation (MoCo), only the proposed method recovers the distinct textures of the manhole cover, demonstrating its robustness. In the cases with sensor-based MoCo, the proposed method achieves the best image quality with almost minimal processing time, demonstrating its efficiency.