High-Precision SAR Imaging of Ship Target with 2D Spatial-Variant Phase Errors via Reversal Conjugate-Chirp Z-Transform and Maximum Likelihood Estimation

Synthetic Aperture Radar (SAR) enables high-resolution imaging of ship target. However, the translational motion and intense three-dimensional (3D) oscillations of the target couple with the platform motion, introducing unknown two-dimensional (2D) spatial-variant (SV) phase errors into the echoes, causing severe defocusing of the target image. This challenges traditional SAR imaging methods for ship target, which can be summarized in the following two aspects: 1) traditional methods struggle to accurately estimate and compensate for phase errors with strong 2D SV characteristics, resulting in poor focusing performance, 2) traditional methods fail to fully utilize the SV signal structure, leading to low algorithmic efficiency. To address these issues, this paper proposes a high-precision SAR imaging method of ship target with 2D SV phase errors based on reversal conjugate-chirp z-transform and maximum likelihood estimation. The proposed method mainly consists of two steps: 1) the reversal conjugate-chirp z-transform is designed to adaptively extracting signals with varying azimuth centroids, thereby ensuring the precision of subsequent phase errors estimation, 2) the maximum likelihood estimation for 2D SV phase errors is derived, which not only enables highly efficient estimation under noisy conditions, approaching statistical optimality, but also offers better theoretical interpretability. Finally, high-resolution imaging of ship target is achieved through matrix-based phase errors compensation. Simulation and measured data validation show that the proposed method not only maintains the superior focusing performance but also achieves high computational efficiency.