Efficient Doppler Parameter Estimation for SAR Imaging of Complex-Motion Ship Target via Phase Gradient and Maximum Likelihood Estimation

Synthetic Aperture Radar (SAR) is capable of acquiring high-resolution images of targets, with advantages such as all-weather, all-day operation, and long-range detection. The complex motion of ship targets introduces two-dimensional (2D) spatial-variant phase errors into the echoes, leading to severe defocusing in SAR images. Existing methods mainly fall into two categories: image-quality-based approaches and parameter-estimation-based approaches. Although these methods can achieve focused imaging of targets, their computational complexity is generally high. To address this issue, this paper proposes an efficient Doppler parameter estimation method for SAR imaging of complex-motion ship targets based on phase gradient and maximum likelihood estimation. The main innovation lies in using the phase gradient to express the chirp rate and the third-order Doppler parameter under Gaussian noise, and combining with maximum likelihood estimation to achieve high-precision Doppler parameter estimation. Subsequently, spatial-variant parameters are inverted via generalized least squares, and a 2D spatial-variant compensation matrices are constructed, thereby enabling efficient focusing of ship targets with complex motion. Both simulated and measured data validate the effectiveness of the proposed method.