A Two-Step Motion Compensation Method for Polar Format Images of Terahertz SAR Based on Echo Data

Terahertz synthetic aperture radar (THz SAR) has great potential in the field of remote sensing due to its high resolution and high frame rate. However, THz SAR is very sensitive to motion errors, making even small 2-D error caused by motion error and polar format algorithm (PFA) seriously affect the image quality. Existing microwave SAR autofocusing methods only estimate the error of a single dimension, which cannot meet the accuracy requirements of THz SAR for 2-D error compensation. In this article, a two-step motion compensation method for polar format images of THz SAR based on echo data is proposed. First, by analyzing the conversion model of polar coordinate format, the node where the 2-D error coupling occurs is determined. On this basis, a coarse compensation based on low-frequency fitting is proposed in front of this node to reduce the influence of PFA on the error coupling of 2-D signals. The method not only preserves the correction of the inherent range cell migration by PFA but also eliminates the interference of PFA to the subsequent error compensation processing. Second, to solve the problem that a single compensation method cannot meet the accuracy requirements of THz SAR, the maximum contrast method after polar coordinate format conversion is used for precision compensation. The effectiveness of the proposed method is verified through simulation and actual measurement data processing of 0.22-THz airborne spotlight SAR system.