Imaging Algorithm for Sliding Spotlight SAR Operating in the Ascending and Descending Periods of Highly Elliptical Orbits

This study pioneers the investigation of synthetic aperture radar (SAR) imaging algorithm during the ascending and descending periods of highly elliptical orbit (HEO). SAR operating in HEO has the potential to enhance observation capabilities of high-latitude maritime routes. However, current state-of-the-art methods struggle to balance full-scene focusing and high efficiency during the ascending and descending periods of HEO. In this article, an SAR imaging algorithm that leverages the unique echo characteristics during these periods is introduced. First, azimuth deramping is employed to address spectrum aliasing. We apply this method to the polynomial model and assess its impact on the model coefficients. Second, the reduction of aperture length is achieved through bulk processing and 2-D frequency-domain resampling. Third, the imaging results are refined by focusing in block unit. Due to the distinct echo characteristics observed during these periods of HEO, the proposed aperture length reduction processing can lead to a significant decrease in aperture length. This enables precise imaging through partitioning without the need for iteration. Thus, the proposed algorithm can achieve accurate focusing while maintaining efficiency. In addition, we propose the reason why frequency-domain resampling causes defocusing in the presence of severe spatial variations in light of the polar format algorithm. Furthermore, the calculation method for the frequency axes after twice resampling is presented. Simulation results validate that the proposed method is effective and efficient for the ascending and descending periods of an HEO with an eccentricity of 0.6.