Time-varying step-transform algorithm for high squint SAR imaging

High-squint angle synthetic aperture radar (SAR) imaging has applications in specific situations such as providing looking angle-dependent scattering information and revisiting an area of interest. It is very difficult for current SAR-imaging algorithms to be used directly in high-squint configurations. This is due to the large range-cell migration and the nonnegligible cubic-phase term in the signal. In this paper, an improved algorithm called the time-varying step-transform algorithm is proposed. It uses different chirp rates to deramp the signal in different subapertures to alleviate the range-focus problem. The interval between subapertures is also varied according to the deramping chirp rate. The nonnegligible cubic-phase term is included in the focusing procedure. The performance of the proposed algorithm in high-squint-angle mode is excellent. By reducing the subaperture length as the squint angle is increased, the performance of the algorithm is almost independent of squint angle. The proposed algorithm is also flexible for multilook processing and motion-error compensation.