Processing of Very High Resolution GF-3 SAR Spotlight Data With Non-Start–Stop Model and Correction of Curved Orbit

A novel imaging algorithm is presented in this article for focusing the very high resolution spaceborne synthetic aperture radar (SAR) data of spotlight mode. The long synthetic aperture time in spotlight mode brings new problems, such as Doppler spectrum aliasing and curved orbit. The imaging results will be filled with ambiguity and would suffer from resolution reduction if Doppler spectrum aliasing is not handled. The error introduced by curved orbit will degrade resolution of the target and introduces asymmetric sidelobes in azimuth direction. The start–stop approximation is no longer applicable, since it introduces two effect, e.g., “fast-time” effect and “slow-time” effect, which will cause defocusing and range-dependent azimuth shift, respectively if not handled. The proposed algorithm combines the two-steps processing approach (TSPA) and the backprojection algorithm (BPA). First, the initial step of TSPA is used to get a high azimuth sampling rate which is higher than pulse repetition frequency to avoid the Doppler spectrum aliasing. Then, the “fast-time” effect of start–stop approximation is corrected in the 2-D frequency domain. Finally, the BPA is used to correct the error introduced by the curved orbit and the “slow-time” effect of start–stop approximation. The experimental results of spaceborne SAR data acquired by Gaofen-3 (GF-3) SAR system demonstrate the feasibility of the proposed algorithm.