# Research on Synthetic Aperture Radar Processing for the Spaceborne Sliding Spotlight Mode
###### Abstract  
Gaofen-3 (GF-3) is China' first C-band multi-polarization synthetic aperture radar (SAR) satellite, which also provides the sliding spotlight mode for the first time. Sliding-spotlight mode is a novel mode to realize imaging with not only high resolution, but also wide swath. Several key technologies for sliding spotlight mode in spaceborne SAR with high resolution are investigated in this paper, mainly including the imaging parameters, the methods of velocity estimation and ambiguity elimination, and the imaging algorithms. Based on the chosen Convolution BackProjection (CBP) and PFA (Polar Format Algorithm) imaging algorithms, a fast implementation method of CBP and a modified PFA method suitable for sliding spotlight mode are proposed, and the processing flows are derived in detail. Finally, the algorithms are validated by simulations and measured data.  
s 201818455 10.3390/s18020455 10.  
on the size of the imaging scene, available partial magnification of any part of the scene of interest, and no frequency domain interpolation is needed to ensure image quality. The biggest drawback of CBP algorithm, however, is the large amount of computation. The fast CBP algorithm has two directions: one is to divide the sub-apertures, and the other is to divide sub-images. The methods improve the efficiency based on the space or time sampling redundancy in the CBP algorithm. Sub-aperture method [9; 10] refers to the technique whereby the whole aperture is divided into several smaller sub-apertures, each corresponding to a narrow range of bandwidth, which means that the sampling frequency in the azimuth domain can be lower to reduce the required interpolation so as to reduce the computation. The image of each sub-aperture is of low resolution, and the high resolution image is finally obtained by their coherent superposition. In sub-image methods [11; 12], the entire imaging is divided into many small sub-images to reduce the spatial bandwidth in both range and azimuth domain. Correspondingly, the signal sampling rate can be reduced in two dimensions in order to improve the computation efficiency. In this paper, the fast realization method of CBP based on Quadtree recursive sub-image segmentation is studied, which is realized by sub-image interception, fast filtering and down-sampling to reduce the computation.  
PFA is a classical spotlight SAR imaging algorithm [13; 14], which is simple and efficient and also very suitable for sliding spotlight mode. Because of the assumption of plane wave front, the residual phase error will cause defocus in PFA to limit the size of the effective imaging scene. Because of the larger range of the imaging scene in sliding spotlight mode, the range of the small effective scene is not sufficient to meet the actual requirements. A modified PFA algorithm based on sub-aperture processing for wide scene with high resolution was proposed by [PERSON] [15], and has been improved in [16], but the overlap rate between two adjacent sub-apertures is very high, and the process is very complex. In this paper, based on the CBP algorithm of Quadtree recursive sub-image segmentation, it is further extended to the PFA algorithm, which greatly improves the effective imaging scene size of PFA. Finally, the proposed algorithms are verified by simulation and measured data.