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 The start-stop approximation is no longer applicable, since it introduces two effect, e.g., "fast-time" effect and "slow-time" effect, which will cause defocusing and range-dependent azimuth shift, respectively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. Index Terms-Backprojection algorithm (BPA), spotlight, synthetic aperture radar (SAR), two-steps processing. I. INTRODUCTION S YNTHETIC aperture radar (SAR) is an important remote sensing tool which has been widely used for earth observation operating in all day and all weather conditions [1]. Modern Manuscript
Accurate and efficient image mosaicking is essential for generating wide-range swath images of spaceborne scanning synthetic aperture radar (ScanSAR). However, the existing methods cannot guarantee the accuracy and efficiency of stitching simultaneously, especially when mosaicking multiple large-area images. In this paper, we propose a novel image mosaic method based on homography matrix compensation to solve the mentioned problem. A set of spaceborne ScanSAR images from the Gaofen-3 (GF-3) satellite were selected to test the performance of the new method. First, images are preprocessed by an improved Wallis filter to eliminate intensity inconsistencies. Then, to reduce the enormous computational redundancy of registration, the overlapping areas of adjacent images are coarsely extracted using geolocation technologies. Furthermore, to improve the efficiency of stitching and maintain the original information and resolution of images, we deduce a compensation of homography matrix to implement downsampled images registration and original-size images projection. After stitching, the transitions at the edges of the images were smooth and seamless, the information and resolution of the original images were preserved successfully, and the efficiency of the mosaic was improved by approximately one thousand-fold. The validity, high efficiency and reliability of the method are verified.
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