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However, the phase error caused by radar platform motion or atmospheric propagation delay restricts its 3D imaging capabilities. The phase error calibration of MB CSAR data is an essential step in the 3D imaging procedure due to the limited accuracy of positioning sensors. Phase gradient autofocus (PGA) is widely utilized to estimate the phase errors but is subject to shifts in the direction perpendicular to the line of sight and long iteration time in some sub-apertures. In this paper, an improved PGA method for MB CSAR 3D imaging is proposed, which can suppress the shifts and reduce computation time. This method is based on phase gradient estimation, but the prominent units are selected with an energy criterion. Then, weighted phase gradient estimation is presented to suppress the influence of prominent units with poor quality. Finally, a contrast criterion is adopted to reach faster convergence. The experimental results based on the measured MB CSAR data (Gotcha dataset) demonstrate the validity and feasibility of the proposed phase error calibration method for MB CSAR 3D imaging.<\/jats:p>","DOI":"10.3390\/rs16224242","type":"journal-article","created":{"date-parts":[[2024,11,14]],"date-time":"2024-11-14T08:06:32Z","timestamp":1731571592000},"page":"4242","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Improved Phase Gradient Autofocus Method for Multi-Baseline Circular Synthetic Aperture Radar Three-Dimensional Imaging"],"prefix":"10.3390","volume":"16","author":[{"given":"Shiliang","family":"Yi","sequence":"first","affiliation":[{"name":"School of Electronics and Communication Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China"}]},{"given":"Hongtu","family":"Xie","sequence":"additional","affiliation":[{"name":"School of Electronics and Communication Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China"}]},{"given":"Yuanjie","family":"Zhang","sequence":"additional","affiliation":[{"name":"School of Electronics and Communication Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China"}]},{"given":"Zhitao","family":"Wu","sequence":"additional","affiliation":[{"name":"School of Electronics and Communication Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China"}]},{"given":"Mengfan","family":"Ge","sequence":"additional","affiliation":[{"name":"School of Electronics and Communication Engineering, Shenzhen Campus of Sun Yat-sen University, Shenzhen 518107, China"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-4038-3053","authenticated-orcid":false,"given":"Nannan","family":"Zhu","sequence":"additional","affiliation":[{"name":"School of Systems Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China"}]},{"given":"Zheng","family":"Lu","sequence":"additional","affiliation":[{"name":"Institute of Remote Sensing Satellite, China Academy of Space Technology, Beijing 100094, China"}]},{"given":"Pengcheng","family":"Qin","sequence":"additional","affiliation":[{"name":"Department of Weapons Engineering, Army Academy of Artillery and Air Defense, Hefei 230031, China"}]}],"member":"1968","published-online":{"date-parts":[[2024,11,14]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","unstructured":"Zhu, J., Song, Y., Jiang, N., Xie, Z., Fan, C., and Huang, X. 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