Hybrid Geometric Calibration Method For Multi-platform Spaceborne SAR Image With Sparse Gcps

Synthetic Aperture Radar(SAR)has the ability of high-accuracy geometric positioning.However,due to the measurement errors of satellite orbit parameters and imaging parameters,the geometric positioning accuracy of SAR images is restricted.Geometric calibration technology can realize the detection and elimination of measurement errors.It is the key to realizing high-accuracy geometric positioning of SAR images,and it is also one of the main ways to improve the geometric positioning accuracy of SAR systems without ground control points(GCPs).In recent years,with the continuous launch of domestic SAR satellites such as the Gaofen-3(GF-3),it is urgent to study the spaceborne SAR geometric calibration technology in order to promote the development and application of domestic SAR satellites.Geometric calibration is able to provide high-accuracy geometric coordinates of spaceborne SAR image through accurate geometric parameters in the Range-Doppler(R-D)model by GCPs.However,it is very difficult to obtain GCPs that covering large-scale areas,especially in the mountainous regions.In addition,the traditional calibration method is only used for single platform SAR images and can't support the hybrid geometric calibration for multi-platform images.To solve the above problems,a hybrid geometric calibration method for multi-platform spaceborne SAR images with sparse GCPs is proposed in this paper.First,we calibrate the master image that contains GCPs.Secondly,the point tracking algorithm is used to obtain the tie points(TPs)between the master and slave images.Finally,we calibrate the slave images using TPs as the control points.We take the Beijing-Tianjin-Hebei region as an example to study SAR image hybrid geometric calibration method using 3 TerraSAR-X images,3 TanDEM-X images and 5 GF-3 images covering more than 235 kilometers in the north-south direction.Geometric calibration of all images is completed using only 5 GCPs.The GPS data extracted from GNSS receiver are used to assess the plane accuracy after calibration.The results after geometric calibration with sparse GCPs show that the geometric positioning accuracy is 3 m for TSX/TDX images and 7.5 m for GF-3 images.The main content and work of this paper are as follows:(1)The rigorous SAR imaging model is researched.In this paper,based on the R-D model,the construction and derivation of R-D model,the polynomial fitting of SAR satellite orbit and the construction and determination of geometric calibration model are described and analyzed in detail.(2)Based on the R-D model,the direct geometric positioning and indirect geometric positioning algorithms are studied,and the mapping relationship between satellite sensor and ground object is established Two corresponding methods of geocoding,namely direct geocoding and indirect geocoding,are introduced to realize the transformation of SAR image from geometric coordinate to projected coordinate.(3)I propose a hybrid geometric calibration method.The core algorithm of this method is described in detail,including point tracking algorithm for TPs and TP's geodetic coordinate obtaining algorithm based on SRTM.The technological process of hybrid geometric calibration is designed,which can realize geometric calibration of multi-scene,multi-platform SAR images with sparse GCPs.(4)Based on the MFC development environment,the hybrid geometric calibration software was developed.We take the Beijing-Tianj in-Hebei region as an example to conduct experiment using 3 TerraSAR-X images,3 TanDEM-X images and 5 GF-3 images,and 82 GPS points were used for accuracy evaluation.