In order to solve the problem of small imaging width and long revisiting time of low orbit SAR,China developed the No.4 land exploration satellite 01,which is a scientific satellite in the medium and long term development plan of national civil space infrastructure(2015-2025),and is the world's first geosynchronous SAR satellite.Using the advantages of the geosynchronous orbit,the revisiting time of the same place can be reduced from the day level of low orbit satellite to the hour level,and the width can be increased from the hundred-kilometer level of low orbit satellite to the thousand-kilometer level,which can provide strong support for the effective implementation of disaster emergency response.For the new system of geosynchronous SAR microwave imaging,a series of key technologies are developed,such as microwave imaging for geosynchronous SAR,ultra-large power space-borne microwave transmission,ultra-large aperture space-borne antenna,large-flexibility and high-precision attitude stability control,intense pulse high-quality large power supply,integrated efficient thermal management for payload and platform,and integrated satellite-ground high-precision orbit determination.The satellite has the capability of quick revisit,large width,all-day and all-weather observation.The overall design of the satellite is introduced,and the technical innovation is summarized.Through the evaluation of the preliminary test results for the satellite in orbit,the geosynchronous SAR images have clear texture and good quality,which can meet the requirements of disaster prevention and reduction,land resource exploration and other tasks.

Spaceborne interferometric synthetic aperture radar (InSAR) technology is an effective method to obtain digital elevation model (DEM) data. The bistatic InSAR configuration of the inclined-geosynchronous (InGEO) transmitter with low earth orbit (LEO) receivers (InGEO-LEO) is a novel InSAR system to acquire terrain information. This novel system is characterised with high resolution, wide swath and timeliness, but the Newton iterative method is time-consuming to solve the bistatic InSAR equations for fast DEM generation. For the conventional LEO bistatic InSAR system, the closed-form solution is an effective method to improve the efficiency of solving InSAR equations, which is invalid in the InGEO-LEO InSAR system because of the significant geometry difference caused by the orbits of InGEO transmitter and LEO receivers. To address this issue, we analyse the bistatic InGEO-LEO geometry in detail and exploit the bistatic InSAR equations to propose an approximate closed-form solution (ACS) for the novel system. Compared with the general Newton iterative method, the ACS significantly improves the efficiency of geolocation for the bistatic InGEO-LEO InSAR system with high precision. Simulation experiments are carried out to verify the effectiveness and superiority of ACS.

Helicopter-borne wide-angle synthetic aperture radar (SAR) can be used in a wide range of military and civil applications because of their high resolution and mobility. The absolute geometric positioning accuracy is an important index for SAR image quality assessment. However, the effectiveness of using traditional geometric calibration methods with helicopter-borne SAR images obtained by time-domain imaging algorithms remains to be verified. Based on the range-Doppler model, a high-precision geometric calibration method for helicopter-borne SAR is proposed in this study and verified by experiments using real data with different resolutions. The results show that after calibration, the positioning accuracy of SAR images with 20 m and 1 m resolutions is improved from 51.33 m to 4.71 m and from 62.49 m to 3.03 m, respectively. These results verify the effectiveness of the proposed method.