Research On The New System Of Medium-earth-orbit SAR

Spaceborne synthetic aperture radar(SAR),which can observe entire planet with all-weather and all-day imaging capacity,has become an critical means for earth observation.With the development of spaceborne SAR technology and the promotion of application requirements,and in order to combine the advantages that high resolution capacity of low-earth-orbit(LEO)SAR and continuous observation capacity of medium or high-earth-orbit SAR,a new system of medium-earth-orbit(MEO)SAR is proposed as an important compromise system.Due to its large orbit eccentricity,the new system of MEO SAR exists issues of complex orbit and SAR characteristics,time-varying orbit heights and beam coverage widths,as well as severe range walk and complex spatial variation when working in the large-squint imaging mode.Therefore,this thesis works on these issues and difficulties in the new system of MEO SAR.The main work of this thesis is as follows:1.The characteristics of the new system of MEO SAR are systematically studied.Aiming at the problem of the relatively complex orbit and SAR characteristics of the new system of MEO SAR,firstly,by introducing several space coordinate systems,the geometric model of the new system of MEO SAR satellite and the earth are established.On this basis,a study is made of the several important characteristics of the new system of MEO SAR.The Doppler parameters expressions of the new system of MEO SAR are mainly derived,and the Doppler characteristic of the target is analyzed.The sub-satellite point trajectory characteristic,beam coverage characteristic and the synthetic aperture time of the point target of the new system of MEO SAR are also analyzed.The ellipse expression of the resolution of the point target is derived using the fuzzy function method and the resolution characteristic is analyzed,the change of the whole-orbit radar transmitting power is analyzed.2.A optimized beam positions design method of the new system of MEO SAR is proposed.Aiming at the problems of time-varying orbit heights and beam coverage widths in the new system of MEO SAR,a beam position design method based on the optimal swath width is proposed,which comprehensively considers factors constraining beam position design such as the swath width,pulse repetition frequency(PRF)limitation conditions,azimuth ambiguity signal ratio,and range ambiguity signal ratio of each beam position at each time of the orbit for optimal swath width and PRF selection.The beam position iteration method based on the optimal swath width is used to complete the design of all beam positions at the orbit time.Finally,The effectiveness of the proposed method is verified by implementing the beam positions design of the strip working mode and sliding spotlight working mode.3.A large-squint imaging algorithm of the new system of MEO SAR is proposed.In order to provide higher resolution and stronger continuous observation capability,the new system of MEO SAR usually needs to work in the large-squint mode.Aiming at the problems of severe range walk and large redundancy of echo data in the large-squint mode,a variable PRF technology was adopted to record the echo and to improve the data storage efficiency.Aiming at the problem of complex azimuth spatial variation of the squinted signal,an imaging algorithm combining an extended omega-K algorithm and a time-frequency joint scale transform are proposed.Firstly an azimuth time scale transformation method is used to complete the azimuth spatial variation correction of the range cell correction(RCM).Then an extended omega-K algorithm is used to correct the RCM of the range spatial variation.At last an azimuth frequency scale transformation method is used to handle the rangeazimuth-coupled spatial variation and the higher-order azimuth variation.Simulation experiments show that the proposed algorithm can achieve scene accurate imaging of the new system of MEO SAR in large-squint mode,the effectiveness of the algorithm is verified.