Research On Key Technologies Of High Timeliness Spaceborne SAR System Design And Data Processing

Spacebome synthetic aperture radar(SAR)is an high-efficiency means of remote sensing because of its all-day and all-weathre capabilities for earth observation.With the continuous promotion of earth observation demand,SAR technology has developed rapidly.In recent years,some high-efficiency novel SAR systems have attracted the attention of researchers gradually.For example,with a geostationary earth orbit satellite as the illuminator and low earth orbit satellite as the receiver,GEO-LEO BiSAR system is characterized with high spatial resolution and real-time performance.The system has the advantages of anti-attack,well-concealment and less cost which are conducive to achieve the industrialization of SAR satellites.In addition,the LEO satellite operates in the sliding spotlight mode(SS)or terrain observation by progressive scans mode(TOPS),which further expands the application of the GEO-LEO BiSAR system by adjusting the satellite attitude or beam pointing center.When the high-resolution observation mission is required,the synthetic aperture time is increased by SS mode.When the large observation mission is required,the beam pointing angle is increased by TOPS mode.High elliptic earth orbit(HEEO)SAR satellite has the characteristics of large orbital eccentricity,which can achieve high-resolution imaging at orbit perigee and monitor high-latitude sensitive areas at orbit apogee.In addition,HEEO SAR has the ability of daily revisit,which improves the surveillance and detection abilities of SAR satellite significantly.By rationally adjusting the system parameters of HEEO SAR satellite,the continuous observation can be realized from the perigee to the apogee of the earth.What’s more,the observation width increases gradually.However,the above two new high-efficiency SAR systems characterized with high timeliness also bring challenges to the traditional SAR system design and data processing algorithms.For HEEO SAR system,the orbital characteristics are significantly different in different orbital segments of an orbital period.So it is necessary to solve the problems of parameter design and data processing of different orbital segments.For the GEO-LEO BiSAR system,the orbit difference of the transceiver platforms leads to the problem of two-dimension spatial variation of the echo.The signal ambiguity problem in GEO-LEO BiSAR SS mode and TOPS is also quite different from that in traditional LEO SAR system,so we have to redesign the dechirp function.In summary,many new technical challenges are faced in system design and signal processing.Firstly,the system parameter design of different orbital segments for HEEO SAR satellite is analyzed,and the common spaceborne coordinate systems and the transformation between them are introduced.Then,we illustrate the system parameter design,azimuth signal ambiguity,two-dimensional spatial variation correction and frequency domain fast imaging algorithm of GEO-LEO BiSAR system.Finally,the fast geolocation technology and geolocation error analysis of GEO-LEO interferometric SAR(InSAR)system working in strip-map mode are introduced.The main contents and innovations of this paper are summarized as follows:(1)HEEO SAR parameters design of an orbit periodBased on the definitions of common spacebome coordinate systems,the orbital characteristics of HEEO SAR system are introduced in the second chapter of this paper first.Then,the orbit parameters and the illuminated area of a whole orbit period studied in detail.Finally,a system parameter design framework based on the slowly varying width is proposed.The orbit characteristics of HEEO SAR are quite different from that of traditional LEO SAR.As a result,the method of system parameter design is also different.In the process of system parameter design of traditional circular orbit SAR satellite,once a set of parameters are designed according to the system requirement indicators,these parameters remain basically constant throughout the orbit cycle.However,the orbit altitude and speed of HEEO SAR satellite have been changing throughout the orbital cycle.The difference is more obvious at perigee and apogee.Therefore,for each orbital segment of HEEO SAR,it is necessary to adjust the off-nadir angle and beam width to ensure that the beam can continuously irradiate the earth.On the basis of effective beam illumination,the parameters are designed according to the system indicators.We take the gradual increase of the illumination width from perigee to apogee as the criterion,this paper proposes a system parameter design framework suitable for HEEO SAR.Under this framework,the illuminated width of each orbital segment increases approximately linearly during the whole orbital cycle from perigee to apogee.Simulation experiments show the effectiveness of this design framework.(2)Parameter design of GEO-LEO BiSAR SS and TOPS modesThe third chapter of this paper firstly introduces the spatial geometry of GEO-LEO BiSAR in earth centered earth fixed(ECEF)coordinate system.Then,for the convenience of system parameter design,the center of the GEO-LEO BiSAR illumination scene is taken as the origin.The target local coordinate system is established.Finally,the design of key parameters of GEO-LEO BiSAR system in SS mode and TOPS mode is proposed.For example,we design an appropriate pulse repetition frequency(PRF)according to the requirements of the range ambiguity to signal ratio(RASR)and azimuth ambiguity to signal ratio(AASR)of the system;we design reasonable transmitted signal bandwidth and azimuth synthetic aperture angle according to the azimuth and range resolution;we design the time-varying beam pointing vectors of LEO satellite under the working conditions of GEO-LEO BiSAR SS or TOPS mode according to the azimuth synthetic aperture angle or azimuth coverage requirement.Finally,satellite tool kit(STK)is used to analyze the performance of the GEO-LEO BiSAR system under the actual configuration,and some reasonable configuration design results are given.(3)GEO-LEO BiSAR SS and TOPS mode frequency-domain imaging methodThe fourth chapter of this paper firstly analyzes the signal model and signal characteristics of GEO-LEO BiSAR,the time and frequency domain expressions of the signal are given.Then,we compare the traditional monostatic SAR system with the GEO-LEO BiSAR SAR,and the two-dimensional spatial variation of the signal needs to be solved.Compared with the signal of GEO-LEO BiSAR working in strip-map mode,it also needs to solve the problem of azimuth ambiguity.In response to the above two problems,the innovative work of this paper is illustrated as follows:■ An azimuth ambiguity removal method based on dechirp function is proposed.Due to the large motion difference of GEO-LEO BiSAR transceiver platforms,the azimuth signal ambiguity removal method utilized in monostatic SAR SS and TOPS modes is no longer applicable.In this paper,a dechirp function suitable for GEO-LEO BiSAR signal is constructed.Convoluting GEO-LEO BiSAR signal with dechirp function to remove the azimuth ambiguity of the range-doppler data.Then,the frequency domain processing such as range compression(RC)and range cell migration correction(RCMC)can be performed in the RD domain to improve efficiency.■ A correction method for rangea/zimuth two-dimension spatial variation of GEO-LEO BiSAR system is proposed.Due to the great difference in the motion characteristics of GEO-LEO transceiver platforms,the azimuth invariation characteristic of the traditional monostatic SAR signal will no longer exist,and the two-dimension variations of the signal needs to be considered.Therefore,this paper deduces the RD expression of GEO-LEO BiSAR signal.By improving the traditional chirp scaling(CS)algorithm,we obtains the CS algorithm suitable for GEO-LEO BiSAR.Finally,the range and azimuth spatial variations of signal are corrected.(4)Fast geolocation method and error analysis of GEO-LEO InSAR systemThe fifth chapter of this paper firstly analyzes the geometric characteristics of GEO-LEO InSAR system working in strip-map mode.Then the corresponding geolocation equations of GEO-LEO InSAR are established.Newton iterative method is used to solve the nonlinear equations.When we solve geolocation equations of the traditional LEO InSAR,the closed solution can be obtained by appropriately deforming the equations to improve the efficiency.However,the closed solution can be no longer applied to GEO-LEO InSAR system directly.To solve the problem,this paper deduces the closed form solution instead of Newton iteration method in GEO-LEO InSAR system.Compared with Newton iteration method,this method can significantly improve the calculation efficiency and maintain high geolocation accuracy at the same time.Finally,the sensitivity equations are derived by using GEO-LEO InSAR geolocation equations in order to analyze the influence of each error source on the geolocation error.