Study On Key Technologies Of HRWS Bistatic SAR System With GEO Transmitter And LEO Receiver

Spaceborne synthetic aperture radar(SAR)is a very effective means of remote sensing because of its day and night and cloud-penetrating operational capabilities for global observation.Spaceborne bistatic SAR(BiSAR)system employed geosynchronous-Earthorbit(GEO)transmitter and low-Earth-orbit(LEO)receiver with azimuth multichannel(AMC),i.e.,GEO-LEO AMC-BiSAR,has the characteristics of good flexibility and realtime performance.In addition,GEO-LEO AMC-BiSAR system has great advantages compared with current spaceborne SARs,such as anti-attack,well-concealment and less cost,which are conducive to achieve the industrialization of light SAR satellites and the standardization of SAR data application,and meet the requirements of high-resolution and wide-swath(HRWS)imaging and interferometric SAR(InSAR)mapping.However,due to the characteristics of large differences of platform orbits,hybrid configurations and strong two-dimensional spatial variation in echo signal,it still faces many new technical problems in system design and SAR/InSAR signal processing.In this dissertation,the main problems and key technologies of system design,bistatic Doppler ambiguity suppression,HRWS bistatic frequency-domain imaging and bistatic InSAR signal processing in GEO-LEO AMC-BiSAR are studied.The main contents and innovations of this dissertation are summarized as follows:(1)System design techniques for GEO-LEO AMC-BiSARIn Chapter 2,the geometric model of arbitrary bistatic configuration in GEO-LEO AMC-BiSAR system is established,and then the planning scheme of bistatic configuration,optimal configuration design and key parameters design method are studied.The main work is as follows:A configuration planning scheme based on ground resolution characteristics is designed.For GEO-LEO BiSAR system,the complex bistatic configuration has significant effect on the imaging performance.Firstly,mathematic models between the indicators related to the imaging performance and the configuration parameters are established based on the geometry.The indicators related to the imaging performance are ground range resolution,azimuth resolution and two-dimensional resolution direction angle.The configuration parameters are the off-nadir angles of satellites,the ground projection of the bistatic angle,the ground projection of the bistatic velocity angle and the ground squint angle of LEO receiver.Then the mathematical relationships are analyzed in detail.According to the constraint relationship,a configuration planning scheme suitable under forward-looking or side-looking of LEO receiver based on ground resolution characteristics is designed.The effectiveness of the proposed method is validated by simulation experiments.A novel optimal configuration design method based on the Simulated Annealing(SA)algorithm is proposed.Firstly,the mathematical models between the performance parameters such as ground resolution characteristics and noise-equivalent sigma zero(NESZ)and the configuration parameters such as the off-nadir angle of LEO SAR satellite,the ground squint angle of LEO receiver and the center time of imaging are established.Then,a general nonlinear multivariate objective function is presented for desired imaging performance.Accordingly,the configuration design problem can be transformed to solve nonlinear multivariate optimization equation.A novel optimal configuration design method based on the SA algorithm is presented.Finally,simulation results are provided for two typical observational missions,to verify the effectiveness of the proposed method.A design method of key parameters for GEO-LEO AMC-BiSAR system is presented.According to the inherent relationship between the performance indicators,such as ambiguity to signal ratio(ASR),NESZ,two-dimensional ground resolution and swath,and the key parameters such as antenna length,pulse repetition frequency(PRF),receiving channel number,signal bandwidth and emission power,the timing and key parameters of each beam are designed according to the given performance requirements.(2)Doppler ambiguity suppression techniques for GEO-LEO AMC-BiSARThe Doppler ambiguity suppression methods for HRWS imaging of GEO-LEO AMCBiSAR in Chapter 3 are studied.The main contents are as follows:A bistatic weighted back-projection algorithm(BWBPA)based on time-domain subimages for GEO-LEO AMC-SAR is derived and presented.Starting from modeling geometry of inclined GEO-LEO AMC-BiSAR,the signal model is analyzed in detail for the first time,and the equivalent positions of the receive channels are obtained.Then we found the spatial-variant residual phase error cannot be compensated accurately by conventional effective phase center(EPC)processing.BPA can well compensate the space-variant phase error.Nevertheless,BPA faces with a technical challenge due to the nonuniform sampling in azimuth.To address the issue,according to the Doppler history of receiving platform,we divide each channel data into multiple sub-apertures,then digital beamfoming(DBF)technology is used to suppress Doppler ambiguities.Simulated data results show the validity of the presented method.A bistatic weighted imaging method based on minimum entropy is proposed.In order to solve the problem of phase error between receiving channels,the sub-band images in each channel are processed by DBF technology,then the image of each channel can be obtained.Then the minimum entropy is used as the cost function,and the optimal values of phase deviations between channels can be estimated by the Newton method with the images of all channels.The simulation and experimental data results verify the effectiveness of the proposed method.(3)Fast frequency-domain imaging techniques for GEO-LEO BiSARTwo-dimension spatial-variant features of the echo signal in GEO-LEO BiSAR caused by nonlinear and nonparallel trajectories of transmitter and receiver raise challenges for BiSAR focusing.In Chapter 4,a curved-trajectory-based slant range model and its equivalent range model without restriction from the “Stop-and-Go” assumption are derived.Then,based on the equivalent range model,a general frequency-domain imaging algorithm for GEO-LEO BiSAR is derived:A nonlinear chirp scaling algorithm based on two-dimensional time-domain perturbation(TP-NLCS)for GEO-LEO BiSAR is proposed.Echo signal of GEO-LEO BiSAR has strong coupling and spatial-variant characteristics in both range and azimuth direction.To address the issue,according to the principle of chirp scaling,a TP-NLCS algorithm based on TP signals is proposed,which can correct the range cell migration(RCM)difference and the variation of azimuth frequency modulation(FM)rate in same range cell.Finally,the high-order phase error and the time-domain residual phase error are derived and compensated.Simulation results show the validity of the proposed method,which can implement GEO-LEO HRWS BiSAR focusing and has fine phase-preserved capability.(4)InSAR signal processing techniques for GEO-LEO bistatic InSAR systemIn Chapter 5,the following work is mainly carried out for GEO-LEO HRWS InSAR processing technology:The scene simulation and signal processing strategy of GEO-LEO InSAR processing are given.Firstly,the formation configuration between LEO SAR receivers is designed by Hill equation.Then,the range frequency domain pulse coherence(RFPC)method is used to simulate echoes for improving simulation efficiency.TP-NLCS imaging algorithm proposed in Chapter 4 is adopted for BiSAR focusing,and the flow of InSAR processing is given.In order to solve the problem of the two-dimensional space-variant problem of image offsets,a multi-level sub-image registration strategy is given,and the SAR/InSAR processing results for the simulation scene data are presented.Two efficient and precise methods for DEM generation in the GEO-LEO across-track InSAR engineering are proposed.Compared with the repeat-pass InSAR,GEO-LEO InSAR system processes more complicated InSAR geometry.To solve the problems,firstly,according to the geometry of GEO-LEO bistatic InSAR system,suitable three-dimensional locating equations are selected for static and inclined GEO SAR transmitters,respectively.Then,according to the conversion relationship between baseline vectors and range vectors,a closed-form solution of geolocation for bistatic InSAR system with static GEO SAR transmitter is derived,and an quasi-closed-form solution of geolocation for bistatic InSAR system with inclined GEO SAR transmitter is presented.The effectiveness of these methods are demonstrated via simulated data.