| Microwave 3-D imaging technique can get much more information than the traditional 2-D synthetic aperture radar(SAR) imaging technique. It can accurately separate and extract the attractive targets’ spatial position information and scattering information from the environment noise and can help us to learn and study the 3-D scattering coefficient distribution of targets in much more detail. So it can be used to analyze and study the targets’ electromagnetic(EM) scattering characteristics outdoor. Thus researching the EM scattering characteristics of targets form the perspective of 3-D SAR imaging is becoming an emerging hot field. Based on the background above, methods of 3-D near field EM scattering characteristics inversion and algorithms of the RCS Near-Field-to-Far-Field transformation(NFFFT) based on SAR imaging are researched in this thesis. And the results of the research can provide some technique supports to the development of RCS measurement system based on 3-D SAR imaging. The main work of the thesis includes:(1). 3-D SAR imaging system mode with fixed transmitter is introduced at first. Then, 3-D resolution of the mode is briefly analyzed and the principle of the linear array antennas 3-D SAR is discussed. Because step frequency(SF) signals can easily generate a larger bandwidth to meet the high range profile resolution, this thesis uses it as the transmitter signal to study the 3-D back projection(BP) algorithm、3-D range Doppler(RD) algorithm and 3-D wavenumber domain algorithm. Then some simulation experiments are done. Through the simulation experiments, which use FEKO software to generate the radar echoes, 3-D high resolution SAR images are gotten and the validity of the algorithms is verified. Then, through the measured and simulated experiments, this thesis has briefly discussed the advantages of the 3-D SAR imaging techniques applied in the RCS measurement.(2). Inversion methods of the near field scattering characteristics based on 3-D SAR imaging is studied in the thesis. Firstly, some concepts about the radar targets scattering characteristics are introduced. Then, starting from the Stratton-Chu electric field integral equation, the mechanism of 3-D SAR imaging, which uses step frequency signal as the transmitted signal, is analyzed from time and frequency domain, respectively. And then the physical meaning of 3-D SAR imaging is explained. Based on the above, the principle of the near field EM scattering characteristics inversion based on SAR imaging is described in the thesis, the inversion algorithm of the near field scattering characteristics is proposed. Then some experiments are done, through which the accuracy of the algorithm is validated. Moreover, in this thesis influences of the system measurement errors and platform motion errors on inversion precision are studied, and the generating reasons for the errors are analyzed. Through the errors analysis, a foundation for the system design of high precise EM scattering characteristics inversion is laid.(3). This thesis lays emphasis on the research of two EM scattering characteristics’ NFFFT algorithms based on 3-D SAR imaging. Firstly, the principle of the RCS NFFFT based on the multi scattering center model is described, and the RCS NFFFT algorithm based on the multi scattering center model is given. And then some experiments are done, through the experiments the accuracy of the algorithm is validated. Then the thesis quantitatively analyzes the errors of the algorithm, the aim of the errors analysis is to improve the RCS NFFFT precise. By the same way, this thesis has studied the principle of the RCS NFFFT based on the targets’ surface electromagnetic current model and presented the transformation algorithm, and then gives out the verified experiments which validate the effectiveness of the NFFFT algorithm for the complex target. At last, the advantages and disadvantages of the two algorithms are compared and analyzed in this thesis. |
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