Research On Synthetic Aperture Imaging And Ground Moving Target Indication Based On MIMO SAR System

Recent years, a new radar conception based on Multi-Input Multi-Output(MIMO) array configuration has drawn much attention. MIMO radar can make full use of spatial diversity and waveform diversity to attain much more equivaluent channels and spatial degree of freedom(DOF) than the number of real antennas. A lot of materials have demonstrated the potential advantages of MIMO radar in target detection, parameter estimation, radar imaging, target recognition, anti-jamming, and et al. Encouraged by the success in radar, the MIMO technique has been applied to Synthetic Aperture Radar(SAR). The newly developing imaging technology based on MIMO radar configuration is able to simultaneously improve the performance of synthetic aperture radar imaging and ground moving target detection by utilizing multiple antennas both at transmission and reception. We have get a relatively clear perceives of MIMO radar in aspect of basic conception, system configuration, signal processing, and performance evaluation. However, the precise signal model, as well as the effect of ground moving target in image domain and the approaches for moving target indication based on MIMO SAR system are rarely investigated. We try to solve the inherent restriction of conventional SAR system, such as high resolution wide swath imaging, slowly ground moving target indication and anti-jamming, the main contents of this thesis are arragemented as follows:In Chapter 1, we firstly summarize the conception, categories and applications in radar imaging based on MIMO radar technologies. We also refine the scientific content of MIMO SAR and elaborate the history and applications of MIMO SAR as well as some key issues.In Chapter 2, we mainly introduce the basic theories about MIMO SAR imaging. Firstly, the array characteristics of MIMO SAR are analyzed from the concept of virtual array, and the results have demonstrated that MIMO radar can improve the array aperture, which means much higher spatial resolution; Secondly, we introduce the conception of equivaluent phase center(EPC), and the phase error resulted from EPC is derived; Thirdly, we take the MIMO SAR system with along-track array configuration as example, the spatial-time hybrid sampling, echo data representation and the imaging strategy are presented; Thirdly, the theory of waveform orthogonality is introduced, and some classical approaches for echoes separation and the influence of waveform orthogonality on imaging performance are presented; Lastly, we analyze the signal-to-noise(SNR) performance by the radar equation of MIMO SAR system.Due to the characteristic of multiple antenna and multiple waveforms configurations, the spatial resolution of MIMO SAR are much more complicated than the traditional SAR system. Thereofre, the conventional approach is not able to character the resolution performance of MIMO SAR. In Chapter 3, we firstly establish the mathematical model of the signal received by MIMO SAR, and then the analytical expression of generalized ambiguity function(GAF) is introduced. Both theory analysis and simulated experiments have demonstrated that: the GAF of MIMO SAR system can be describled by the product of spatial ambiguity function and the monostatic SAR generalized ambiguity function. In other word, the resolution of MIMO SAR not only depends on the signal bandwith and the length of the synthetic apterture, but also the manifold of transmitting and receiving arrays.Except for the orthogonality of the multiple transmitted waveforms, however, for MIMO SAR imaging, additional requirements are needed, such as: a large product of timewidth and bandwidth to achevie high range resolution and SNR, a constant envelope so as to reduce the required peak transmit power, high Dopper tolerance, and being easy to be produced and processed. In order to fulfill the strict requirements of the orthogonal waveform sets, we propose two kind of orthogonal waveform sets based on OFDM signal in Chapter 4. The first kind is the interleaved subcarriers OFDM(I-OFDM) waveform sets, which exploit the inherent orthogonal feature between different subcarriers to yield two two I-OFDM signals with ideal orthogonality. However, the I-OFDM only comprises two orthogonal signals, which is not suitable for MIMO SAR system with more than two transmtters. We then propose a set of random subcarriers OFDM(R-OFDM) waveforms, which exploit the orthogonal feature by turn-on or turn-off the subcarriers randomly. The performance of autocorrelation and crosscorrelation of the two kind waveform sets are analyzed.In Chapter 5, we present a way to overcome the contradiction between high resolution and wide swath imaging due to the inherent restriction of conventional SAR system. The main idea is to make full use of the spatial information contained in MIMO array. We firstly establish the signal model of MIMO SAR with along-track multiple antennas, and the phase relationship between different transmit-receive pairs is derived; then the iterative adaptive approach(IAA) is introduced to reconstruct the unambiguity Doppler spectrum. Lastly, a detailed strategy for MIMO SAR imaging is presented.In Chapter 6, we mainly focus on the technique of ground moving target indication based on MIMO SAR system. Firstly, the signal models for both stationary and moving target are established, and then an adaptive approach for clutter suppression is imposed. Due to the spatial steering vector is a function of the radial velocity of moving target, therefore variety of spatial beamforming technique could be used for the radial velocity estimation of the moving target. We introduce the Iterative Adaptive Approach(IAA) to estimate the spatial spectrum of the data coming from the same resolution cell, and then the radial velocity could be estimated. The simulated experiments have dementrated the adavantage of the IAA approach over the conventional delay-and-sum approach.In Chapter 7, we make a summary of the thesis, and several problems which are still needed to be exploited further are proposed.To summarize, we take newly developing conception of MIMO SAR as the object, and have explored the system conception, signal model, waveform design, SAR imaging and ground moving target indication and etc. The research may enrich system info of SAR, and provide some constructive reference for recognizing, understanding, and designing the future MIMO SAR system.