Study On Moving Targets Detection And Imaging Technique In High Resolution Synthetic Aperture Radar

Synthetic aperture radar (SAR) can obtain two-dimensional image of the target, through wideband pulse compression and synthetic aperture techniques to improve the resolution of range and azimuth. With the demand of military or civilian, and the development of the SAR technology, moving target detection and imaging in the SAR system becames possible.Ground Moving Target Indicator (GMTI) in SAR has been an important focus in international SAR technique research field. At present, there are still some issues of these technologies that have not been solved well and should be improved, as followed: first, both the small aperture antenna and the maneuvering flight of radar platform lead to the doppler spectrum spreading of ground clutter, slowly moving target has been submerged in the main lobe of clutter, which becomes difficult to be detected. Second, the radial velocity causes migration of target location, the track velocity leads to defocusing in azimuth, acceleration and self-rotation could increase the doppler frequency complexity of echoes.Third, high resolution of azimuth direction makes migration through resolution cells (MTRC) phenomenon more serious, it improves the difficulty of correction. Fourth, Low Pulse Repetition Frequency (PRF) of SAR is used to avoid range ambiguity, but decreases the detection performance of radial velocity. How to effectively suppress the ground clutter to improve detection performance, and improve the estimation accuracy of the target doppler parameter become the key technologies needed to solve in SAR-GMTI. On the research background of high resolution SAR for MTI and imaging, the primary works of the dissertation introduced as followed: Problems with MTI and imaging under high-resolution SAR were analyzed.First of all, the target echo wave model was established to analyze the echo doppler frequency characteristics, and point out the principle which causes target location offset and image defocusing. Furtherly, from radar systems, ground clutter characteristics and maneuvering of the target, the factors were analized which impact performance of moving target detection, and explore the key technical problems which need to be considered in the imaging procedure of moving targets. Finally, this paper sum up the importance and difficulty of clutter suppression, target detection and imaging focus in single-antenna SAR, and prepare for the following research work, which are moving target detection and imaging technique.To solve the difficult problems of slow moving target detection single-antenna SAR system, uncertainty of azimuth position for moving target perpendicular to range direction and other issues, the subaperture processing was proposed. Through the analysis of the characteristics of subaperture and synthesis array, the subarray model was constructed, which is equivalent to synthesis array, to study the imaging and MTI methods thought subaperture. With the spectrum between moving target and ground clutter, the problem above was solved by spectrum splitting method. The target was located, depending on the image sequence of subaperture, and phase-time relationship of subaperture. Through the processing of subaperture, the width of ground clutter spectrum reduce and the detection performance.enhance.For the detection of the slow ground moving target in single-antenna SAR system, we study the subaperture method for clutter reduction. The select method of subaperture in wide-beam SAR system was provided, and the phase compensation, time alignment, image registration methods were given. The method of clutter cancellation and interference process was applied to suppress the clutter in subaperture echoes which improves the detection performance of slow moving target. Further more, MTI methods under low Signal to Noise Ratio (SNR) was studied, and the simulation and measured data have verified the effective of the methods.For the doppler parameter estimation of moving target. The doppler centroid tracking method was put up, and doppler de-ambiguous method base on migration estimation to solve the problem of doppler centroid estimation; the theory of Matching Fourier Transform (MFT) was descripted, and derive the expression of high-level MFT. Finally, a novel doppler rate estimation method from MFT through the analysis of MFT statistical characteristics on doppler parameter estimation.By introducing the concept of the instantaneous rate of doppler frequency, we propose a time-frequency rate distribution method. Based on the complex structure of the time delay model, it is presented to estimate the high-order phase of acceleration target echoes. Through the doppler parameter estimation, we obtain the radial, tangential velocity and acceleration of the target, and provide necessary conditions for high-quality imaging. For non-uniform linear moving targets, a imaging algorithm with high-order match function is proposed.Simulation and Experiment prove it effective.Research on high-resolution imaging of moving target, marine vessels were studied, with analyzing doppler frequency characteristics of target under three-dimension rotation (yaw, pitch and roll), and reveal the key technology with the existence of rotation. Motion compensation is an important procedure for moving target imaging, we propose a compensation method through the doppler parameters estimation in short coherent processing interval, and compensate the translational and rotational component separately, finally obtain high quality images which also reflect the movement of the target at different time. The method can resolve the defocusing problem using traditional SAR processing method when exists three-dimension rotation.Through the study of this dissertation, the methods of ground clutter reduction and target motion parameter estimation in high resolution SAR-GMTI, improve the performance of target detection and imaging. The current study is extremely useful for the development of whole SAR-GMTI system and has great potential to be employed in military area.This dissertation finally summarizes the research work, and discusses those potential research topics and directions in future.