Study On The Imaging Techniques For High Resolution Sar Systems

One of the goals of Synthetic Aperture Radar (SAR) is to obtain the high resolution image with well focused quality. As the wide application of SAR techniques, all kinds of SAR systems are required to have high resolution imaging ability in different operation mode, and obtain the well focused images in various flying conditions in practice. In this dissertation, some theoretical and technical problems are studied, which are met in the application of three different SAR systems when they working on the high resolution imaging mode. The three SAR systems include ultra-wideband SAR (UWB SAR), spotlight SAR and highly squinted SAR.The main content of this dissertation is summarized as follows:1. Generalized expression of SAR frequency domain algorithms is derived and the imaging performances of different algorithms for the high resolution SAR imaging are analyzed. Based on the SAR universal echo signal model, the generalized deduction of SAR frequency algorithms is presented. With the deduction, a new explanation of SAR imaging principle of the extended Omega-K algorithm (EOKA) from signal processing viewpoint is given, which is easier to be understood as comprared to the former explanations. At last, using the example of UWB SAR, comparison analyses of the performances of different frequency algorithms for high resolution SAR imaging are carried out, which form the basis for following study.2. Analyses of radar platform motion error on SAR imaging processing are carried out. The main work includes: (1) Theory analyses of the forward velocity error on SAR imaging are carried out. Based on the results, we conclude that as the SAR resolution higher or carrier frequency lower, the influence of forward velocity error worse. (2) Based on the sinusoidal trajectory deviation model, the expression of echo spectrum obtained in nonideal conditions is derived, and the season that why different trajectory deviations cause different defocus phenomenon in SAR image is found out. Moreover, a conclusion is drawn by thoery confirmation that as the amplitude or frequency of the trajectory deviation increases, it has worse influence on the SAR imaging. The analysis results form the basis for following study.3. The motion compensation (MoCo) method for high resolution UWB SAR based on raw data is studied. For the MoCo problem of low frequency UWB SAR without available measured motion data, a MoCo method based on raw data is proposed. The main works include: (1) According to the characteristic of UWB SAR raw data, a modified MoCo method using the Doppler frequency rate estimation is proposed, which improves the MoCo accuracy of UWB SAR raw data. (2) In order to remove the impacts of forward velocity error on echo range focusing, a subaperture modified Stolt interpolation processing is proposed. (3) In order to compensate the spatial variant high-order phase error in the UWB SAR real image, an image subpacthes focused method is proposed, which improves the focused quality of whole real image.4. The MoCo method for high resolution UWB SAR mounted on small size aircraft based on the measured motion data and raw data is studied. In order to compensate the large amplitude and high frequency motion error, a three-step MoCo method based on the low accuracy Global Position System (GPS) data and raw data is proposed. In order to obtain the better coarse focused result based on GPS data, a modified two-step MoCo approach is proposed, which not only has better performance on compensating the low/moderate frequency motion errors, but also has better performance on compensating the high frequency motion errors. By smoothing the trjactory deviation data with lowpass filter, the impacts of measure error induced by the GPS system is removed, and the way of the cutoff frequency determination is given.5. The imaging algorithm for high resolution squinted spotlight SAR based on the deramping-based techniques is studied. In order to resovle the imaging problem of squinted spotlight SAR in presence of the azimuth spectrum folding phenomenon, an extended two-step focusing approach is proposed. The main works includes: (1) the spectrum folding problem encountered in squinted spotlight SAR case is analyzed, and the reason of the squint angle has impact on the azimuth coarse focusing is found out. (2) To overcome this problem, a nonlinear shift method for Doppler centroid correction is proposed, and the tranditional two-step focusing approach is successfully extended for the squinted spotlight SAR imaging. (3)To resolve the azimuth mismatched filtering problem of the coarse focused echo sginal, an azimuth subscenes precise focusing method is proposed, which improves the performance of the extended two-step focusing approach for the wide swath high resolution squinted spotlight SAR imaging.6. The imaging algorithm for high resolution highly squinted SAR is studied. In order to resolve the range cell curvature correction (RCCC) problem, a bulk RCCC method with integrated the range blocks is proposed, which improves the accuracy of the range focusing. To overcome the azimuth depth of focus (ADOF) problem induced by the linear range walk correction, a modified azimuth mathed filtering based on the nonlinear chirp scaling (NLCS) technique is proposed, which achieves the high accuracy azimuth compression for the wide swath high resolution higly squinted SAR.Some research results achieved in this dissertation have been applied on the raw data imaging, which are collected by the first domestic airborne high resolution low frequency UWB SAR system and the first domestic high resolution low frequency UWB SAR system mounted on small size aircraft, and a large number of high resolution images with high focused quality are obtained. The imaging results prove that the obtained research results have well applicable value in practice.