| High-resolution has always been the main direction of the theoretical research and technological development of synthetic aperture radar(SAR).However,with the improvement of the resolution,the models and algorithms based on ideal linear trajectory cannot meet the requirements of imaging accuracy.The nonlinear problem of the real trajectory results in two-dimensional spatial-variation errors in the envelope and phase term,which seriously affects the final imaging quality.This thesis focuses on two conventional systems,spaceborne and airborne SAR,and studies some key issues in the high-resolution SAR imaging process.The main contents of this thesis are summarized as follows:Firstly,a high-resolution and efficient imaging algorithm based on a nonuniform PRF(Nu PRF)design is proposed to address the problems of orbital curvature and azimuth spatial-variation velocity in side-looking spaceborne SAR.Based on the nonlinear trajectory,a high-accurate equivalent acceleration model is established.The impact of azimuth spatialvariation velocity is eliminated by performing Nu PRF design at the radar hardware system,which effectively avoids the degradation of imaging efficiency caused by conventional interpolation processing.Then,the highorder error terms induced by the orbital curvature are compensated uniformly using the center point as a reference.After converting the nonlinear trajectory into the ideal linear trajectory,high-resolution imaging is completed under the real-time processing framework of overlapped subaperture algorithm(OSA).The experimental results show that this algorithm is effective in dealing with the nonlinear trajectory and achieving high-resolution imaging of all targets.Secondly,a high-resolution imaging algorithm based on a modified range migration algorithm(MRMA)and Nu PRF design is proposed to address the problems of orbital curvature and two-dimensional spatial-variation velocity in squint spaceborne SAR.Compared with the side-looking mode,both the velocity and orbital curvature are range spatial-variation,which is nonnegligible due to the severe two-dimensional coupling in squint mode.Therefore,it is insufficient to use the central point parameters for unified processing,while the residual errors can lead to severe defocusing in the image.Based on the squint range model,the azimuth spatial-variation velocity is eliminated through the Nu PRF design.After uniformly compensating for the non-spatial-variation orbital curvature errors,the Stolt interpolation kernel is modified to simultaneously solve the problems of the range spatialvariation velocity and range spatial-variation higher-order error terms during the interpolation process.Then,the high-resolution imaging is completed after eliminating the residual effects of the nonlinear trajectory.The experimental results demonstrate the high-resolution imaging effect and efficiency advantage of the proposed algorithm.Thirdly,aiming at the problem of envelope defocus and low signalto-noise(SNR)caused by motion errors in squint airborne SAR,a highresolution autofocusing algorithm based on a hybrid MD-PGA framework is proposed.In the first step,the map drift(MD)algorithm is used iteratively to estimate and compensate for most of the low-frequency error roughly,which does not require a high SNR.In the second step,the phase gradient autofocus algorithm(PGA)is used to accurately estimate and compensate for the residual errors based on a relatively high SNR.The final high-resolution SAR imaging is obtained after eliminating all the motion errors.The experimental results show that the proposed algorithm can achieve high-resolution autofocusing in the situation of severe defocus and low SNR.Finally,a full-aperture high-resolution autofocusing algorithm based on IQE-NCS is proposed to solve the problem of high-order azimuth spatialvariation motion error in wide-beam squint airborne SAR.The azimuth spatial-variation error is established as a fourth-order polynomial model to ensure accuracy.A cubic-order and fourth-order nonlinear chirp scaling(NCS)is introduced to compensate for the high-order azimuth spatialvariation error based on the polynomial model.After the derivation and analysis,the autofocusing problem is gradually transformed into a parameter optimization problem based on image quality evaluation(IQE).Then,the multi-parameter dimensionality reduction and separation are performed according to the parameter characteristics.And the final high-resolution image is obtained after determining the optimal parameters.The experimental results demonstrate that the proposed algorithm can effectively improve the imaging quality of wide-beam SAR and achieve the goal of fullaperture high-resolution autofocusing. |
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