Research On Highly Squinted SAR Imaging Based On Electromagnetic Model

As an active microwave sensor,synthetic aperture radar(SAR)can be used to obtain a high-resolution SAR image of a desired scene at different squint angles and frequencies on different platforms.It is able to work at any time of day or night under all weather conditions.Therefore,it has unique advantages and broad application prospects in civilian and military fields.In terms of the processing of common side-looking and low-squint(the squint angle is not larger than 45°)SAR echo,the traditional SAR imaging algorithms can meet the requirement of focus quality.However,for the highly squinted SAR imaging(the squint angle is greater than 45°),the large squint angle makes the SAR imaging algorithms become more complex,and the traditional SAR imaging technology is difficult to satisfy the demand of SAR imaging.Thus,the study on the SAR imaging technology is carried out through analyzing the signal model and echo characteristics of the highly squinted SAR in this thesis.On the other hand,for the investigation of SAR imaging algorithms,the present researches mainly focus on proposing algorithms and validating them using a point target or measured data,and the theoretical simulation of SAR images from a rough sea scene,a complex target or a composite target-ocean scene is rarely.In fact,by applying the simulation of SAR images to a rough sea scene,a complex target or a composite target-ocean scene can not only make us understand the interaction mechanism between the electromagnetic wave and targets,but also provide a strong foundation for the marine remote sensing,target detection and recognition and SAR image interpretation.Based on the squint echo model and characteristics,this thesis studies the highly squinted SAR imaging algorithms and presents three modified highly squinted SAR imaging algorithms.Finally,the feasibility and superiority of the proposed algorithms are analyzed through simulation results.The main work of this thesis includes the following aspects:A procedure for simulating the highly squinted SAR images of a composite target is proposed by combining the electromagnetic scattering theory and the working principles of a SAR.In this process,the combination of geometrical optics and physical optics(GO-PO)method is used to analyze the scattering characteristics of the target,which lays the foundation to obtain the SAR echo efficiently.Then a SAR imaging algorithm can be employed to obtain the SAR image.On the basis of the combination of the capillary wave medication facet scattering model and the GO-PO method,a simulation method of highly squinted SAR images from a composite ship-ocean scene is proposed according to the work principle and imaging features of SAR.First,the composite scene is divided into small facets,then the facet-based electromagnetic scattering model is used to calculate the spatial distributed radar cross section.Then the SAR echo can be generated,based on which the SAR image can be readily obtained.This method can accurately reflect the electromagnetic scattering characteristics of a composite ship-ocean scene,and it can meet the application demand of SAR image interpretation,target recognition and detection.The traditional frequency scaling(FS)algorithm is unable to compensate the second range compression(SRC)error completely in highly squinted mode,which leads to the defocusing of SAR images.Therefore,this thesis proposed an amendatory FS algorithm based on the traditional one,which uses the nonlinear frequency scaling operation to eliminate the SRC error.Together with the simulation procedure of SAR echo from a target and a composite ship-ocean scene,the highly squinted SAR images are simulated and the effects of the incident angles and the squint angles are analyzed.A modified range Doppler(RD)algorithm is proposed based on the traditional one to deal with highly squinted SAR echoes.The RD algorithm is a commonly used algorithm.However,with the increasing of squint angle,the range cell migration(RCM)increases and it no longer meets the requirement of highly squinted SAR imaging.Accordingly,this thesis analyses the feature of the high squinted SAR echo and it shows that the SAR echo has a large range walk and small range curvature.Moreover,the effect of the approximation distance between the SAR platform and the target on the echo phase is analyzed.According to the analyses,it is found that the higher-order Taylor expansion should be employed to meet the requirement of high resolution.Then,the modified RD algorithm is proposed,and it avoids the interpolation operation,which improves the processing efficiency by about 70 times.Finally,the simulated SAR images of complex targets show the good ability of the proposed algorithm.An improved fourth-order spectral analysis(SPECAN)algorithm is proposed,which is able to deal with highly squinted SAR echoes.Based on the traditional SPECAN method,the improved method is proposed by considering the characteristics of highly squinted SAR echo.Firstly,the impact of RCM on the azimuthal phase is studied by analyzing the highly squinted SAR echo.Then,a fourth-order phase mode is employed to enhance the accuracy of azimuthal phase.Moreover,this improved algorithm only contains the fast Fourier transform operation and the complex multiplication operation,which makes it able to efficiently process the highly squinted SAR echo.Finally,the effectiveness of the improved method is verified by simulation results.