Research On Airborne Highly Squint SAR Imaging Method

Airborne highly quint Synthetic Aperture Radar(SAR)has the capability to detect targets in the front squint area of the aircraft in advance.Compared with side-looking SAR,the flexibility and detection range are greatly enhanced.The range cell migration of airborne highly quint SAR is very large,which results in the serious range-azimuth coupling and the strong spatial-variant of linear range cell migration.In addition,the non-ideal motion of the airborne platform will cause the range cell migration errors and spatial-variant phase errors in the echo,which will seriously affect the image quality.Therefore,in order to achieve high-resolution highly squint SAR image,it is of great significance and value to study the imaging algorithm of airborne highly squint SAR.Focusing on the key problem involved in airborne highly squint SAR imaging methods,the theoretical research and numerical simulation are carried out in this thesis.The main contents are as follows:1.The echo and motion error models for airborne highly squint SAR are built.According to the geometrical configuration of highly squint SAR,the author analyzes the range cell migration characteristics and Doppler characteristics.Then,the range cell migration error and the spatial-variant characteristics of Doppler phase error are studied.Finally,the author analyzes the effects of motion error on echo characteristics,frequency spectrum and imaging results.2.The high-precision range cell migration correction method for highly squint SAR is studied to solve the problem of the spatial-variant range cell migration and high-order errors.This work achieves correction of range cell migration with accuracy smaller than a range resolution by the spatial-variant linear range cell migration correction,the high-order range cell migration correction and the unknown range cell migration errors compensation.3.A Doppler centroid estimator for highly squint SAR based on phase center point locking is proposed.The phase center point of Doppler signal is tracked and locked by binary stage optimization.The proposed method can estimate the Doppler centroid more exactly and rapidly.4.Focusing algorithm for highly squint SAR imaging based on optimal nonlinear chirp scaling(CS)is proposed to solve the problem of nonlinear spatial-variant and unknown Doppler parameters of highly squint SAR.Through the nonlinear CS processing and the optimal estimation of the scaling coefficient,the high resolution imaging of the airborne highly squint SAR with motion errors is realized.The effectiveness of the research contents above are verified by simulation experiments.The simulation results demonstrate that the methods proposed above are able to solve the key problem in airborne high-resolution highly squint SAR imaging effectively.