Study On Imaging And Motion Compensation In Highly Squinted Missile-borne SAR

In modern battlefield,missile-horne radar is gaining more and more attention.To achieve high-precision navigation and attack,the missile-borne radar usually has the following three working modes:low-resolution searching,high-resolution imaging,and high-resolution tracking.During low-resolution searching,the radar beam performs large-swath scanning and target detection to select some targets of interest.In high-resolution imaging mode,the high-resolution microwave images of the selected potential targets are generated,which is used for automatic target recognition.Based on the recognition result,the most important target is detected.Then the radar turns into the high-resolution tracking mode.The target position information is real-time measured for navigation guidance,and optimal attack point is selected for final attack.This dissertation studies the synthetic aperture imaging technique,which is used in the high-resolution imaging mode.The main content of this dissertation is summarized as follows.1.The first part focuses on the fast time-domain imaging for high-squint SAR.An adaptive fast factorized back-projection(AFFBP)algorithm is proposed.In AFFBP,target detection technique,which is used as a discrimination tool to retain the target pixels and reject the clutter and noise pixels,is integrated into the sub-aperture imaging chain during FFBP implementation,aiming to speed up the overall image formation time.Due to the characteristic of spotlight acquisition,an adaptive processing scheme is developed for multiple sub-aperture images detection.With the integrated target detection technique,only the target pixels need to be interpolated and accumulated coherently.Without loss of focusing performance,the number of interpolations is dramatically reduced.The proposed AFFBP algorithm has distinctive superiority for sparse scene reconstruction,such as the maritime target imaging.2.The second part studies the range-variant autofocusing technique.Based on the conventional map-drift(MD)method,the extended MD(EMD)algorithm is proposed,which is capable of estimating and correcting range-variant phase error induced by motion error in high-squint SAR.The conventional MD method is suitable for broadside mode,and only deals the spatial-invariant motion error.In high-squint SAR,motion error usually presents non-negligible range-variance,which limits the application of MD method.The proposed EMD algorithm mainly contains two processing steps:1)the range-invariant phase-error is firstly retrieved by a squinted MD(SMD)estimator;and 2)a squinted range-dependent MD(SRDMD)estimator is proposed to derive the residual range-variant phase-error.By correcting the phase-errors obtained from SMD and SRDMD,the highly squinted SAR data can be finally focused.3.The third part presents the 3-D trajectory deviations estimation algorithm,which adopts the weighted total least square(WTLS)kernel to improve the estimation precision.First,multiple local scenes at different illumination directions within the radar beam are selected and synthesized using back-projection algorithm,generating multiple local-images.Then the selected local-images are autofocused based on the criterion of maximum sharpness,outputting multiple local phase error functions.A gradient function considering smoothness regularization is developed to avoid local-minimum sharpness-maximizing solution.In the next,the local phase error functions are combined to retrieve the residual 3-D trajectory deviations by a WTLS kernel.By updating the trajectory parameters,well-focused SAR image can be obtained.4.The fourth part proposes a 2-D space-variant autofocusing algorithm,which can estimate and compensate the range and cross-range variant phase errors simultaneously.A 2-D space-variant phase error model is established.To resolve this phase error model,multiple local-images are extracted automatically through a 2-D sliding window operation.Then these local-images are autofocused by the weighted squint phase gradient autofocus(WSPGA)kernel,generating multiple local phase error functions.Considering estimation accuracy,the derived local phase error functions are grouped to determine the unknown space-variant parameters in the phase error model using a weighted total least square(WTLS)method.In the next,pixel-wise phase error correction can be performed within the procedure of FFBP formation.As an excellent result,the highly-squinted SAR data with unknown motion errors can be focused precisely.5.The fifth part studies the problem of the moving target imaging in high-squint SAR.A high-order phase error correction-based ground moving target imaging(HPC-GMTIm)algorithm is proposed.The maneuvering motion of military target can not be neglected,which causes high-order phase terms in the echoed data.In the proposed HPC-GMTIm algorithm,the whole coherent processing interval(CPI)is divided into multiple subapertures.In each subaperture,the target velocity is assumed to be constant.This assumption is acceptable since the subaperture CPI is very short.As a result,the subaperture signal can be reduced to a three-order phase function.From this three-order phase function,the instantaneous Doppler frequency(DF)can be obtained,using Hough transform and fractional Fourier transform for Doppler centroid and Doppler rate estimation,respectively.The next step is to estimate the signal phase in the whole CPI.The subaperture DF is combined into a linear system of equations,and the whole CPI phase coefficients are determined by solving a total least square problem.The HPC-GMTIm algorithm can estimate and correct more high-order phase terms in comparison with the existing methods.