Research On Imaging Algorithm For The Forward-Looking And Squint High Resolution SAR

With the ability of day&night,all wearth condition,and long range imaging on observing areas,synthetic aperture radar(SAR)plays an important role in civil and military applications.To obtain the SAR image of the airborne platform squited fronter areas,researches on the novel SAR systems have been carried out in the SAR community at home and abroad.By means of adjusting the beam pointing,squinted SAR can realize the imaging of the front areas of the airborne platform without flying this area,that is,squint SAR has the capability of imaging on the front target.Besides,azimuth synthetic aperture can be obtained based on the the radar antenna rotating.Thus,helicopter-borne forward-looking rotating array synthetic aperture radar(HBFL-RoASAR)can achieve two dimensional imaging.It has more advantages such as high spatial and temporal resolution,short revisit period,forward-looking imaging capacity,simple structure and low cost,and two dimensional images can be obtained without the necessity of any movement of the helicopter platform.Therefore,as a new imaging configuration for the front targets of airborne platform,the HBFL-RoASAR and highly squinted SAR could be widely used in both military and civilian fields,and have become hot topic in recent years.This thesis starts the research around the principle and imaging algorithm of the HBFL-RoASAR and highly squinted SAR.The main contents and key innovations in this thesis are as follows:(1)The linear range cell migration(RCM)correction and inherent range-dependent squint angle in the case of high-resolution highly squinted synthetic aperture radar(SAR)imaging result in two-dimensional(2-D)spatial-variant RCMs and azimuth-dependent Doppler parameters(i.e.,highly varying Doppler centroid and frequency modulation(FM)rates).To overcome these problems,a new imaging algorithm for highly squinted high-resolution SAR is proposed.First,the linear RCM is removed with reference range RCM to mitigate the range-azimuth couples in the range frequency and azimuth-time domain.And then,a new perturbation function is designed in the extended azimuth nonlinear chirp scaling(EANLCS)algorithm to overcome the azimuth-dependent RCM and to equalize Doppler parameters.Next,a modified CS(MCS)algorithm is proposed to correct the range-dependent RCM and realize range and azimuth large swath high-resoltion imaging.The analysis of computational complexity,imaging scene extended performanece,and incorporating the motion compensation into the proposed algorithm are provided.Finally,simulation results demonstrate the effectiveness of the proposed algorithm.(2)Due to the dependence of the azimuth resolution of HBFL-RoASAR on the azimuth reconstruction angle,to enhance high resolution of azimuth dimension,it is necessary to expand the slant range expression to higher order polynomial.However,the accurate analytical expression of the two-dimensional spectrum is impossible to obtain in this case,and thus,it is difficult to design and implement an efficient frequency domain imaging algorithm.To overcome this problem,a new approximation phase equation is expressed according to Taylor expansion of two variable functions.Based on this new expression,a high-resolution HBFL-RoASAR imaging algorithm based on the generalized keystone transform(GKT)is proposed,in which the problem of the accurate two-dimensional spectrum analytical expressions derivation is avoided.To efficiently implement the proposed algorithm,the nonuniform fast Fourier transform(NUFFT)is utilized to substitute the interpolation in GKT to reduce the computational complexity.Finally,computer simulated results validate the effectiveness of the proposed HBFL-RoASAR imaging algorithm.