Study On Computationally Efficient Method For SAR Focusing Of Fast Maneuvering Targets

Synthetic aperture radar(SAR)can perform high-resolution imaging of the observation area of interest.It has all-time and all-weather characteristics and is widely used in military and civilian fields.With the growing demand of SAR users for moving target information,ground moving target indication has become an important function in modern SAR systems.It can improve the capability of SAR systems to sense moving target information.However,many fast maneuvering targets,such as vehicles speeding on the road,weapons with high maneuvering on the battlefield,and speedboats moving at high speed on the sea,are bound to appear in the scene due to the increasing complexity of the observation environment.The fast maneuvering targets have the characteristics of high value and threat.The imaging of fast maneuvering targets is of great significance in the applications of battlefield situation assessment,traffic monitoring,and antiterrorism at sea.Nevertheless,the relative movement of fast maneuvering targets is highly complicated.The resulting defocus and Doppler ambiguity will be serious and complicated.The moving target focusing is difficult due to their noncooperative property,which poses a great challenge to SAR systems.For this reason,this dissertation studies the problems of serious defocusing of moving target energy,focusing difficulties caused by complex Doppler ambiguity,and multidimensional parameter search leading to excessive calculation complexity and affecting algorithm implementation.The main contributions of this dissertation are summarized as follows.First,aiming at the problems of serious two-dimensional migration and Doppler ambiguity of moving targets,a corresponding fast moving target signal model is established in accordance with the relative motion model of SAR and moving target,and the characteristics of the signal model are analyzed in detail.A fast moving target-focusing method based on an efficient radial velocity-matching operation is proposed.This method avoids the problem of azimuth Doppler spectrum splitting by constructing a spectral compression function.After the second-order Keystone transform(SOKT)is used to eliminate the range curvature migration(RCM)accurately,the efficient velocity-matching operation and Lv's distribution are applied to estimate the target signal parameters.The moving target is focused on the range-Doppler domain.The theoretical analysis and simulation results show that the proposed method can accurately eliminate the range migration(RM)and linear Doppler frequency migration(LDFM)of moving targets.It has good focusing performance in the presence of Doppler ambiguity.Compared with the traditional multidimensional parameter search method,the proposed method shows greatly reduced computational complexity.The simulation data-processing results verify the effectiveness of the proposed method.Second,aiming at the disadvantage that the traditional SOKT can eliminate only RCM and the effect of LDFM still exists,a modified SOKT(MSOKT)is proposed.This transform can accurately compensate RCM and LDFM,simplifying the focus-processing steps.A fast moving target-focusing method based on MSOKT is proposed.After the Doppler center shift and range walk migration are eliminated by using the proposed radial velocity-matching function,the remaining RCM and LDFM are simultaneously eliminated using MSOKT.The focusing results in the range and azimuth time domain are obtained using the azimuth reference function.This method can accurately eliminate RM and LDFM.The proposed algorithm exhibits low computational complexity because the combined processing of range and azimuth simplifies the algorithm steps and the nonuniform fast Fourier transform(NUFFT)is used to accelerate the implementation.Compared with traditional Keystone transform(KT)-based methods,the proposed method shows strong tolerance of Doppler ambiguity.The simulation and measured data-processing results verify the effectiveness of the proposed algorithm.Third,aiming at the problems of complex RM and Doppler frequency migration(DFM)of fast maneuvering targets and complex and diverse distribution of azimuth Doppler spectrum,a signal model of the fast maneuvering target is constructed,then a focusing method for ground fast maneuvering targets is proposed.This method considers not only low-order RM and DFM but also the effects of high-order quadratic RCM and quadratic DFM.For Doppler ambiguity,the proposed method considers that the Doppler spectrum occupies not only one and two pulse repetition frequency(PRF)bands but also multiple PRF bands.The mismatch of the proposed RCM compensation function is eliminated through the improved azimuth bandwidth compression function combined with SOKT.A corresponding identification procedure based on the analysis of the multitarget cross-interference characteristics is proposed to eliminate the influence of cross-term spurious peak.The proposed method can obtain well-focused performance because the effect of the third-order phase is solved.Two constant factors are introduced to expand the applicability of the proposed method.The proposed method has low computational complexity,given that it can be implemented using the fast Fourier transform,fast inverse Fourier transform,and NUFFT.The simulation and measured data-processing results verify the effectiveness of the proposed focusing method and spurious peak identification procedure.Fourth,aiming at the problems that the approximate compensation by using a priori information,such as scene parameter and platform speed,for the method proposed in the third part affects the application of this method to different scenarios,a generalized third-order signal model of the maneuvering target is established.The model has a wide applicability,for the focusing of not only SAR ground fast maneuvering targets but also for ground-based radar high maneuvering targets.A modified generalized high-order KT(MGHOKT)based on MSOKT is proposed.This transform shows wide application potential.Based on MGHOKT,an efficient focusing method for a generalized third-order range model maneuvering targets is proposed.The proposed method can accurately compensate for the effects of high-order RM and DFM caused by the generalized third-order phase.The proposed method can process multiple targets simultaneously without the search of Doppler ambiguity number for each target and effectively identify multitarget cross terms.The proposed method has strong tolerance of complex Doppler ambiguity,including Doppler center blur and Doppler spectral ambiguity.The computational complexity of the proposed method is low because only two simple processing steps are required after range compression.The simulation and measured data-processing results verify the effectiveness of the proposed method.