Research On Forward Scatter Radar Air Moving Target Parameter Estimation And SISAR Imaging

As a kind of bistatic radar of special topology,forward scatter radar employs the enhancement of radar cross section(RCS)in forward scatter region and has great advantages in detecting stealth targets and low-slow-small targets compared with traditional monostatic radar.Moreover,if applying the shadow inverse synthetic aperture radar(SISAR)imaging theory,it can extract target shadow profiles for recognition.Thus the forward scatter radar has attracted attentions of researchers around the world in recent years.At present,most of the forward scatter radar signal processing techniques,including parameter estimation and SISAR imaging,are only applicable for ground target detection in cases of short baselines.However,forward scatter radars for air target detection have longer baselines,higher target velocities and longer observation time.Therefore,studies on its special echo features,signal modeling and processing methods are urgently desired.In this thesis,investigations on signal modeling and analysis,motion parameter estimation and SISAR imaging for forward scatter radar detection of air moving targets were made.Main contents and productions are as follows:1)Modeling and characteristics analysis of forward scattering echoes of air moving targets were investigated.Traditional ground-target forward scattering signal model has not considered the impact of scattering phase,thus it is not accurate enough for describing air-target echoes.To solve this problem,a complex scattering coefficient echo model was proposed based on the analytical deduction of complex scattering coefficient.The novel model separately expresses the target scattering characteristics and Doppler,greatly facilitating the theoretical analyses of echo characteristics.Then,theoretical and simulation investigations on the amplitude and phase characteristics of forward scattering echoes of air moving targets were made based on the complex scattering coefficient model.Theoretical explanations for the periodical fluctuation of far-field echo amplitude,periodical saltation of far-field echo phase,shadow blocking of nearfield echo amplitude,and slow variation of near-field echo phase were given,respectively.At last,the boundary between near-field forward scattering and far-field forward scattering was quantitatively defined,which laid the theoretical foundation for corresponding signal processing algorithms.2)Air target motion parameter estimation methods for forward scatter radar were studied.Since the scattering coefficient and random initial phase are not considered in current parameter estimation methods for ground-surveillance forward scatter radar,methods for estimation of Doppler variation rate based on time-frequency analysis and match filtering were proposed.In addition,a novel method for global motion parameter estimation based on least square was also given.These methods effectively avoid the interference of scattering phase and initial phase.Then,to overcome the disadvantages of low robustness and low accuracy for current track initialization methods,a 3-dimensional track initialization method based on least square was proposed.This method not only guarantees the practical implementation but also greatly improves the performance of track initialization.3)Accurate SISAR imaging techniques for forward scatter radar air target detection were investigated.A novel radio holographic signal(RHS)reconstruction method,which was composed of a modified Hilbert transform and a fitting of main lobe envelop,was proposed.This modified RHS reconstruction method can accurately reconstruct the RHS.Meanwhile,to solve the motion compensation problem in large-diffraction angle SISAR imaging,the time-scale transform relationship between complex scattering coefficients and carrier frequencies was discovered.Next,a method for large-diffraction angle SISAR motion compensation and imaging using dual frequencies was given,by which the history of slant range sum can be accurately estimated and the accurate motion compensation in large-diffraction angle SISAR can be realized.4)The target detection and imaging techniques for passive forward scatter radar based on global navigation satellite system(GNSS)were exploited.Firstly,the GNSS forward scattering signal models were built based on theoretical deductions.It was also proved that the Doppler signal was linear-frequency modulated.Meanwhile,the GNSS SISAR imaging method was given based on complex scattering coefficient model.In outfield experiments,GNSS forward scattering signals of airliner targets were collected.SISAR shadow profiles using GNSS signals were obtained for the first time.The processing results of experiment data verified the correctness of signal modeling and the possibility of GNSS SISAR imaging.