The Research On High Precise Scattering Center Modelling For Radar Targets

In recent years,scattering center is one of the important topics in the research of the electromagnetic scattering characteristics of radar targets and has attracted a great deal of attention from radar fields.The model of ideal point scattering center cannot satisfy the requirements of the techniques of high resolution radar,and more accurate scattering center models for complex radar target are urgently needed.With development of the high resolution radar,the physical information of targets that represented by radar echo and radar target images becomes richer.How to extract,understand and use these information is very important for enhancing performance of radar target detection,tracking and recognition.Scattering centers are effective bridges which connect radar echo characteristics or radar images features with the physical parameters of radar targets.Therefore,scattering centers are of great value for radar signal processing and subsequent applications,such as radar images understanding and automatic target recognition.The scattering centers of real targets have complex attributes,i.e.,the scattering amplitudes and positions of scattering centers depend closely on the frequency,observation direction and polarization of the incident waves.This paper aims to propose a method of high precise scattering center modelling to characterize various attributes of multiple scattering centers of the complex targets.The main contents of this paper are as follows:Firstly,a scattering center model for streamlined target is proposed.The existing methods of scattering center models do not include the models of sliding scattering centers of streamlined targets.Based on the thorough analysis of scattering characteristics,the scattering center features of streamlined targets are presented.Then based on these features,an accurate scattering center model for streamlined targets is proposed.Through analysis,the relationships between the location and amplitude of scattering centers on the target and the target geometry and radar orientation are derived.The simulation results verify the accuracy of this model.This model is also applicable to streamlined targets with arbitrary convex surface.Secondly,the method for scattering center modeling based on the model of induced currents is proposed.The existing methods of scattering center modelling are always based on radar images,for example,to extract the scattering centers directly from radar images by image processing,or to predict the parameteric models of scattering centers based on scattering mechanism and then estimate unknown parameters of scattering centers model by optimal matching of radar image.Considering the truth that the characteristics of scattered fields are quite complicated,however,the change of induced currents is relatively slow.Based on this,this paper proposes an approach for scattering center modellong by using induced currents.In this method,the mathematical model for the surface current are firstly built,based on which,models of scattering centers are further derived.The advantage of this method is that none radar image processing is involved,so parameters of scattering centers not affected by the quality and resolutions of radar image.Through comparison with existing methods,it is found that this method achieves higher accuracy,and it is able to describe the weak scattering centers,such as creeping wave equivalent scattering center.Thirdly,a scattering center model for electrically large deep cavity is proposed.The existing scattering center models are manily specific to targets with convex surface,and there is rare literature about scattering centers induced by multiple reflections inside the deep cavities.Lots of numerical results show that although the ray theory has errors in the calculation of the scattered waves from complex targets,it can derive the equivalent positions of the scattering center accurately.In this paper,the ray-paths of multiple reflections inside the cavities are studied through ray-tracing method,and the relationships between incident angles and the geometric parameters of the cavities,reflection times and the propagation distances are derived.Analytical description of the equivalent locations of the scattering centers is then acquired.The simulation results verify the accuracy of this model.In addition,numerical results show that the scattering amplitudes of scattering centers due to multiple reflections inside the cavities are much larger than those induced by diffracted waves from edges and end points.They are dominant scattering centers for targets with deep cavity,and should be considered carefully during post-processing of scattered signals.Finally,a method for estimating the parameters of the scattering centers from under sampling scattered waves is proposed.In this method,segmentally compressive sampling is used.In the estimation of parameters of the scattering centers,scattered data under wide scan range of aspect angles are needed(where the time-frequency representation is used in analysis)and even those under both wide band and scan range(where 2D image is used in analysis).For the electrically large target,the calculation complexity of the scattered field is very large,thus computation time is very long,which can hardly meet practical requirements.Based on the study on the aspect dependences of differnet types of scattering centers,this paper proposes a method to reduce the amount of scattered field data by using segmentally compressive sampling theory.According to the aspect dependences of scattering centers,the aspect angle range is divided appropriately,and within the zone where localized scattering centers or sliding scatterin centers dominate,only scattered fields on selected aspect angles according to the random under sampling theory are computed.A series of signal and image processing,such as time-frequency transformation,filter processing and image transformation are used in the post-processing of randomly unde-sampled data to extract the parameters of scattering cneters.The simulation results show that the data reduction rate can reach less 30% of that under critical sampling.The problem of complex computation is well relieved.In order to ensure the accuracy of all the results in this dissertation,the full wave numerical method which is used to compute scattered fields is the finite element-boundary integral-multilevel fast multiple algorithm(FE-BI-MLFMA),which has been well validated by analytical results and measured results.