Research Of High Frequency Method For Composite Scattering From Targets And Rough Surface

In computational electromagnetics(CEM),the scattering problem of a composite scene composed of targets and the environment is a challenging subject.In applications,the environment is usually modeled by the rough surface.Then the composite scattering from targets and rough surface has important theoretical significance and engineering application value in many fields such as remote sensing.Among many CEM methods,the high frequency method have received extensive attention and research because of its accuracy and high efficiency.This dissertation studies the application of high frequency method especial shooting and bouncing ray method(SBR)on anylyzing composite scattering of targets and the rough surface.The main research contents and innovations of this dissertation can be summarized into following aspects.(1)In order to overcome the obstacle that kd-tree construction consumes too much time in SBR,a point-based kd-tree construction method(PKCM)is proposed to replace the traditional surface-based kd-tree construction method.In SBR,to improve its effciency,an adaptive space partitioning structure named kd-tree is introduced to reduce the search time for intersections.For ensuring the performance of kd-tree,Surface Area Heuristic(SAH)was used to determine the optimal splitting position of each partition while building kd-tree.Because the value of SAH is depended on facet information,global search and calculate the SAH value of all condidate positions is inevitable and then consumes much time in kd-tree construction.In this case,PKCM replaces the facet information in SAH with vertex information,thereby simplifying the calculation of SAH and greatly narrowing the search range.The simulations show that the kd-tree construction time of PKCM is about 10% compared with that of traditional construction method.(2)In order to improve the accuracy of high frequency method in analyzing the scattering problem when space exsists the dielectric medium,this dissertation first introduces volume equivalence principle into the high frequency method,and then volume equivalent shooting and bouncing ray(VESBR)is proposed to analyze the scattering problem of the scene that space exsists dielectric medium.Based on the analytic solution of a plane wave incidence on an infinite metal plate,the traditional high frequency method is proposed and developed.According to surface equivalence principle,traditional high frequency method can calculate the scattering of metal structure and thin coated structure.But it is unable to correctly consider the contribution and propagation of electromagnetic waves inside the medium,so it is not suitable for analyzing electromagnetic scattering of dielectric and composite objects with thick dielectric medium.Based on volume equivalence principle,VESBR comprehensively considers the propagation and contribution of the electromagnetic wave inside the dielectric medium.To analyze the propagation of electromagnetic waves,based on the assumption that electromagnetic waves travel in straight line,VESBR only consider the particle property of electromagnetic waves and ignores their wave property.Then,a new formula is derived to analyze the propagation of reflection and transmission waves at all types of dielectric boundaries when a plane wave is incident.For quickly calculating the volume integral,VESBR transforms the volume integral in a ray tube into surface integrals on its surface based on Gauss’ s law.Compared with full wave methods,simulations show that VESBR is accurate and efficient for anlalyzing the scattering of pure dielectric objects,composite objetcs with arbitrary thickness of medium and hierarchical inhomogeneous objetcs.(3)On the basis of VESBR,this dissertation proposes dynamic volume equivalent shooting and bouncing ray(DVESBR)to efficiently and accurately analyze the scattering of ships sailing on the sea.When ships sailing on the sea,sea surface is varied because of the effect of many factors such as sea wind,and ships are moved.Therefore,the geometric model of the scene is changed at different moments.Besides,due to the draft depth of ships is greater than zero,ships are partially buried in the sea that it is unable to find out a closed surface to surround the scene which means surface equivalence principle is invalid.In general,the geometric model of targets and sea surface are constructed independently,and the model of sea surface is rough surface.In order to account the contribution of volume equivalent sources when using surface geometric model,DVESBR deduces the volume equivalent physical optics(VEPO)based on VESBR.In addition,in order to ensure the correct boundary conditions,tradition CEM methods must treat the geometric model to eliminate the effects of overlapping part.In DVESBR,with a new separated ray tracing strategy,ray tracing replaces geometric model processing to eliminate the effects of overlapping part.Therefore,DVESBR avoids wasting time to treat geometric model at each moment.In this case,DVESBR first time solves the three dimensional scattering problem of targets partially buried in a rough surface,and quickly calculate the scattering of dynamic scenes.Simulations prove the accuracy and efficiency of DVESBR for calculating composite scattering from targets and rough surface.Based on the SBR framework,this dissertation analyzes several critical problem in using high frequency methods when calculating composite scattering from targets and rough surface.And many new methods have been proposed to overcome the obstacles of high frequency method,such as the low efficiency in solving electrically large problems,the inaccuracy when space exsists dielectric media,and difficulty when solving the scattering problem of targets partially buried in rough surfaces etc.Finally,this dissertation realizes the efficient and accurate calculation of composite scattering from targets and rough surface on both static and dynamic conditions.