Research On Time-Domain Algorithm For Transient Response Of Target Above Half Space

The research on the composite electromagnetic scattering characteristics of the target above half space is widely used in the target detection and recognition,radar imaging,resource detection,space-based telemetry and remote sensing and so on.The calculation method of target electromagnetic scattering of the half space needs to consider the irradiated wave reflected by the interface and the coupling between the target and the background,and its scattering characteristics are quite different from those in free space.The calculation method of the transient electromagnetic scattering of the targets over the layered half space is proposed in this paper.The innovational works of this paper are the calculation of the time-domain reflection and transmission coefficients of the layered lossy half space,the fast calculation of time-domain far-field of the arbitrarily oriented dipole over the half space,the improvement of the Finite Difference Time Domain(FDTD)method and the Time Domain Shooting and Bouncing Rays(TDSBR)method of the half space.The main contributions of the dissertation are as following:(1)The calculation of the time-domain reflection and transmission coefficients of layered lossy media.In the transient algorithm of composite electromagnetic scattering of the target above half space,the key problem is to obtain the time-domain reflected and transmitted waves.Firstly,the time-domain reflection and transmission coefficients of the layered lossy media are deduced using the time-domain transmission mode method.Secondly,the total time-domain reflection and transmission coefficients of the layered lossy media combined with the recurrence formula of time-domain reflection and transmission coefficients are obtained.Then,the time-domain reflection and transmission coefficients of the layered electrical and magnetic lossy media and uniaxial anisotropic media are derived.The proposed methods is almost an analytical algorithm with high calculation accuracy and clear physical meaning,which lays a solid foundation for the introduction of incident wave above the layered half space and the transient far-field extrapolation from the transient scattering of the targets over the layered half space.(2)The time-domain analytical expressions of the far radiation fields from an arbitrary polarized electric dipole above the half space are deduced when the observation point is far from the interface.The electromagnetic scattered fields from targets above the half space can be equivalent to the radiated fields from the electric and magnetic dipoles above the half space.Therefore the radiation problem of the dipole above half-space is one of the fundamental problems of the electromagnetism.In order to avoid the calculation of the complex half space Green’s function and improve the calculation efficiency of the radiation field of the dipole above the half space,the time-domain analytical formulas of the far radiated fields from an arbitrary polarized dipole in a pulse waveform above half-space is deduced.The fast calculation of the transient radiation field of arbitrarily oriented dipole over the half space is realized.The numerical results verify the accuracy and validity of the proposed method.(3)The research on the algorithm of the transient composite scattering characteristics of the target over the layered half space based on the FDTD method.The traditional electromagnetic scattering calculation of targets above the half space mostly adopts integrated modeling,which consumes a lot of computing resources.In order to improve the computational efficiency,the calculation area of the FDTD method in this paper only includes the target itself.The incident wave in the FDTD method includes the direct incident wave and the reflected wave from the half space interface.According to the equivalence principle,the incident wave is introduced into the calculation region along the direction of the incident wave and the direction of the reflected wave in the half space by the FDTD connection boundary.Then,the equivalent surface electromagnetic current on the extrapolated boundary is calculated.The compound transient scattering field of the target above the half space is calculated by using the formula of the time-domain far-field of the dipole over the half space.The broadband scattering characteristics of the target can be obtained by the Fourier transform.(4)The research on the algorithm of the transient scattering characteristics of the target based on the analytical integral TDSBR method.In order to solve the electromagnetic scattering calculation of electrically large targets,the TDSBR algorithm based on analytic integral is proposed in this paper.Compared with the traditional SBR,this new method obviously reduces the number of ray tubes and can saves computer memory.It is an efficient method for analyzing the scattering problem of electrically large objects.Then,in order to calculate the transient scattered field of the electrically large targets of the lossy half space,the TDSBR algorithm is improved.In ray tracking,the time-domain reflection field can be calculated by the time-domain reflection coefficient.After tracking,the time-domain current on the target surface is extracted,and the transient electromagnetic scattering characteristics of the target are calculated and analyzed using the time-domain analytical expressions of the far radiation fields from a dipole above half-space.