Application Of Finite Difference Time Domain Method And Physical Optics Method In Composite EM Scattering From Rough Surface And Target

In this dissertation,the research on composite electromagnetic(EM)scattering of rough surface and target is carried out based on the two methods: finite difference time domain(FDTD)method and physical optics(PO)method.FDTD is a numerical method in time domain.It simulates the propagation of EM wave and the interaction between EM wave and matter by recurrence of difference between electric field and magnetic field in space and time.Therefore,it does not need to solve Green's function and matrix equation.FDTD is simple and flexible.It can deal with models with complex structure and medium composition.However,it is difficult to simulate the scattering from electrically large scatters quickly and efficiently.PO is an approximate method in frequency domain.It calculates the induced currents on the surface of scatterers basing on geometrical optics approximation.The scattering field of PO method is represented as an integral of the induced currents.Its physical concept is clear and easy to program.PO can solve the problem of electrically large scatters quickly with less computer resource.However,the results obtained in large scattered angles are inaccurate.In this dissertation,FDTD is applied to calculate the EM scattering of a target coated with double negative material(DNM)and the rough surface.PO is applied to calculate the EM scattering of a target and large size rough surfaces.The main work is as follows:1.Maxwell equations in the DNM model are reconstructed by introducing auxiliary polarized EM current density with Drude dispersion model.The propagation characteristics of EM wave in one-dimensional(1D)and two-dimensional(2D)DNM are simulated by FDTD method.The radar cross section(RCS)of 2D target coated with DNM is calculated by FDTD.The result shows that the DNM coating can effectively reduce the RCS of target.The composite scattering characteristics of 1D rough surface and target coated with DNM are analyzed by using FDTD.Moreover,the stealth mechanism of DNM is discussed in detail by analyzing the influence of plasma frequency,plasma collision frequency and thickness on the composite scattering coefficient.2.Combining PO with PO,the PO-PO method is proposed.In this method,PO is applied to calculate the scattering of a target and a rough surface respectively.The coupling fields between the rough surface and the target is solved iteratively by combining the idea of multi-path and Huygens' principle.This method is adopted to calculate the composite scattering of 1D rough surface and 2D target.The comparisons with the results from traditional method of moment(Mo M)show that the method is accurate and suitable for the computation of electrically large models.The effects of rough surface parameters,target parameters and incident wave parameters on the scattering coefficient are analyzed in detail.In addition,the Doppler spectra of a high-speed flying missile over time-varying sea surface are calculated by using this method.The influence of wind speed,incident angle,flying target height and coupling effect on Doppler spectra is analyzed in detail.3.Since PO is inaccurate in calculating primary induced currents on rough surface with large roughness,a more accurate integral equation method(IEM)is used to calculate EM scattering from rough surface with large roughness.Combining IEM and PO,an IEM-PO hybrid method is developed.The bistatic normalized radar cross section(NRCS)of rough sea surface with and without target is calculated by using this method in the high wind speed case.The results are compared with that obtained from Mo M.It shows that the two methods agree very well in most angles.Furthermore,the influence of different wind speeds,incidence angles,target scales on NRCS is discussed in detail.4.A Bi-iterative physical optics(BIPO)method with iteration on the upper and lower surfaces of a rough surface is proposed.In this method,the equivalent currents on upper surface of a rough surface are introduced to the lower surface of the rough surface according to the boundary conditions.Thus,the induced currents on the upper surface and a target above the rough surface are calculated by the iterative process in PO-PO method.Meanwhile,the induced currents on the lower surface and a target below the rough surface are also calculated by the same iterative process.The comparison result of BIPO and Mo M shows that BIPO can be used to calculate the semi-buried target model in rough surface,the composite model of layered rough surface and target,and layered rough surface model.The NRCS of different types of semi-buried target on sea surface,and a target above two-layer soil surface are calculated by using this method.The influence of target size and roughness on NRCS is analyzed in detail.5.The PO-PO method of 2D situation is extended to the three-dimensional(3D)situation.The formulas for calculating the composite scattering coefficients of 3D object and 2D rough surfaces are derived.The PO-PO method is investigated to calculate the difference field scattering coefficients(D-RCS)of a 3D target above 2D rough surface.The D-RCS of a coated target above rough surface is calculated by modeling the coated target as a layered structure.The PO-PO method of 3D situation is validated by comparing with commercial software FEKO and a numerical method.In addition,the scattering of 2D layered rough surface is calculated by this method,and the influence of different rough surface parameters on the NRCS is analyzed.To speed up PO-PO in iterative computation,the coupled scattering calculation is accelerated by using Open MP which is based on multi-core parallel high performance computation.The result shows that the parallel code based on Open MP can effectively reduce the computational time.