Three-dimensional Target With Random Rough Underlying Surface Electromagnetic Scattering Fdtd Method

Research on electromagnetic scattering from a target in a complex environment has been applied in wide fields. In the limit of approximation conditions, analytic methods have difficulties to solve scattering from a complex rough surface and the model of an isolated target above a rough surface. In the recent years, two dimensional model of a rough surface and the model of a target above a rough surface have been researched widely by electromagnetic numerical methods. In numerical methods tapered incidence was introduced to overcome the edge effects of a finite rough surface simulated in computers. However, the more areas of a rough surface should be computed with the larger incident angle. In the simulation of scattering from rough surfaces, a larger number of unknowns should be computed in three dimensional model than the two dimensional model. Thus, it is difficult to solve these problems using numerical methods on a personal computer.Instead of a tapered incidence, we take a rough surface as a periodic structure to eliminate its edge effects. And the relationship between the dimension of a numerically simulated three dimensional rough surface and its correlation length was discussed, so that the simulated rough surface could satisfy characteristics of our needed rough surface. By using the periodic surface method, the computed area of a rough surface remains a period cell and will not change with the incident angles. Thus a large number of computer memory is saved so that scattering from three dimensional model of rough surface can be solved on a personal computer. In the case of an oblique incidence, periodic boundary condition (PBC) is difficult to be used due to the time delay along the periodic directions. "Sin/cos technique" is adopted to solve this difficulty. The structure is excited once by a sin signal and once by a cos signal. The PBC for the oblique incidence can be implemented by an appropriate combination of these two excitations in an appropriate way. Finally, scattering from a three dimensional rough surface is studied by using Finite- Difference Time-Domain (FDTD) method.Having computed scattering from a rough surface, we constructed a composite scattering model of an isolated target above a rough surface by FDTD method. PBC is used to eliminate the edge effects of a finite rough surface. However it could not be used to compute the target above a rough surface in the same way. Thus, we first computed the scattered fields from a rough surface in the near zone. Then, the scattered fields and incident fields were both introduced to excite the model of a target above a rough surface. Since PBC was not used in the composite model, the period of the target would not be created. Scattered fields from a target and a rough surface in the near zone were computed in the time domain by FDTD method. Finally, omnidirectional scattering from the composite model was computed by near-to-far zone transform.The multipath phenomenon will decrease ability of radars to detect a target above/on a rough surface. Narrow band(CW) analysis of the multipath effects has been studied by many researchers. However, CW radars have limited ability to detect targets above rough surface. Therefore, ultrawide band/short pulse radars have become of interest. Sin/cos technique used before is a single frequency technique, and loses the wide band capability of the FDTD method because of the sin and cos excitations. Thus, we can only study the scattering from the structure by a single frequency excitation. To implement the wide frequency band computation capability of FDTD, constant transverse wave (CTW) was introduced in the paper. Because the wavenumber is kept constant in the direction of periodicity in each simulation, there is no delay in the transverse plane and PBC can be implemented directly in the time domain. This method is also called spectral method. Using spectral FDTD (SFDTD), Wideband scattering computation model of a rough surface has been presented in this paper. Because CTW was introduced, PBC could be easily implemented in the case of an oblique pulse incidence so that edge effects were eliminated by periodic surface method. Responses from a three dimensional rough surface excited by CTW were computed in time domain. By Fourier fast transform, bistatic wideband scattering from the three dimensional rough surface was studied. Subsequently, SFDTD computation model of a target above a rough surface is presented to study the wideband scattering from this structure. At every time step, scattered fields from the rough surface in the near zone were used to excite the structure of a target and the rough surface. Scattering results from the structure were acquired both in time domain and in the wide frequency band domain.The work of this dissertation simulated scattering from two models of a randomly rough surface and a target above a rough surface excited by both monochromatic wave sources and wide-band wave sources in the three dimensional case, and provided a new set of feasible numerical methods for scattering computation of a randomly rough surface and the composite model of a target and a underlying rough surface.