Investigation On Composite Electromagnetic Scattering From The Rough Surface And The Target Using Time Domain Integral Equations

In this dissertation,investigation on the stability and fast algorithms in marching-on-in-time(MOT)scheme on composite scattering from the randomly rough surface and the target using time domain integral equations is reported.The magnetic vector potential is calculated by second-order central finite difference with small numerical errors to improve numerical stability.Thus,a stable numerical algorithm is obtained for solving the electromagnetic scattering of 2D target,the rough surface,and the composite model of the rough surface and the target.A TDIE–TDKA hybrid method is proposed to solve the electromagnetic scattering from the rough surface and the targets for improving computation efficiency.Numerical results show that the TDIE–TDKA method not only can be applied to a composite model of ship floating on sea surface but also is suitable to the scene of multiple targets located above the rough surface.A multi-region TDIE algorithm is introduced to calculate the composite electromagnetic scattering from rough surface with a target above it for further improving numerical calculation efficiency.The main achievements of the study reported in the dissertation are summarized as follows.The explicit MOT solution is adopted to solve the time-domain electric-field integral equation(TD–EFIE)where the magnetic vector potential is approximated by backward finite difference.The transient currents of 2D targets,the rough surface,and the composite model of the rough surface and the target are calculated.The effects of 2D target size,time step size,splitting scale,rough surface roughness,and distance between target and rough surface on the algorithm stability are discussed.Simulation results illustrate that long time step,large target size,smooth rough surface correspond to stable transient currents.Instability of transient currents increases dramatically for the composite model of a target located above a rough surface.Small target size and close distance between the target and the rough surface indicate high stability of the transient currents of composite model.The numerical instability of TDIE is improved by calculating the magnetic vector potential by using second-order central finite difference.The transient currents of 2D targets,the rough surface,and the composite model of the rough surface and the target are used to verify the stability of the algorithm.Numerical results obtained for the 2D targets,the rough surface,and the composite model of the rough surface with the target are stable.The TDIE–TDKA hybrid algorithm is proposed to the study of composite electromagnetic scattering from a ship located on sea surface.In this hybrid method,composite model is divided into TDIE and TDKA regions,respectively.The ship and its adjacent sea surface are selected as the TDIE region,and the rest of the electrically large rough sea surface is the TDKA region.The surface current interactions between the TDIE and the TDKA regions are considered in the solving process to guarantee calculation accuracy.Numerical results demonstrate that the hybrid method can accelerate the computation considerably unlike the conventional TDIE.The influence of wind speed,ship size,and depth of the ship immersing in sea surface on the transient scattered fields is discussed in detail.A theoretical model of TDIE–TDKA hybrid algorithm for a rough surface with multiple targets is established.In the scheme,each of the targets is one TDIE region and the underlying rough sea surface is chosen as TDKA region.Considering the interactions between different regions including TDKA region and each TDIE region,a matrix equation is given and used to solve the composite transient scattering from multiple targets and underlying rough sea surface.Numerical results confirm that the hybrid algorithm is applicable to the composite model of rough surface with multiple targets.The influence of target numbers,heights,sizes,and distances and wind speed on the transient scattered fields of composite model is discussed in detail.A multi-region TDIE hybrid algorithm is proposed and used to analyze the transient scattering characteristics of the rough surface with a target above it.In the multi-region model,the rough surface is divided into multiple regions depending on the intensity of the incident wave along the rough surface.The amplitude of the tapered plane wave at the center of the rough surface is much larger than that at the edge.The center regions of the rough surface including the target are chosen as the dominant region.The regions where the intensity of incident wave is small are set as the subordinate regions.The transient scattered fields of the dominant and subordinate regions are calculated by the TDIE and TDKA methods,respectively.Mutual couplings between different regions are also considered in the solving process to ensure accuracy.The calculation efficiency of the multi-region TDIE algorithm is further improved by comparing it with the TDIE–TDKA hybrid method.