谐振区地海表面上三维运动目标的近、远场电磁散射特性研究

Composite electromagnetic scattering from a moving object above a land or sea surface is studied by using FDTD (Finite Difference Time Domain) method. Firstly, we calculate the scattering of a single moving object combining the FDTD method with the relativistic boundary conditions. And an improved method is proposed to accelerate computation in this project. Secondly, we take a rough surface as a periodic structure to eliminate its edge effects and calculate a three dimensional model of rough surface with large incident angles. Then we construct a electromagnetic scattering model for a moving object above a rough surface. Scattered fields from a rough surface are computed in the FDTD region. And the scattered fields and incident fields are both introduced to excite the model of a moving object above a rough surface. Scattered fields from a target and a rough surface in the near zone are computed. And omnidirectional scattering from the composite model is computed by near-to-far zone transform. Thirdly, wideband electromagnetic scattering model of a moving target above a rough surface is studied. To implement the wideband computation capability of FDTD, the sin/cos modulation gaussian pulse is applied to periodic boundary conditions. Then scattering from the composite model is computed in time domain. By Fourier fast transform, bistatic wideband scattering from the the composite model is acquired. Furthermore Wideband scattering from the model is also calculated by spectral FDTD. Finally, scattered fields outside of FDTD computation region can be expanded by means of the spherical-multipole expansion. Near-field to near-field transform is devoloped to acquire near-field properties of a moving object above a rough surface. By contrast with experimental results we can testify the correctness and efficiency of the method.The theory base of detecting moving targets above a land or sea surface and stealthy technique can be presented in the project.

本课题基于FDTD方法对运动目标在地海表面上的复合电磁散射进行预估。首先，结合相对边界条件研究单独运动目标的电磁散射，并提出改进算法。然后，采用周期性延拓面法消除边缘绕射，计算三维大角度入射情形下的粗糙面散射，并根据二次激励法建立运动目标与粗糙下垫面复合模型的电磁散射。利用近远场变换，由计算区域内的电磁场外推出复合模型的远区散射场。接着，为研究粗糙面上运动目标的宽频带散射特性，将调制高斯脉冲波运用于斜入射情形下的周期边界条件，计算复合模型的时域瞬态散射波形，通过傅立叶变换获得宽频带散射。同时用Spectral FDTD方法研究宽频带散射，对比两种方法的数值结果，相互验证。最后，基于球多极展开方法，利用FDTD计算区域内的散射场值求解计算区域外散射场的球多极展开式，外推有限距离处的近场散射分布，与实验测量数据对比验证。本课题的研究为地海面上运动目标的识别及对目标实施隐身措施提供理论依据。

随着空间技术与深空探测的发展，对高速运动目标进行电磁散射特性分析得到了越来越多的关注，特别在雷达探测高速运动目标的领域。研究高速运动目标电磁散射时需考虑相对论理论。本项目首先提出了改进的相对边界条件与FDTD结合方法，计算了运动目标的时谐散射场，大大提高计算效率并节省计算机内存。随后模拟了运动目标的瞬态回波响应，由傅立叶变换获得其宽频散射频谱。结果显示回波的振幅和频率都随其速度的大小和方向而改变。随后，在此模型基础上对运动目标边界运用相对边界条件，基于FDTD法建立了地表面上运动目标的电磁散射模型，模拟了地表面上运动目标的近场分布，并利用近远场外推技术获得其在不同运动速度下的双站散射系数。地表面的多路径效应是接收GNSS信号的主要误差源，为此我们提出多路径半天球MHM模型，利用环境不变条件下多路径效应的时空重复性，消除地表面的多路径误差，能实时应用于GNSS高精度数据处理。先前，采用相对边界条件的方法不易于对非金属复杂形状的运动目标进行电磁模拟，因此我们利用洛伦兹变换，将实验坐标系下入射波激励运动目标的场景转换到静止坐标系下，使运动目标相对坐标系是静止的。在此静止坐标系下应用FDTD方法计算相对静止目标的电磁散射，获取其近区散射场，并外推出远区散射场，最后再将静止坐标系下的散射场转换到实验坐标系。本项目具体分析了运动目标的双站电磁散射和多普勒随运动速度的变化情况，结果显示高速运动目标的相对论性电磁散射不可忽略。洛伦兹变换还可与其他电磁计算方法结合，我们接着利用FEM方法具有占用内存少及易于模拟复杂形状目标的优势，结合洛伦兹变换计算了涂覆雷达吸波材料运动目标的电磁散射。给出了不同运动速度对相同涂覆材料吸波效果的影响。最后，拓展了项目的研究方向，结合遗传算法对天线进行优化设计，为今后在电磁计算中实现参数优化奠定理论基础。本项目的研究为雷达有效探测及识别高速运动目标提供电磁理论基础。