雪层覆盖的粗糙地面与上方矩形截面柱复合电磁散射的时域有限差分法研究

采用土壤介电常数的四成分模型和雪的介电常数模型分别表示实际的地面和雪层的介电特性, 应用指数型分布粗糙面模型和Monte Carlo方法模拟实际的粗糙地面, 运用时域有限差分方法研究了雪层覆盖的粗糙地面与上方矩形截面柱的复合电磁散射问题. 得出了复合电磁散射系数的角分布曲线, 计算了双站复合散射系数随土壤与雪层粗糙度参数、介电参数、 矩形截面柱几何参数、介电参数等的变化情况, 并做了详细分析与讨论. 得到了雪层覆盖的粗糙地面与上方矩形截面柱复合电磁散射特性.     

大尺度粗糙地面的电磁散射特性

综合考虑空间区域分割、消息传递、负载平衡及同步和边界处理等因素,给出了计算随机粗糙地面电磁散射的并行时域有限差分算法流程,并在大型高性能并行机上应用。理论分析和仿真结果表明：该算法可以快速准确地计算大尺度随机粗糙地面的电磁散射特性;通过与经典解析方法的比较,验证了该算法的正确性和实用性,在大尺度情况下拟合结果更好,更能体现地面的统计特性。     

海面与其上方双矩形截面柱复合散射的混合算法研究

采用Pierson-Moscowitz(PM)海谱和Monte Carlo方法模拟实际的粗糙海面, 基于矩量法和基尔霍夫近似的混合算法研究了海面与其上方双矩形截面导体柱的复合电磁散射特性, 得出了复合散射系数的角分布曲线, 计算了复合散射系数随海面参数、矩形截面柱参数以及入射波参数的变化情况, 并做了详细分析与讨论, 得到了PM谱海面与其上方双矩形截面柱复合散射特性. 结果表明, 与单纯的矩量法相比较, 采用基于矩量法和基尔霍夫近似的混合算法, 既可获得较高的准确性, 同时又可减少计算时间和内存占用量, 而且粗糙面尺度越大该优势越明显.     

三维随机粗糙面与目标复合电磁散射的FDTD方法

用时域有限差分方法(FDTD)研究三维周期性延拓的随机粗糙面与上方目标复合电磁散射.用周期性延拓消除数值计算中截取有限大小粗糙面产生的边缘效应,讨论一个周期单元粗糙面的边长与其相关长度之间的关系.给出在FDTD方法中向粗糙面加载入射波的方法,建立了粗糙面上单个三维目标的复合散射FDTD计算模型.数值结果给出粗糙面与目标散射的近场分布,应用近远场变换得到全方位散射角的双站散射系数.比较了三维与二维散射模型的区别.结果显示当粗糙面上放置目标时,其后向散射显著增强.     

基于时域有限差分法的各向异性铁氧体圆柱电磁散射分析

采用时域有限差分法(FDTD)求解了各向异性铁氧体材料覆盖金属圆柱的雷达散射截面(RCS).提出一种新的磁各向异性材料的离散方法,推导了磁化铁氧体的FDTD公式,并将其应用于分析二维铁氧体覆盖金属圆柱的散射.通过与移位算子法(SO-FDTD)对比证明了此方法的有效性.应用此方法求解了不同铁氧体参数影响下金属圆柱的RCS.数值结果表明,该方法正确可行,具有较高的计算效率.     

用二维时域有限差分法计算机翼雷达散射截面

应用二维时域有限差分(FDTD)程序计算二维翼型NACA0012和金属方柱的双站雷达散射截面,与国内外文献上的结果进行比较,证明该程序的有效性.而后计算二维不同形状机翼的双站雷达散射截面(RCS)和不同后掠角时后掠翼和三角翼的双站RCS,得出不同形状机翼有不同的RCS分布;改变后掠角的大小可以改变机翼的RCS分布.因此,根据设计要求,可以通过选用不同机翼形状或不同大小的后掠角,使飞行器达到RCS减缩.     

FDTD方法分析高功率微波粗糙地面散射特性

采用时域有限差分方法(FDTD)对高功率微波(HPM)近地面传输进行了仿真,其中提出的一种完全匹配层-广义完全匹配层混合吸收边界,较好地抑制了常规FDTD方法在计算低掠射角入射时产生的边缘绕射现象。将理想水平地面数值计算结果与解析法结果对比验证计算模型的正确性,通过数值计算结果分析高功率微波粗糙地面的散射特性。理论分析和仿真结果表明:粗糙地面对散射系数变化影响较明显,从宏观角度上看,散射系数曲线同理想水平地面散射系数曲线相似,即随掠射角的增大散射系数先减小后增大;从微观角度上看,由于粗糙地面的作用,其散射系数曲线并不存在布鲁斯特角,且在某一区域并非单调增加或减小。     

FDTD方法分析高功率微波粗糙地面散射特性

采用时域有限差分方法(FDTD)对高功率微波(HPM)近地面传输进行了仿真,其中提出的一种完全匹配层-广义完全匹配层混合吸收边界,较好地抑制了常规FDTD方法在计算低掠射角入射时产生的边缘绕射现象。将理想水平地面数值计算结果与解析法结果对比验证计算模型的正确性,通过数值计算结果分析高功率微波粗糙地面的散射特性。理论分析和仿真结果表明：粗糙地面对散射系数变化影响较明显,从宏观角度上看,散射系数曲线同理想水平地面散射系数曲线相似,即随掠射角的增大散射系数先减小后增大;从微观角度上看,由于粗糙地面的作用,其散射系数曲线并不存在布鲁斯特角,且在某一区域并非单调增加或减小。     

雪层覆盖土壤表面与半埋柱体宽带复合散射FDTD方法

为满足半埋目标测量和检测的需要,应用指数型分布粗糙面模型和Monte Carlo方法模拟雪层、土壤表面,运用时域有限差分方法研究雪层覆盖的土壤表面与半埋矩形截面柱复合模型的宽带电磁散射,得出宽带复合散射系数的频率响应曲线,计算宽带复合散射系数随雪层表面、土壤表面高度起伏均方根、相关长度,雪层类型、雪层厚度,土壤层含水量,矩形截面柱几何参数、倾角、埋藏深度,电磁波入射角等参数的变化. 结果表明,雪层表面高度起伏均方根、相关长度对宽带复合散射系数影响较小,其它参数对宽带复合散射系数均有显著影响且较为复杂.     

Sound scattering at fluid-fluid rough surface

Extinction theorem was used to deduce the first order scattering cross-section including the double scattering effects for the fluid-fluid rough surface. If the double scattering effects are neglected in the present method, the scattering cross-section agrees with the result obtained by the perturbation method based on Rayleigh hypothesis. Calculations of scattering strength were carried out, and comparisons with the first-order perturbation method based on Rayleigh hypothesis were also done. The results show that double scattering effects are obvious with the increase of the root mean square of surface height and the grazing angle when the valid condition k ₁ h < 1 is satisfied. Supported by the National Basic Research Program of China (Grant No. 2005CB422307)     

Application of a multiregion model to the EM scattering from a rough surface with or without a target above it

An efficient multiregion model is introduced to calculate the electromagnetic scattering from a perfectly electrical conducting(PEC) rough surface with or without a PEC target above it.In the multiregion model,the rough surface is split into multiple regions depending on their position along the rough surface.Two intermediate regions are chosen as the dominant region.If a target is located above the rough surface,the target will also be included in the dominant region.The method of moments(MOM) is only adopted on the dominant region to ensure validity.Hence,the new model can greatly reduce the number of unknowns associated with full MOM analysis.The induced electric currents on the other regions are obtained by approximately considering the mutual coupling between different regions along the rough surface.Compared with the published hybrid method,this new model is not only suitable for EM scattering from a target above a rough surface but also applicable for just rough surfaces.Several numerical simulations are presented to show the validity and efficiency of the multiregion model.     

Investigation on electromagnetic scattering from rough soil surface of layered medium using the small perturbation method

Electromagnetic scattering from a rough surface of layered medium is investigated, and the formulae of the scattering coefficients for different polarizations are derived using the small perturbation method. A rough surface with exponential correlation function is presented for describing a rough soil surface of layered medium, the formula of its scattering coefficient is derived by considering the spectrum of the rough surface with exponential correlation function; the curves of the bistatic scattering coefficient of HH polarization with variation of the scattering angle are obtained by numerical calculation. The influence of the permittivity of layered medium, the mean layer thickness of intermediate medium, the roughness surface parameters and the frequency of the incident wave on the blstatic scattering coefficient is discussed. Numerical results show that the influence of the permittivity of layered medium, the mean layer thickness of intermediate medium, the rms and the correlation length of the rough surface, and the frequency of the incident wave on the bistatic scattering coefficient is very complex.     

分形大地土壤表面与部分埋藏目标复合散射特性

采用土水混合物介电常数的Topp方程模型表示大地土壤的介电特性,应用带限Weierstrass-Mandelbrot分形粗糙面模型和Monte Carlo方法模拟大地土壤表面,运用矩量法研究了带限分形大地土壤表面与部分埋藏矩形截面柱复合模型的电磁散射,得出了复合散射系数的角分布曲线;计算了复合散射系数随带限分形大地土壤表面分维、空间基频、高度起伏均方根、土壤含水率、矩形截面柱几何参数、埋藏深度、倾角、入射波频率等的变化情况,并做了详细分析与讨论;结果表明,土壤表面分维等参数对复合散射系数的影响是非常复杂的,镜像附近的角分布曲线具有明显的分形特征。     

Investigation of composite electromagnetic scattering from ship-like target on the randomly rough sea surface using FDTD method

Composite electromagnetic scattering from a two-dimensional (2D) ship-like target on a one-dimensional sea surface is investigated by using the finite-difference time-domain (FDTD) method. A uniaxial perfectly matched layer is adopted for truncation of FDTD lattices. The FDTD updated equations can be used for the total computation domain by choosing the uniaxial parameters properly. To validate the proposed numerical technique, a 2D infinitely long cylinder over the sea surface is taken into account first. The variation of angular distribution of the scattering changing with incident angle is calculated. The results show good agreement with the conventional moment method. Finally, the influence of the incident angle, the polarization, and the size of the ship-like target on the composite scattering coefficient is discussed in detail.     

双尺度动态分形粗糙海面的电磁散射及多普勒谱研究

利用更为符合海面实际散射场计算的双尺度法分析并计算了一维动态分形粗糙海面的后向散射截面，同时与微扰法有关结果进行了比较.研究了动态分形海面后向散射信号的多普勒谱分形特征，分析了不同极化状态下后向散射截面和多普勒谱谱宽随入射角及分维的变化规律，最后详细讨论了多普勒谱中心频率与海面Bragg谐振频率间的关系. 关键词： 双尺度模型 分形海面 电磁散射 多普勒谱     

大尺度分层介质粗糙面电磁散射的特性研究

首先建立大尺度分层介质粗糙面散射的物理模型, 基于Stratton-Chu积分方程和Kirchhoff近似导出了粗糙面散射场的计算公式. 采用高斯随机粗糙面来模拟实际的分层介质粗糙面, 通过数值计算得到了正下视单站雷达接收到的后向散射回波. 理论推导了散射场强度与表面粗糙度之间的定量关系, 并从数值仿真的角度分析了表面和次表面的粗糙度对散射回波的影响, 给出了散射场随粗糙度变化的曲线. 最后考察了分层介质的电特性参数(介电常数和电导率)对分层粗糙面散射场的影响, 并对计算结果做出了分析.