Effect of shadowing on electromagnetic scattering from rough oceanwavelike surfaces at small grazing angles

A hybrid moment-method/geometrical-theory-of-diffraction technique (MM/GTD) has been implemented to numerically calculate the electromagnetic scattering from one-dimensionally rough surfaces at extreme illumination angles (down to 0° grazing). The hybrid approach allows the extension of the modeled scattering surface to infinity, avoiding the artificial edge diffraction that prevents use of the standard moment method at the smallest grazing angles, Numerical calculation of the backscattering from slightly rough large-scale surfaces approximating ocean wave features shows that roughness in strongly shadowed regions can contribute significantly to the total backscatter at vertical polarization. This is observed when the shadowing obstacle is several wavelengths high, and the magnitude of the shadow-region contribution does not depend on the radius-of-curvature of the shadowing feature. Strongly shadowed roughness does not significantly contribute to the backscatter at horizontal polarization, although weakly shadowed roughness near the incidence shadow boundary does. The calculations indicate that a shadowing-corrected two-scale model may be able to predict the distributed-surface portion of the sea-surface scattering from the ocean surface at grazing angles down to about 15°, but at lower grazing the shadowing and large-scale curvature of the surface prevent the establishment of a Bragg resonance and invalidate the model     

A note on the scattering from a slightly rough surface

A Fourier transform approach is used to derive the bistatic radar scattering cross section of a slightly rough perfectly conducting infinite surface. A perturbation expansion is used to apply the boundary conditions, and the scattered fields are asympotically evaluated by means of the method of stationary phase. The resultant expression for the radar cross sectionsigma^{0}is shown to agree with that obtained using the method as outlined by Rice.     

Wave scattering from a large sphere with rough surface

Wave scattering from a rough perfectly conducting sphere is considered. Use is made of the reformulated current method in which the object is replaced by a current distribution radiating into an unbounded media. The scattered field components are obtained in terms of the joint characteristic function defining the roughness.     

A study of scattering from an object below a rough surface

A numerical model is applied in a Monte Carlo study of scattering from a three dimensional penetrable object below a lossy dielectric rough interface. Both time and frequency domain results are investigated to illustrate the relative importance of coherent and incoherent scattering effects in the sample problem considered. Results show that introducing a reduced transmission coefficient is reasonable for object coherent scattering predictions in this example, and that incoherent object/surface interaction effects approximately follow a simple scaling behavior as surface roughness is increased.     

介质涂覆机翼与下垫粗糙面后向复合散射研究

本文以杏仁体结构的等离子体涂覆机翼为例就等离子体涂覆目标隐身效果做探索性的研究.利用互易性定理结合高频方法的快速混合算法获取锥形波入射下的微粗糙度粗糙面与其上方等离子体涂覆机翼的后向雷达散射截面;数值结果以等离子涂覆机翼与下垫高斯粗糙面为例,讨论了等离子体角频率、电子碰撞频率等的变化对等离子体隐身效果的影响,数值结果为研究等离子体参数对等离子体消隐性能的影响提供了理论参考.     

低掠角入射动态分形粗糙海面与船目标的双站散射数值模拟

为快速数值计算动态粗糙分形海面上有船目标时的双站散射，该文将广义前后向迭代法（GFBM）与谱积分加速算法（SAA）结合用于求解磁场积分方程（MFIE）。避免了电场积分方程（EFIE）数值计算的不稳定性。数值模拟了TE锥形波低掠角入射在一维动态分形粗糙导体海面以及船目标存在时的双站散时，讨论了多次散射传播的双疲散射与动态分形海面和船目标特征参数的关系。     

Electromagnetic scattering from a dielectric cylinder buried beneath a slightly rough surface

An analytical solution is presented for the electromagnetic scattering from a dielectric circular cylinder embedded in a dielectric half-space with a slightly rough interface. The solution utilizes the spectral (plane-wave) representation of the fields and accounts for all the multiple interactions between the rough interface and the. buried cylinder. First-order coefficients from the small perturbation method are used for computation of the scattered fields from the rough surface. The derivation includes both TM and TE polarizations and can be easily extended for other cylindrical buried objects (e.g., cylindrical shell, metallic cylinder). Several scattering scenarios are examined utilizing the new solution for a dielectric cylinder beneath a flat, sinusoidal, and arbitrary rough surface profile. Results indicate that the scattering pattern of a buried object below a slightly rough surface differs from the flat surface case only when the surface roughness spectrum contains a limited range of spatial frequencies. Furthermore, the illuminated area of the incident wave is seen to be a critical factor in the visibility of a buried object below a rough surface.     

A numerical study of scattering from an object above a rough surface

A numerical model is applied in a Monte Carlo study of scattering from a three-dimensional penetrable object above a lossy dielectric rough interface. The model is based on an iterative method of moments solution for equivalent electric and magnetic surface current densities on the rough interface and equivalent volumetric electric currents in the penetrable object. Both time-and frequency-domain results are investigated to illustrate the relative importance of coherent and incoherent scattering effects in the sample problem considered. Results show that a four-path model using a reduced-reflection coefficient can be reasonable for coherent scattering predictions and that incoherent object/surface interaction effects can make significant contributions to received cross sections.     

Fast iterative approach to difference scattering from the target above a rough surface

The difference field radar cross section (d-RCS) has been defined to analyze the scattering from the target above a rough surface, which takes account of scattering from the target and multiinteractions of the target and underlying rough surface. The d-RCS removes the effect of the finite illuminated surface length under the tapered wave incidence. In this paper, the electric field integral equations (EFIEs) of the difference-induced current J/sub sd/ on the rough surface and the induced current J/sub o/ on the target are derived. A small section of rough surface toward the target in the specular direction is taken to speed up computation of the scattering contribution E/sub s0/ from the moderate rough surface to the target. Then, an iterative approach is developed to solve the EFIEs of the induced currents, directly, and yields the bistatic d-RCS. A finite rough surface length for numerical iteration is taken, corresponding to the dependence on the maximum scattering angle. Using the Monte Carlo method to generate rough surface, the bistatic d-RCS of the target, e.g., a cylinder or a square column, above a Pierson-Morkowitz rough surface is numerically simulated. The induced currents on the target and the d-RCS are discussed, and compared with the case of the target in free-space.     

低掠入射时分形粗糙面散射的高阶微扰解

针对粗糙表面低掠入射散射特性求解的大误差问题,采用高阶微扰法研究了分形布朗运动粗糙表面在低掠入射条件下的散射特性,计算了高阶微扰法在低掠入射时的修正值,比较了高阶场量和阴影函数对低掠入射的修正效应,讨论了不同分维数下低掠入射时的散射系数,分析了低掠入射下极化指数与不同幂次正割函数的拟合逼近问题,修正了Ngo等人的结论.结果表明,高阶微扰法能有效求解掠入射角下分形粗糙表面的散射特性.     

粗糙地面上相邻两树干的电磁散射模型

用有限长多层介质圆柱模拟实际的树干,建立了基于粗糙地面上相邻两树干电磁散射模型。用迭代方法和等效原理,求解了多层介质圆柱的散射矩阵。用互易原理求解了相邻有限长多层介质圆柱包括其二次散射场在内的解析表达式。     

粗糙铜表面对低频太赫兹波的散射实验

搭建了以0.2 THz返波管振荡器源、热释电探测器、小型自动旋转光学平台等组成的太赫兹波目标散射特性实验测试系统,对两种不同粗糙度铜盘表面的散射特性进行了测试,表明:太赫兹金属粗糙目标散射中导体表面的感应电流产生电磁散射和粗糙导体表面引起的朗伯散射是同时存在的;在斜入射时这种近似于朗伯体的金属粗糙表面几乎可以被看成镜体,但随着目标表面的粗糙度变大,反射变弱,散射增强,主峰向小于反射角的方向偏移;在垂直入射的情况下,散射角小于40度时散射曲线下降较快,超过40度散射曲线变化变得很缓慢,但在50度附近很多材料都会出现一个小的散射峰.     

On the far-field approximation for scattering from randomly rough surfaces

Various authors have justified the far-fields approximation for rough surface scattering using one of the classical approximations for the scattered fields, usually considering either the coherent scattered field or the incoherent scattered intensity. An exact expression for the field scattered from a perfectly conducting rough surface is considered. The expression for the incoherent scattered intensity is formally derived, and a condition under which the far-field approximation is valid is found, independent of specific approximations for the surface or scattered fields or for the surface height statistics. The condition so derived is, under many circumstances, substantially less restrictive than that derived before in the general case. Furthermore, the previous results may be easily recovered by further specialization of our result.     

基于GPU的并行FDTD方法在二维粗糙面散射中的应用

利用显卡(Graphics Processing Unit, GPU)加速时域有限差分(Finite-Difference Time Domain, FDTD)法计算二维粗糙面的双站散射系数, 介绍了FDTD的理论公式以及计算模型.采用各向异性完全匹配层(Uniaxial Perfectly Matched Layer, UPML)截断FDTD计算区域.重点讨论了基于GPU的并行FDTD计算粗糙面双站散射系数的并行设计方案计算流程.在NVIDIA GeForce GTX 570显卡上获得了50.7×的加速比.结果表明:通过对FDTD计算粗糙面散射问题的加速, 极大地提高了计算效率.     

The scattering behavior of a slightly rough surface moving parallel to its mean plane with uniform velocity

The scattering of plane electromagnetic (EM) waves from a perfectly conducting, slightly rough surface moving in vacuum with uniform and constant velocity is examined by combining the boundary perturbation method and the special, homogeneous Lorentz transform. The direction of motion is assumed to be parallel to both the incidence plane and the mean plane of the moving rough surface. The analysis is confined to first-order scatter. The end result is the determination of the velocity-dependent expressions for the bistatic and backscatter incoherent cross sections per unit area of the scattering surface. The expressions for bistatic geometry are only valid for scattering inside the plane of incidence. A numerical example demonstrates the effect of the motion on the scattering behavior of the rough surface.     

Multiple scattering and depolarization by a randomly rough Kirchhoff surface

A matrix approach has been developed to compute bistatic scattering coefficients which include shadowing and multiple scattering effects for a randomly rough Kirchhoff surface. The method permits the computation of these coefficients in terms of the existing single-scatter bistatic scattering coefficients. Thus the effects of multiple scattering are readily recognized from the expressions obtained. The bistatic scattering coefficients are shown to satisfy energy conservation to at least two significant figures. It is observed that while polarized backscattering is dominated by the single-scattering process, the depolarized backscattering is due to multiple scattering. Unlike depolarization by slightly rough surfaces or volume scattering, the angular behavior of the depolarized backscattering is similar to that of the polarized backscattering. The transitional behavior of the relative dominance between single and multiple scattering for the polarized and depolarized scattering coefficient as a function of the azimuth angle is illustrated.     

On the control of edge diffraction in numerical rough surface scattering using resistive tapering

The use of resistive loading to remove edge effects in electromagnetic scattering from rough surfaces with finite conductivity has been considered. An electric field integral equation formulation using impedance boundary conditions was implemented to model the conductivity of sea water at X band. The resistive loading was added over surface sections within three wavelengths of the modeled edges. A resistive taper synthesized to control the sidelobe level in scattering from flat, perfectly conducting plates proved better able to reduce edge diffraction than a power-law taper of a type that is often used. The calculated scattering from test profiles that model breaking water waves using the resistive loading show good agreement with those found using a reference scattering approach provided that the local grazing angle on the loaded surface section is greater than 20/spl deg/.     

复杂陆地粗糙面及其上方坦克目标复合散射研究

研究了复杂陆地粗糙面及其上方坦克目标的复合散射特性.基于Monte Carlo方法,利用高斯谱函数模拟不同统计特性的陆地表面,运用加权反正切函数滤波处理以构造边界和连接相邻区域,构建了复杂陆地分区域复合粗糙面的几何模型.提出了一种快速精确求解复杂陆地粗糙面及其上方坦克目标复合散射的混合方法,利用迭代物理光学法求解目标与环境间的多次耦合,运用等效电磁流法修正棱边绕射.分析了复杂陆地粗糙面的散射特性,研究了入射角、均方根高度、相关长度和粗糙面表面结构对复合散射特性的影响.本研究为探测复杂环境上方的坦克目标提供了理论基础,数值仿真结果对雷达目标的探测与识别具有借鉴意义.     

二维分形粗糙面电磁散射的分形规律研究

采用二维归一化带限Brown分形函数来模拟二维分形粗糙面,利用基尔霍夫近似给出了该粗糙面的电磁散射场和散射强度方差的计算公式,主要讨论了散射场分布与分维的关系,获得了散射场波峰拟合线的斜率与分维D满足线性关系这一重要规律。     

Electromagnetic scattering interaction between a dielectriccylinder and a slightly rough surface

An electromagnetic scattering solution for the interaction between a dielectric cylinder and a slightly rough surface is presented in this paper. Taking the advantage of a newly developed technique that utilizes the reciprocity theorem, the difficulty in formulating the secondary scattered fields from the composite target reduces to the evaluation of integrals involving the scattered fields from the cylinder and polarization currents of the rough surface induced by a plane wave. Basically, only the current distribution of isolated scatterers are needed to evaluate the interaction in the far-field region. The scattered field from the cylinder is evaluated in the near-field region using a stationary phase approximation along the cylinder axis. Also, the expressions for the polarization current induced within the top rough layer of the rough surface derived from the iterative solution of an integral equation are employed in this paper. A sensitivity analysis is performed for determining the dependency of the scattering interaction on the target parameters such as surface root mean square (RMS) height, dielectric constant, cylinder diameter, and length. It is shown that for nearly vertical cylinders, which is of interest for modeling of vegetation, the cross-polarized backscatter is mainly dominated by the scattering interaction between the cylinder and the rough surface. The accuracy of the theoretical formulation is verified by conducting polarimetric backscatter measurements from a lossy dielectric cylinder above a slightly rough surface. Excellent agreement between the theoretical prediction and experimental results is obtained