高阶基尔霍夫法求解导体粗糙表面的散射特性

在分析粗糙表面电磁散射特性的基础上,提出了一种考虑粗糙表面协方差函数Taylor展开的高阶基尔霍夫近似(KA)法,解决了经典KA近似的大逼近误差问题.应用9阶高度的KA和传统的KA,对比研究了不同照射频率和均方根高度下的后向散射系数,并比较了遮挡函数的修正效应.同时应用高、低阶KA计算了典型粗糙面的后向散射系数,并分别与测量值和矩量法的数值解进行了比较.结果表明,9阶表面高度展开的切平面KA不仅拓宽了KA的适用条件,还具有更高的精度范围,从而证明了高阶均方根高度展开的KA近似的有效性.     

Backscattering from a randomly rough dielectric surface

A backscattering model for scattering from a randomly rough dielectric surface is developed. Both like- and cross-polarized scattering coefficients are obtained. The like-polarized scattering coefficients contain single scattering terms and multiple scattering terms. The single scattering terms are shown to reduce to the first-order solutions derived from the small perturbation method when the roughness parameters satisfy the slightly rough conditions. When surface roughnesses are large but the surface slope is small, only a single scattering term corresponding to the standard Kirchhoff model is significant. If the surface slope is large, the multiple scattering term will also be significant. The cross-polarized backscattering coefficients satisfy reciprocity and contain only multiple scattering terms. The difference between vertical and horizontal scattering coefficients increases with the dielectric constant and is generally smaller than that predicted by the first-order small perturbation model. Good agreements are obtained between this model and measurements from statistically known surfaces     

The relationship between the Kirchhoff approach and small perturbation analysis in rough surface scattering theory

Theoretical analyses of the scattering of the electromagnetic waves from rough surfaces have progressed using basically the Kirchhoff approach (KA) and the small perturbation method (SPM) of Rice. It is demonstrated that the latter method (SPM) is actually a special case of the former (KA). Reasons why SPM agrees with experimental data near grazing angle are given and arguments are made for the invalidity of the SPM second-order depolarized term.     

粗糙面下方金属目标复合电磁散射的快速算法

为快速有效计算粗糙面下金属目标的复合电磁散射,提出了一种基于前后向迭代算法(FBM)和共轭梯度(CG)法的快速互耦迭代算(CCIA).首先建立目标与粗糙面的耦合积分方程组,并采用矩量法将其离散为矩阵方程.其次针对得到的耦合积分方程,用FBM求解粗糙面表面电流分布,用CG法求解目标表面电流分布,目标和粗糙面的相互作用通过更新两方程的激励项完成.最后,计算了高斯粗糙面下方无限长金属圆柱目标的复合电磁散射系数,当目标尺寸趋于零或目标深度趋于无穷时的结果与单独介质粗糙面相一致,验证了该数值方法的正确性;同时,讨论了不同粗糙面情况下该方法的收敛性,并分析了不同粗糙面媒质、目标尺寸和目标位置对双站散射系数的影响.     

三类新型随机粗糙面模型的快速散射特性分析

基于表面粗糙度对雷达目标电磁散射特性的影响,为真实地模拟实际自然场境,提出了三类粗糙环境中表面粗糙度不同的新型分区域复合随机粗糙面模型.采用在同一粗糙面上使用不同均方根高度和相关长度的方法进行建模,替代了传统的单一谱函数模型.应用稀疏矩阵平面迭代/规范网格法快速计算了此三类新型随机粗糙面模型的电磁散射特性,重点分析了不同表面粗糙度模型对于环境电磁散射特性的影响,为研究、分析和探测符合实际自然环境的地形、地貌提供了理论依据.     

Resolution of a controversy surrounding the Kirchhoff approach and the small perturbation method in rough surface scattering theory

It is shown that the small amplitude limit of the mean backscatter cross section for a perfectly conducting random rough surface calculated from the first two terms in the iterative series solution of the surface current integral equation gives the same results as the small perturbation method. This demonstration resolves a long-standing controversy in rough surface scattering theory.     

Radar determination of the surface properties of a randomly rough surface

This letter conveys a simple method for remote sensing of the surface properties, both physical and geometrical, of a randomly rough surface by means of a polychromatic bistatic radar. Difficulties in evaluating the properties of a randomly rough surface by the conventional method of calculating the scattering cross section or the average power are discussed.     

New results on coherent scattering from randomly rough conducting surfaces

An approximate solution for the average field scattered by a perfectly conducting randomly rough surface having a correlation length much smaller than the electromagnetic wavelength is presented. The analysis is based on the use of a substitute surface which gives rise to the same describing equations as the true surface relative to the average scattered field. The substitute surface comprises large nonoverlapping fiat areas having random elevations with respect to the mean planar surface and arbitrary correlation between adjacent areas. The average scattered field is shown to depend upon the number of interacting areas and the surface roughness. For a given range of surface roughness there is a specific number of interacting areas which dominate the average scattered field. It is demonstrated how this number can be computed and how a continuous curve of average scattered field as a function of surface roughness is obtained. Of particular importance is the quantitative correspondence established in this paper between the surface roughness and the degree of multiple interaction on the rough surface.     

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.     

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.     

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     

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.     

分形海面电磁散射特性的基尔霍夫近似

利用二维Weierstrass带限函数模拟了粗糙海面的形状,讨论了风速、粗糙度等参量对海面形状的影响;针对模拟的分形海面形状,从亥姆霍兹（Helmholtz）积分出发,利用基尔霍夫近似推导了二维分形海面的电磁散射系数,并进行了数值模拟,同时对数值模拟结果进行了分析,讨论了海面形状对电磁散射的影响,所得结论可用于目标探测技术的研究。     

Application of stochastic second-degree method to electromagnetic scattering from randomly rough surfaces

Application of a stochastic second-degree method in combination with the banded matrix canonical grid (BMIA/CAG) method for two- dimensional electromagnetic scattering from PEC randomly rough surfaces is presented. This method can improve convergence while preserving the computational attractiveness of the BMIA/CAG method. Numerical examples illustrate the effectiveness of the proposed method.     

Gaussian rough surfaces and Kirchhoff approximation

Electromagnetic scattering is often solved by applying the Kirchhoff approximation to the Stratton-Chu scattering integral. In the case of rough surfaces, it is usually assumed that this is possible if the incident electromagnetic wavelength is small compared to the mean radius of curvature of the surface. Accordingly, evaluation of the latter is an important issue. This paper generalizes the groundwork of Papa and Lemon (see ibid., vol.36, p.647-50, May 1988) by computing the mean radius of curvature for Gaussian rough surfaces with no restriction on its correlation function. This is an interesting extension relevant to a variety of natural surfaces. Relations between the surface parameters and the mean radius of curvature are determined and particular attention is paid to the relevant small slope regime     

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.