粗糙介质面对高斯波束的散射

该文研究了当入射场为光波束时粗糙介质面的散射问题。利用波束的平面波谱展开方法及基尔霍夫近似理论导出了散射场和非相干截面计算公式,并对后向非相干截面进行了数值计算,最后对其结果进行了分析讨论。     

Scattering, transmission, and absorption by a rough resistive sheet - E polarization

Motivated by the use of thin conducting textiles in absorbers, the interaction of electromagnetic waves with a rough resistive sheet is examined. Expressions for the fields above and below the sheet are derived in terms of an equivalent current using Gaussian beam illumination. These field expressions are then used with equations derived from the conservation of energy to find the distribution of scattered, transmitted, and absorbed power using an exact numerical solution and the Kirchhoff approximation. As a consequence of the lack of refraction, the Kirchhoff single scattering solution produces strong coherent transmitted intensities at the forward angles with an attendant minimum in the incoherent intensity. A second minimum in the incoherent intensity is observed in the antispecular direction. Fields arising from multiple scattering interactions are observed independent of the Kirchhoff single scatter contributions at these angles for surfaces of large root-mean-square slope. An increase in the absorption in the sheet is also observed as surface roughness is increased. The increase in absorption is believed due to multiple scattering interactions on the surface.     

Coherent scattering of a spherical wave from an irregular surface

The scattering of a spherical wave from a rough surface using the Kirchhoff approximation is considered. An expression representing the measured coherent scattering coefficient is derived. It is shown that the sphericity of the wavefront and the antenna pattern can become an important factor in the interpretation of ground-based measurements. The condition under which the coherent scattering-coefficient expression reduces to that corresponding to a plane wave incidence is given. The condition under which the result reduces to the standard image solution is also derived. In general, the consideration of antenna pattern and sphericity is unimportant unless the surface-height standard deviation is small, i.e., unless the coherent scattering component is significant. An application of the derived coherent backscattering coefficient together with the existing incoherent scattering coefficient to interpret measurements from concrete and asphalt surfaces is shown.     

A new approach to the analysis of rough surface scattering

A novel approach to the problem of scattering by a randomly rough surface is developed based on combining normalization and the method of smoothing. Previous uses of smoothing have been forced to assume small surface heights in order to guarantee the dominance of the scattered field by its average or specular part. The term that causes the average scattered field to be so sensitive to the height is identified, and is normalized out of the basic integral equation. Smoothing is then applied to the normalized integral equation with a subsequent reintroduction of the normalizing factor after smoothing. The results are shown to extend beyond existing approximations, and they contain none of the divergent integral behavior that has recently been observed with a pure second order iterative approximation. An examination of the conditions under which the technique provides very accurate results shows that it is essentially an extension of the so-called composite surface scattering model     

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     

Focal region fields of distorted reflectors

The problem of the focal region fields scattered by an arbitrary surface reflector under uniform plane wave illumination is solved. The physical-optics (PO) approximation is used to calculate the current induced on the reflector. The surface of the reflector is described by a number of triangular domain-wise fifth-degree bivariate polynomials. A two-dimensional Gaussian quadrature is employed to numerically evaluate the integral expressions of the scattered fields. No Fresnel or Fraunhofer approximations are made. The relation of the focal fields problem to surface compensation techniques and other applications are mentioned. Several examples of distorted parabolic reflectors are presented and discussed     

水泥路面激光散射特性研究

为了通过激光散射特性识别不同粗糙程度水泥路面, 设计了路面粗糙度测量系统。使用千分表测量水泥路面高度分布, 计算高度均方根与相关长度。根据以上参量采用功率谱频域滤波生成随机粗糙表面以模拟水泥路面, 通过切平面近似将粗糙面离散为大量微面元, 由菲涅耳公式计算本地场, 利用蒙特卡罗方法获取不同偏振光入射条件下粗糙面双向与后向散射光强度统计平均值。基于虚拟仪器技术进行高精度自动化激光散射测量, 并根据实验数据对理论模型进行了验证。结果表明, 双向散射小粗糙度水泥路面散射光强度在镜像方向散射角40°附近具有峰值90lx, 在镜像方向两侧逐步递减, 大粗糙度水泥路面光学特性近似为朗伯体, 散射光强度在各散射角方向变化不大; 后向散射在垂直入射时, 小粗糙度水泥路面散射光强度峰值为103lx, 随散射角增大逐渐递减, 大粗糙度水泥路面具有朗伯体散射特征。此研究结果可为车辆自主驾驶方案路面信息感知提供参考。     

Numerical simulation of scattering from three-dimensional randomlyrough surfaces

Randomly rough surface patches in three dimensions are generated on the computer. The FD-TD method is used to compute scattering from surface patches by converting the Maxwell's equations into difference equations using a central difference approximation for the space and time derivatives. The volume of grids above the rough surface is divided into the total field and the scattered field regions. In between these two regions, obliquely incident waves are generated. To reduce computation, the volume of grids is chosen to be small, and a transformation is used to convert the scattered field into far zone fields for bistatic scattering coefficient calculations. Possible errors near the edge of the surface due to the use of a relatively small volume are suppressed by introducing a windowing function. Very good agreements are obtained between the results obtained by this method and those calculated by an integral equation method (IEM) for scattering from randomly rough perfectly conducting and dielectric surfaces     

粗糙球体和锥体目标激光散射非相干分量比

为了研究激光散斑对目标探测的影响,采用物理光学近似方法,进行了平面波激光照射在粗糙球体和圆锥体目标时对散射场统计特性的理论分析,推导了粗糙体目标散射场量的二阶统计矩, 数值计算了粗糙球体和锥体的非相干散射分量比随粗糙度、散射角、半径及目标材料的变化情况。结果表明,散射角的变化对粗糙球体散射非相干分量比有影响,粗糙度变大,目标的非相干分量占总散射分量的比重越大;随着粗糙球体半径变小,球体表面越粗糙;圆锥体目标散射非相干分量比的峰值位置随粗糙度变化而不同,但其峰值均位于镜反射方向上;金属类材料比非金属抛光铝材料的非相干分量比小,且半径变化与非相干分量比成正比。该研究结果可为更复杂目标激光散射特性和激光散斑探测、识别的研究提供一定的参考价值。     

MMW scattering by rough lossy dielectric cylinders and tree trunks

The millimeter-wave (MMW) radar backscatter response of a rough, lossy, dielectric cylinder is examined both analytically and experimentally. Models for both the coherent and incoherent components of the scattered field are developed based on the geometric-optics approximation. The accuracy of the analytical models and their applicability in predicting the backscatter response of tree trunks are determined experimentally by measuring the radar backscatter at 95 GHz from a rough conducting cylinder and from a section of a tree trunk. In both cases, very good agreement was achieved between the model predictions and the measurements     

Depolarization and scattering of electromagnetic waves by irregular boundaries for arbitrary incident and scatter angles full-wave solutions

Explicit expressions are presented for the radiation fields scattered by rough surfaces. Both electric and magnetic dipole sources are assumed, thus excitations of both vertically and horizontally polarized waves are considered. The solutions are based on a full-wave approach which employs complete field expansions and exact boundary conditions at the irregular boundary. The scattering and depolarization coefficients axe derived for arbitrary incident and scatter angles. When the observation point is at the source these scattering coefficients are related to the backscatter cross section per unit area. Solutions based on the approximate impedance boundary condition are also given, and the suitability of these approximations are examined. The solutions are presented in a form that is suitable for use by engineers who may not be familiar with the analytical techniques and they may be readily compared with earlier solutions to the problem. The full-wave solutions are shown to satisfy the reciprocity relationships in electromagnetic theory, and they can be applied directly to problems of scattering and depolarization by periodic and random rough surfaces.     

Bistatic scattering by arbitrarily shaped objects with rough surface at optical and infrared frequencies

The scattering cross sections for arbitrarily shaped dielectric objects with rough surface are determined for optical and infrared frequencies using the Kirchhoff approximation. The formula of the coherent scattering cross section is derived, and numerical method of incoherent scattering cross section is given. As a specific example, the infrared laser scattering cross sections of rough spheres are calculated at 1.06 μm.     

Sommerfeld and Zenneck wave propagation for a finitely conductingone-dimensional rough surface

Starting with Zenneck and Sommerfeld wave propagation over a flat finitely conducting surface has been extensively studied by Wait (see IEEE Antennas Propagat. Mag., vol.40, p.7-24, 1998) and many other authors. We examine propagation over a finitely conducting rough surface, also studied by many people including Feinberg (1944), Bass and Fuks (1979), and Barrick (see Radio Sci., vol.6, p.517-26, and vol.6., p.527-33). This paper extends the multiple scattering theories based on Dyson and Bethe-Salpeter equations and their smoothing approximations. The theory developed here applies to rough surfaces with small root-mean-square (RMS) heights (σ<0.1λ). We limit ourselves to the one-dimensional (1-D) rough surface with finite conductivity excited by a magnetic line source, which is equivalent to the Sommerfeld dipole problem in two dimensions (x-z plane). With the presence of finite roughness, the total field decomposes into the coherent field and the incoherent field. The coherent (average) field is obtained by using Dyson's equation, a fundamental integral equation based on the modified perturbation method. Once the coherent field has been obtained, we determine the Sommerfeld pole, the effective surface impedance, and the Zenneck wave for rough surfaces of small RMS heights. The coherent field is written in terms of the Fourier transform, which is equivalent to the Sommerfeld integral. Numerical examples of the attenuation function are compared to Monte Carlo simulations and are shown to contrast the flat and rough surface cases. Next, we obtain the general expression for the incoherent mutual coherence functions and scattering cross section for rough conducting surfaces     

Diffuse like and cross-polarized fields scattered from irregularlayered structures-full-wave analysis

The full-wave solution for scattering from two-dimensional (2-D) irregular layered structures is expressed as a sum of radiation fields, the lateral waves and the surface waves. Only the radiation far fields are considered in this work. The lateral waves and surface waves are ignored since excitations of plane waves are considered and the observation points are in the far fields. The scattering coefficients appearing in the full-wave generalized telegraphists' equations for irregular layered structures are proportional to the derivatives of the surface heights at each interface. Using a first-order iterative procedure to solve the generalized telegraphists' equations, the diffusely scattered fields from irregular layered structures are expressed as a sum of first-order fields scattered at each rough interface. In this paper, the like and cross-polarized diffuse scattered fields are derived for three medium irregular structures with 2-D rough interfaces. The thickness of the coating material or thin film between the two interfaces is arbitrary, however, in this work it is assumed to be constant. Thus, in this case, both interfaces are rough and there are five different scattering processes identified in the full-wave results. A physical interpretation is given to the five different scattering mechanisms that contribute to the diffusely scattered fields. This work can be used to provide realistic analytical models of propagation across irregular stratified media such as ice or snow covered terrain, remote sensing of coated rough surfaces, or the detection of buried objects in the presence of signal clutter from the rough interfaces     

The use of fractals for modeling EM waves scattering from rough seasurface

A rough surface model based on fractal geometry is presented for the study of surface scattering. In particular, the Pierson-Moskowitz spectrum is incorporated into this model to represent a fully developed sea surface. The Kirchoff approximation is used to evaluate the scattered field from this rough surface. Some interconnection are found between the surface model developed and the statistical characteristics of the scattered field. These include: 1) the relationship between the surface correlation length and the surface fractal dimension; 2) the relationship between the shape parameter of the K-distribution and the surface fractal dimension; 3) the mean value of the scattered amplitude as a function of the surface fractal dimension; and 4) the effect of the incident angle on the scattered field     

Radar images of rough surface scattering: comparison of numericaland analytical models

Rough surface scattering theories are investigated through analysis of radar images. Backscatter results from 10 GHz to 14 GHz under tapered wave illumination are considered for one-dimension (1D) random rough surface realizations which satisfy an impedance boundary condition. Back-projection tomography is applied to form two-dimensional (2D) synthetic aperture radar images from deterministic surface scattered field data at multiple incidence angles and frequencies. Numerical predictions of surface backscattered fields are obtained from an accelerated forward-backward (FB) method and the resulting images are compared with those obtained from approximate scattering theories such as the physical optics (PO) approximation, the small slope approximation (SSA), and the nonlocal SSA (NLSSA). The resulting radar images illustrate scattering sources associated with single and multiple scattering on the boundary, and a ray tracing analysis confirms the locations of time-delayed image points due to double reflections. For single scattering effects, the images demonstrate excellent agreement between analytical and numerical methods in both horizontal and vertical polarizations. For surfaces with RMS height 2.0 cm and correlation length 7.5 cm at normal incidence, multiple-scattering effects are observed and successfully captured when the lowest-order NLSSA is employed     

Rough surface scattering using specular point theory

The expression for the average scattering cross section for a random surface is derived using the stationary phase approximation for the scattered field due to specular points on a finitely conducting rough surface. A previous error in the literature is corrected by showing that the proper result is proportional to the average value of the product of the number of specular points per unit area and the principal radii of curvature at the specular point, rather than the product of the average value of the number of specular points and the average value of the radii of curvature. When the correct expression for the average value of the product is inserted in the expression for the scattering cross section, the result is in total agreement with the answers obtained when the averaging and stationary phase processes are interchanged. This analysis explicitly accounts for shadowing.     

A numerical simulation of scattering from one-dimensionalinhomogeneous dielectric random surfaces

An efficient numerical solution for the scattering problem of inhomogeneous dielectric rough surfaces is presented. The inhomogeneous dielectric random surface represents a bare soil surface and is considered to be comprised of a large number of randomly positioned dielectric humps of different sizes, shapes, and dielectric constants above an impedance surface. Clods with nonuniform moisture content and rocks are modeled by inhomogeneous dielectric humps and the underlying smooth wet soil surface is modeled by an impedance surface. In this technique, an efficient numerical solution for the constituent dielectric humps over an impedance surface is obtained using Green's function derived by the exact image theory in conjunction with the method of moments. The scattered field from a sample of the rough surface is obtained by summing the scattered fields from all the individual humps of the surface coherently ignoring the effect of multiple scattering between the humps. The statistical behavior of the scattering coefficient σ° is obtained from the calculation of scattered fields of many different realizations of the surface. Numerical results are presented for several different roughnesses and dielectric constants of the random surfaces. The numerical technique is verified by comparing the numerical solution with the solution based on the small perturbation method and the physical optics model for homogeneous rough surfaces. This technique can be used to study the behavior of scattering coefficient and phase difference statistics of rough soil surfaces for which no analytical solution exists     

双程传输中粗糙面散射场统计特性的研究

为了研究激光散斑对目标探测的影响,采用广义惠更斯-菲涅耳原理,进行了双程传输中激光束照射远场目标时在接收平面上散斑的统计特性的理论分析,导出了汇聚光束及准直光束照射情况下散斑场的互相关函数、平均散射强度以及强度协方差的表达式。结果表明,随着波束特征半径以及传输距离的减小,会聚波束照射时接收平面上散斑场的互相关函数下降迅速,而准直波束正好相反,总体上准直波束比会聚波束下降要快得多;强度协方差函数的变化规律与互相关函数的变化规律基本一致。     

Estimation of Incoherent Scattered Field by Multiple Scatterers in Random Media

This paper proposes a method to estimate directly the incoherent scattered intensity and radar cross section (RCS) from the effective permittivity of a random media. The proposed method is derived from the original concept of incoherent scattering. The incoherent scattered field is expressed as a simple formula. Therefore, to reduce computation time, the proposed method can estimate the incoherent scattered intensity and RCS of a random media. To verify the potential of the proposed method for the desired applications, we conducted a Monte‐Carlo analysis using the method of moments; we characterized the accuracy of the proposed method using the normalized mean square error (NMSE). In addition, several medium parameters, such as the density of scatterers and analysis volume, were studied to understand their effect on the scattering characteristics of a random media. The results of the Monte‐Carlo analysis show good agreement with those of the proposed method, and the NMSE values of the proposed method and Monte‐Carlo analysis are relatively small at less than 0.05.