LUDWIG算法的改进与应用

Ludwig积分在雷达散射截面(RCS)计算中被广泛应用.传统的Ludwig积分针对基于三角形面元的模型剖分技术,因此主要用于平面片积分.三角形面片由于是平面片,因此无法对复杂模型准确建模.本文改进了Ludwig积分,并将其用于基于NURBS(非均匀有理B样条)技术的曲面积分.通过计算实例可以看出改进后的Ludwig积分形式适用于基于曲面建模技术的物理光学(PO)算法.     

一维微粗糙面与其上方金属平板的复合电磁散射研究

研究了一维导体随机粗糙面与其上方金属平板的复合电磁散射。应用互易性原理使求解复合目标的二次散射场简化为求解包含平板上的极化电流和微粗糙面散射场的积分方程。利用物理光学近似和粗糙面微扰法分别计算了平板上的感应电流和粗糙面的电磁散射场,导出了复合散射模型单、双站散射的计算公式并给出了单站数值计算结果,讨论了后向复合散射截面随入射频率及平板尺寸、位置的变化关系。     

计算电大尺寸目标物理光学散射场的快速算法

针对电大尺寸目标高频散射场的仿真，采用物理光学（physics optics，PO）算法来求解.由于PO积分为高振荡积分，传统的数值求积方法非常耗时，文中提出了数值最速下降路径（numerical steepest descent path method，NSDP）算法来计算.首先，通过对振幅函数和相位函数二次拉格朗日函数插值，得到二次曲面片上PO积分标准形式.其次，通过变换积分路径，将高振荡PO积分转化为最速下降路径上的积分，大大减少了计算复杂度.NSDP算法进一步将PO积分转变为驻相点、谐振点和顶点的贡献，具有鲜明的物理意义.数值算例证明了NSDP算法具有精度误差可控和频率无关的特性.     

NURBS曲面建模的电大目标的宽带RCS快速计算

该文采用物理光学方法(PO),快速计算了非均匀有理B样条 (NURBS) 曲面建模的电大目标的时域瞬态散射和宽带雷达截面(RCS)。通过对频域物理光学散射场表达式进行逆傅里叶变换推导出卷积形式的瞬态散射表达式;对频域物理光学积分进行逆傅里叶变换得到时域物理光学积分的表达式。为了避免数值积分的使用,将NURBS曲面等参数离散为一组三角面片,运用Radon变换得到了时域和频域物理光学积分的精确闭式表达式。遮挡消隐时使用改进的z-buffer方法进行了加速。对时域瞬态散射场快速傅里叶变换得到目标的宽带RCS。文中计算了高斯脉冲平面波入射下模型的瞬态散射响应和宽带RCS,数值结果表明该文方法具有很高的计算精度,且计算速度快于传统时域物理光学法(TDPO)。     

基于NURBS建模技术的迭代MoM-PO方法分析 电大尺寸平台天线方向图

本文采用基于非均匀有理B样条(NURBS,Non-Uniform Rational Bezier Spline)曲面建模技术的物理光学方法结合矩量法(Method of Moments-Physical Optics)分析位于电大尺寸平台附近天线的辐射方向图.文章推导了基于有理贝齐尔曲面的物理光学散射场计算公式.采用驻相法计算有理贝齐尔曲面上的物理光学感应电流积分.利用物理光学散射场迭代矩量法区域的电压矩阵.通过与传统平面片建模的物理光学方法的计算结果对比,说明本文方法的有效性和优点.     

任意形状电大散射体附近天线受扰方向图的快速分析

该文采用基于非均匀有理B样条曲面(NURBS)建模的物理光学方法结合矩量法(MoM-PO)分析任意形状电大散射体附近天线的受扰方向图。采用插值驻相点技术加快了方向图的计算速度。文章推导了基于有理贝齐尔曲面的物理光学散射场计算公式,采用驻相法(SPM)计算有理贝齐尔曲面上的物理光学感应电流积分从而得到物理光学散射场,并利用物理光学散射场迭代矩量法区域的电压矩阵。通过与传统平面片建模的物理光学方法的计算结果对比,说明该文方法的有效性和计算速度快的优点。     

阻抗劈散射的MM-PO混合算法研究

本文系统地研究了各向异性阻抗劈绕射的矩量法-物理光学(MM-PO)混合算法.首先研究了任意各向异性阻抗面的物理光学模型,推导出表面物理光学等效电磁流计算式.其次,提出了一种有效的含Hankel函数的弱振荡被积函数无穷积分处理方法.最后,将作者已公开发表的修正绕射电流基函数用于各向异性阻抗劈散射场研究,数值结果和已知的一致性绕射理论结果高度吻合.     

二维有耗介质目标重建的Newton迭代方法

本文给出了一种由已知的散射场数据重建二维非均匀有耗目标的复介电常数的迭代算法。由积分方程出发，利用点匹配技术导出了依赖于未知参数的解析逆散射公式。由此可以以解析的形式计算场量对未知参数的导数（Jacobian和Hessian矩阵）。本文采用Newton优化方法迭代求解逆散射问题，具有二次收敛特性。为了克服逆散射中解的不适定性，连续采用多个方向的TM波照射目标，并采集目标区域外的散射场数据，以及采用共轭梯度法（CGM）求解逆问题，数值结果表明了本文所提方法的可行性和灵活性。     

随机粗糙面上目标散射差值计算的快速互耦迭代方法

用粗糙面上方有目标和无目标时空间散射场的差值计算的雷达散射截面,称为差场雷达散射截面.本文推导TE波入射下电场积分方程(EFIE),直接求解散射差场.本文提出目标与粗糙面之间的互耦迭代的计算方法,散射场纳入了目标与粗糙面之间复杂的相互作用,给出了迭代过程中纳入的粗糙面长度的选择.用Monte-Carlo方法,计算了P-M谱粗糙海面上方二维圆柱和方柱的散射,说明目标的几何结构对散射方向图的影响.     

半空间复杂目标的高频分析方法

该文研究了半空间内复杂导体目标散射的高频求解方法。分析半空间电磁波的传播规律,将半空间并矢格林函数引入传统的物理光学方法,等效电磁流法方法中,推导出半空间物理光学算法、半空间等效电磁流算法,同时结合图形电磁学和射线追踪方法,分别对半空间复杂目标的消隐和多重散射进行考虑,与半空间目标面元、棱边散射场相叠加,快速有效地计算了半空间复杂目标的雷达散射截面。数值结果证明了方法的有效性和准确性。     

Method based on physical optics for the computation of the radar cross section including diffraction and double effects of metallic and absorbing bodies modeled with parametric surfaces

A method to compute the monostatic radar cross section (RCS) of complex bodies modeled by nonuniform rational B-spline (NURBS) surfaces is presented. The bodies can be covered by any kind of radar absorbing material (RAM) with electric and/or magnetic losses. Physical optics (PO) is used to obtain the scattered field of each surface. Fresnel coefficients are included in the stationary phase method (SPM) in order to take into account the effect of the RAM material. The contribution of diffraction by edges and double effects is also considered, improving the results of the PO approach. The diffraction is computed by the equivalent current method (ECM). A combination of geometrical optics (GO) with PO and ECM is used for the double reflection and double interaction between edges and surfaces respectively. Some simple cases are shown to validate the proposed method. The reliability of the method to analyzing the effect of covering a realistic target with RAM is also illustrated.     

Equivalence of surface current and aperture field integrations for reflector antennas

The "extinction theorem" is used to prove that the fields of reflector antennas determined by integration of the current on the illuminated surface of the reflector are identical to the fields determined by aperture field integration with the Kottler-Franz formulas over any surfaceS_{a}that caps the reflector. As a corollary to this equivalence theorem, the fields predicted by integration of the physical optics (PO) surface currents and the Kottler-Franz integration of the geometrical optics (GO) aperture fields onS_{a}agree to within the locally plane-wave approximation inherent in PO and GO. Moreover, within the region of accuracy of the fields predicted by PO current or GO aperture field integration, the far fields predicted by the Kottler-Franz aperture integration are closely approximated by the far fields obtained from aperture integration of the tangential electric or magnetic field alone. In particular, discrepancies in symmetry between the far fields of offset reflector antennas obtained from PO current and GO aperture field integrations disappear when the aperture of integration is chosen to cap (or nearly cap) the reflector.     

Modeling tree effects on path loss in a residential environment

A theoretical model is proposed to compute the path loss in a vegetated residential environment, with particular application to mobile radio systems. As in the past, rows of houses or blocks of buildings are viewed as diffracting cylinders lying on the Earth and the canopy of the trees is located adjacent to and above the houses/buildings. In this approach, a row of houses or buildings is represented by an absorbing screen and the adjacent canopy of trees by a partially absorbing phase screen. The phase-screen properties are found by finding the mean field in the canopy of the tree. Physical optics (PO) is then used to evaluate the diffracting field at the receiver level by using a multiple Kirchhoff-Huygens integration for each absorbing/phase half-screen combination     

Uniform PO and PTD solution for calculating plane wavebackscattering from a finite cylindrical shell of arbitrary crosssection

The plane wave backscattering from a perfectly conducting three-dimensional shell of arbitrary cross section has peen studied. A uniform physical optics (PO) solution, valid across the reflection limits, is derived. The solution, derived from an asymptotic evaluation of the PO integral, includes end-point contributions that account for the diffracted field on edges. It can be improved by the fringe fields derived from an analytical integration of the equivalent edge currents of the physical theory of diffraction (PTD). It is computationally efficient for electrically large shells and compares very well with the finite-element method     

Application of physical optics to the RCS computation of bodiesmodeled with NURBS surfaces

The paper presents a method for the computation of the monostatic radar cross section (RCS) of electrically large conducting objects modeled by nonuniform rational B-spline (NURBS) surfaces using the physical optic (PO) technique. The NURBS surfaces are expanded in terms of rational Bezier patches by applying the Cox-De Boor transform algorithm. This transformation is justified because Bezier patches are numerically more stable than NURBS surfaces. The PO integral is evaluated over the parametric space of the Bezier surfaces using asymptotic integration. The scattering field contribution of each Bezier patch is expressed in terms of its geometric parameters. Excellent agreement with PO predictions is obtained. The method is quite efficient because it makes use of a small number of patches to model complex bodies, so it requires very little memory and computing time     

Special problems in applying the physical optics method forbackscatter computations of complicated objects

The backscatter computation of complicated objects is carried by the physical optics (PO) method, known as the vector Kirchoff approximation. The object is described by a geometrical model using flat plates (panels). These panels can be nonperfectly conducting and multilayered. The PO solution for the scattering matrix of a single multilayered panel is evaluated in detail using the Fresnel reflection coefficients. An example of the computed reflection coefficient of a two-layered medium is presented. The phase integral of the PO method is solved analytically. The hidden-surface problem is discussed, and the procedure for the treatment of doubly reflecting panels is described. For an ideal conducting cube with additional surfaces that generate shadow and double-reflection effects, the computed radar cross section (RCS) is compared with measurements. Computational results of the RCS for nonperfectly conducting panels are given     

Diffraction by an arbitrary-angled dielectric wedge. I. Physicaloptics approximation

A complete form is presented of the physical optics solution to diffraction by an arbitrary dielectric wedge angle with any relative dielectric constant in cases of both E- and H-polarized plane waves incident on one side of two dielectric interfaces. The solution, which is obtained by performing the physical optics (PO) approximation to the dual integral equation formulated in the spatial frequency domain, is constructed by the geometrical optics terms, including multiple reflection inside the wedge and the edge diffracted field. The diffraction coefficients of the edge diffracted field are represented in a simple form as two finite series of cotangent functions weighted by the Fresnel reflection coefficients. Far-field patterns of the PO solutions for a wedge angle of 45°, relative dielectric constants 2, 10, and 100, and an E-polarized incident angle of 150° are plotted in figures, revealing abrupt discontinuities at dielectric interfaces     

时域物理光学法分析均匀介质目标的瞬态散射

提出将时域物理光学法(TDPO)应用于计算电大均匀介质目标的时域散射场。将菲涅尔反射系数应用到频域物理光学近似中,由逆傅里叶变换推导出介质TDPO的表达式,从而使TDPO能够分析电大均匀介质目标的瞬态响应。同时给出了三角面片建模下入射波的遮挡消隐方法。计算了典型目标的瞬态散射响应和宽带雷达散射截面(RCS),与其他方法求得的结果吻合良好,验证了介质TDPO的正确性。     

A physical optics version of the geometrical theory of diffraction

The authors derive a diffraction coefficient which is suitable for calculating the filed diffracted by the vertices of perfectly conducting objects. This diffraction coefficient is used to calculate the field scattered by the corner of a metallic sheet. Two diffraction coefficients, one for edges and one for vertices, are derived by solving the appropriate canonical problems using the physical optics (PO) approximation. The diffraction coefficients are calculated by first using the PO approximation which consists of calculating the total field on the surface of an object from the incident field according to the laws of geometrical optics, and then calculating the scattered field by employing this total surface field in a vector diffraction integral. The validity of the diffraction coefficients has been investigated by comparing their predictions with experimental measurements of the scattered field from a single corner of a rectangular metal sheet, and good agreement was found