基于BP神经网络的介质圆柱体逆散射方法研究

将人工智能技术应用于介质圆柱体电磁逆散射问题研究,通过BP神经网络将原逆散射问题转化为一个回归估计问题,重构了目标的几何与电磁参数。在TM波的照射下,设置多个目标散射场的观测点,以散射场的幅值作为BP网络的输入,相应的几何与电磁参数作为输出,经过适当的训练,建立了介质圆柱体逆散射模型,并以此模型重构了已知探测范围内的介质圆柱体的半径、相对介电常数及电导率。比较结果显示了该方法的有效性和准确性,为目标的实时逆散射研究提供了一种有效方法。     

沙尘暴中圆柱形导体目标对电磁波的散射

研究了尘暴介质中导体目标对电磁波的散射特性 ;当入射波沿任意方向极化时 ,得出了毫米波段散射场的表达式及目标表面的电流分布 ,导出了目标的微分散射宽度 ,结果表明 :散射场具有较强的后向散射增强效应 ,导体表面的纵向电流大于横向电流     

用TLM法模拟二维三维电磁散射近场和远场

数值模拟二维等腰直角三角形导电柱体和三维方形导电柱体对高斯脉冲平面波的电磁散射近场和远场分布。数值结果表明在三角柱体的电磁散射近场中镜面反射并不明显,但在远场却非常明显并且成为最主要的散射方向。三维电磁散射的数值结果表明,二次辐射是电磁散射的主要机理,并且在多个导体目标情况下,各自的散射场将互相干涉形成总的散射场。     

有限频宽条件下三维理想导体逆散射成像的计算研究

本文在保加斯基 -刘易斯 (Bojarski-Lewis)方法的基础上对有限频宽条件下三维理想导体逆散射成像的计算进行了研究 ,详细分析了频率孔径的选择、中间函数F的精确计算以及包络提取等问题 ,然后对典型理想导体进行了模拟计算 ,采用物理光学法计算散射场 ,并根据此数据进行了逆散射成像 ,结果吻合很好。     

用APRGA求解二维导体柱电磁逆散射

从电磁散射的积分方程出发，利用点匹配法和脉冲基函数求解电磁散射问题，以测量的散射场和计算的散射场的偏差程度为目标函数，将待优化变量设置为描述导体柱轮廓的形状函数的傅立叶展开式系数，Ai0，Ai1…AiN/2,Bi1,Bi2…BiN/2,k=1,2…k，通过参数自适应实数遗传算法（APRGA）对待优化变量进行优化，使目标函数达到最小值来对自由空间中导体柱族进行电磁成像，证实了APRGA比BRGA有更好的收敛性能和成像精度，更强的抗随机噪声干扰能力。     

相邻目标电磁散射特性的研究

本文研究了植被环境中相邻两目标的电磁散射问题.在互易原理的基础上,得到了计入相邻目标二次散射的散射场的积分表达式.推导得出了相邻有限长导体圆柱的二次散射场的闭式解,分析计算了其前向散射特性,并与矩量法的数值结果进行了比较.     

多极化前向散射RCS分析及其对目标分类识别的影响

在前向散射情况下,基于阴影逆合成孔径雷达(SISAR)成像原理可以获得运动目标的轮廓像,从而对运动目标进行分类与识别。为了研究多极化对前向散射雷达运动目标识别的影响,该文根据前向散射阴影逆合成孔径原理,建立了目标前向散射雷达截面积(RCS)与目标轮廓像谱信息之间的联系,首次将多极化引入到前向散射目标的分类识别中;并借助电磁仿真软件CST,仿真得到了多极化条件下目标的前向散射RCS曲线。通过分析仿真结果发现同一个目标在不同极化情况下具有不同的前向散射RCS旁瓣曲线,此种差异对应于目标轮廓像的差异;联合多极化产生的前向散射RCS旁瓣差异可以获得更多关于目标轮廓的特征信息。仿真结果验证了多极化能够提高前向散射目标分类识别的能力。     

复杂飞行目标电磁散射特性及计算

本文讨论了复杂飞行目标电磁散射分析及计算的一种方法，利用曲面拟合法的目标几何外形进行拟合，以样条函数拟合目标外形，并进行分元，利用物理光学近似技术求解每一分元的散射场，经相位综合，求得目标总射场，数学模型和实例证明了方法的正确性。文中讨论限于全金属导体目标的单站散射情况。     

二维多体散射系统分析

本文提出一种多体散射系统的普遍化的分析方法,原则上能处理任意形式的多体散射系统。利用系统散射方程把多体问题归结为一组单体问题,然后用有限元法对各个单体问题逐个进行分析。本文着重讨论二维多体系统,并给出由介质涂敷导体往构成的三体散射系统的计算实例。     

基于迭代散射算法的柱体阵列散射场分析

该文基于迭代散射算法(ISP)对柱体阵列的散射场进行分析。通过矢量柱面波函数展开,根据理想导体表面边界条件,建立柱体表面入射场与散射场的关系式。将前一次迭代时柱体阵列的近区散射场作为下一次迭代的入射场,推导出柱体阵列散射场系数间的迭代关系。通过分析不同迭代次数下2~4个柱体的散射场,确定3次迭代即可保证算法的准确度。对比数值结果表明,迭代散射算法具有与矩量法(MoM)结果同等的准确度,并具有明显优于矩量法的计算速度。     

An integral equation approach to electrical conductance tomography

A reconstruction procedure for electrical conductance tomography developed by solving a linear Fredholm integral equation of the first kind is discussed. The integral equation is obtained from a linearized Poisson's equations. Properties of the integral equation are discussed, and problems associated with numerical solution of the equation are treated. The reconstruction requires only one matrix multiplication and therefore can be computed in a short time. Test results of the algorithm using both simulated and measured data are presented.     

A numerical solution to full-vector electromagnetic scattering bythree-dimensional nonlinear bounded dielectrics

This paper deals with electromagnetic scattering by nonlinear dielectric objects. In particular, a numerical approach is developed that is aimed at determining the distributions of the electromagnetic field vector inside a three-dimensional nonlinear, inhomogeneous, isotropic scatterer illuminated by a time-periodic incident electric field vector. An integral-equation formulation for the full-vector scattering problem is considered, and the nonlinear effect is taken into account by introducing equivalent sources and a Fourier-series representation. A system of integral equations (for each harmonic vector component and for the static term) is obtained that includes the internal electric field distribution as the unknown. After discretization, the solution is reduced to solving an algebraic system of nonlinear equations. Some preliminary numerical results are reported concerning scatterers that exhibit a specific (quadratic) dependence of the dielectric permittivity on the total electric field. The harmonic components of the scattered electric field outside the objects are also computed     

An asymptotic solution of EM backscattering from a conducting sphere coated with a composite material

An asymptotic high frequency solution for the electromagnetic (EM) backscattered field produced by a plane wave incident on a perfectly conducting sphere coated with a thin composite material is derived in this paper. For the formulation of the incident and the reflected field the characteristics of the wave transformation and the line integral via the stationary phase method were applied, respectively, and the obtained results are cast in the ordinary ray formats of a geometrical optics field. Based on the Watson transform technique, the diffracted field is also formulated from the residue series solution of the problem and presented in a form suitable for the numerical calculation. The numerical results obtained from the derived asymptotic solution show excellent agreement with those from the rigorous eigenfunction solution.     

Microwave imaging for a dielectric cylinder

The problem of reconstructing both the shape and the relative permittivity of a homogeneous dielectric cylinder from the measurement of scattered field is numerically simulated. The Newton-Kantorovitch algorithm and the moment method are used to solve a set of nonlinear integral equations. Numerical results show that, with multiple incident directions, good reconstruction is obtained. This algorithm can be applied at a single frequency without limitation on the value of dielectric constant. The effect of random noise on imaging reconstruction is also investigated     

用修正玻昂迭代法重建复杂介质结构

着重阐述了将修正玻昂迭代法结合高效的数值模式匹配法用于感应测井中的反演与剖面成象。首先利用玻昂近拟将非线性问题线性比，然后应用吉洪诺夫正则化与迭代方法求解电磁场的积分方程。     

A path integral time-domain method for electromagnetic scattering

A new full wave time-domain formulation for the electromagnetic field is obtained by means of a path integral. The path integral propagator is derived via a state variable approach starting with Maxwell's differential equations in tensor form. A numerical method for evaluating the path integral is presented and numerical dispersion and stability conditions are derived and numerical error is discussed. An absorbing boundary condition is demonstrated for the one-dimensional (1-D) case. It is shown that this time domain method is characterized by the unconditional stability of the path integral equations and by its ability to propagate an electromagnetic wave at the Nyquist limit, two numerical points per wavelength. As a consequence the calculated fields are not subject to numerical dispersion. Other advantages in comparison to presently popular time-domain techniques are that it avoids time interval interleaving and it does not require the methods of linear algebra such as basis function selection or matrix methods     

用变形玻恩迭代法反演电导率的二维非均匀分布

本文把变形玻恩迭代方法用于求解二维轴对称逆散射问题。该方法提供一种迭代收敛较快的反演成像算法。利用对称轴上２０ｋＨｚ电场的测量值在二维非均匀介质中对电导率分布进行反演。首先给出对于未知电导率分布的非线性积分方法，并用玻恩近似使积分方程线性化，然后用吉洪诺夫与正则化方法求出电导率分布。在迭代过程中，数值模式匹配法用于求解正演场．数值实例表明，用简单的对称轴上测量场能得到好的好的成像结果。     

Electromagnetic coupling through small apertures in a conducting screen

The electromagnetic field coupling through small apertures illuminated by an arbitrary incident plane wave is discussed for general aperture shapes. A set of new integral equations in a form highly amenable to numerical solution techniques is derived. Based on the application of the Rayleigh series method, an analytical solution is obtained for the first few terms of the expansion of the apertureE- field of a circular aperture. Numerical results are also constructed for the aperture field and the diffracted field of small rectangular apertures and compared with those of the circular apertures.     

Microwave imaging within the second-order Born approximation:stochastic optimization by a genetic algorithm

This paper addresses the problem of reconstructing the location, shape, and dielectric permittivity distribution of an inhomogeneous dielectric object from measurements of the field scattered by the object. The object is an inhomogeneous infinite cylinder of arbitrary cross section illuminated by a transverse magnetic incident electric field. The approach is based on the Lippmann-Schuringer integral equation for the electromagnetic inverse scattering problem, approximated by applying the second-order Born approximation, which allows an extension of the range of contrast values that can be accurately imaged. The numerical approach is developed in the spatial domain and makes use of a multi-illumination multiview processing. In particular, the inverse problem is recast in a global nonlinear optimization problem (including a penalty function), solved by a stochastic method based on a genetic algorithm. In this paper, the mathematical formulation of the approach is described and the results of several dielectric reconstructions are reported, including comparisons with analogous reconstructions performed within the linearized (first-order) Born approximation     

Confocal microwave imaging for breast cancer detection: delay-multiply-and-sum image reconstruction algorithm

A new image reconstruction algorithm, termed as delay-multiply-and-sum (DMAS), for breast cancer detection using an ultra-wideband confocal microwave imaging technique is proposed. In DMAS algorithm, the backscattered signals received from numerical breast phantoms simulated using the finite-difference time-domain method are time shifted, multiplied in pair, and the products are summed to form a synthetic focal point. The effectiveness of the DMAS algorithm is shown by applying it to backscattered signals received from a variety of numerical breast phantoms. The reconstructed images illustrate improvement in identification of embedded malignant tumors over the delay-and-sum algorithm. Successful detection and localization of tumors as small as 2 mm in diameter are also demonstrated.