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。     

二维有耗介质目标重建的迭代－共轭梯度方法

本文提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。连续采用多个方向的ＴＭ平面波照射目标，并分别采集目标区域外的散射场数据。本文利用矩量法．（ＭＯＭ）将积分方程变成矩阵方程，我们以Ｂｏｒｎ近似为初始值，通过多次迭代，实现目标特性的重建。通常，逆问题多是不适定的。为了克服解的不适定性，我们采用共轭梯度法（ＣＧＭ）求解逆问题。最后，通过计算机模拟，给出了一些数值重建结果。     

二维有耗介质目标重建的迭代——共轭梯度方法

本文提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。连续采用外个方向的ＴＭ平面波照射目标，并分别采集目标区域外的散射场数据。本文利用矩量法将积分方程变成矩阵方程。我们以Ｂｏｎ近为初始值，通过多次迭代，实现目标特性的重建。通常，逆问题多是不适定的。     

IPO-MoM混合法分析开槽电大目标的电磁散射

本文利用迭代物理光学法（ＩＰＯ）与数值方法相结合的混合方法计算二维开槽民大目标的电磁散射,首先根据等效原理将散射场进行分解,分别采用ＩＰＯ法和矩量法（ＭｏＭ）计算槽缝填充的电大目标和槽缝的散射,并在口径处应用广义网络原理处理耦合问题,数据结果表明本文方法是准确和高效的。     

分层媒质中浅二维目标的探测

本文探讨利用时域散射场信息探测分层媒质中法二维目标、分层媒质中目标的探测具有明显的应用价值．但相对自由空间而言，这种探测要困难得多。本文给出一种基于体等效原理的探测分层媒质中钱二维目标的方法。这是一种选代方法．从一阶玻恩近似开始迭代．每次迭代过程中正问题和逆问题各计算一欢，正问题采用FDTD方法。利用体等效原理，得到反映时域散射场和目标介电常数分布之间关系的体积分方程，经傅立叶变换并离散化后得到反演方程。为了克服问题的病态．采用了伪求逆技术、文中列举了几个具有代表性的例子，最后讨论了有关的几个问题。     

IPO—MoM混合法分析开槽电大目标的电磁散射

本文利用迭代物理光学法（ＩＰＯ）与数值方法相结合的混合方法计算二维开槽民大目标的电磁散射,首先根据等效原理将散射场进行分解,分别采用ＩＰＯ法和矩量法（ＭｏＭ）计算槽缝填充的电大目标和槽缝的散射,并在口径处应用广义网络原理处理耦合问题,数据结果表明本文方法是准确和高效的。     

非均匀介质薄筒的电磁逆散射

本文采用傅里叶级数展开的方法,处理截面为窄圆环的二维损耗介质体的电磁逆散射问题,根据介质体的散射场解析地表达出它的电参数。模拟计算得到满意的结果。     

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。     

基于随机平均梯度下降和对比源反演的非线性逆散射算法研究

采用非线性对比源反演(CSI)算法求解电磁逆散射问题时,在每次迭代过程中都涉及到求解散射场数据关于对比源和总场的微分,即Jacobi矩阵,该矩阵求解导致算法存在计算代价大和收敛速度慢等问题。该文在CSI框架下,采用一种基于随机平均梯度下降的对比源反演算法(SAG-CSI)代替原来的全梯度交替共轭梯度算法来重构介质目标介电常数的空间分布信息。该方法在每次迭代中只需计算随机抽取的部分测量数据在目标函数中的梯度信息,同时目标函数对未抽中的测量数据的梯度信息保持不变,用以上两部分梯度信息共同求解出目标函数的最优值。由模拟数据结果表明,该方法与传统CSI方法在成像精度相比拟的情况下,降低了计算代价并提高算法收敛速度。     

用BP神经网络技术重构介质圆柱体电磁参数

将BP神经网络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.设置多个目标散射场观测点,分别以目标不同的电磁参数及其响应下的散射电场的幅值作为BP网络的输出与输入,采用L-M训练算法,经过适当的训练,建立起逆散射模型,再以新的散射电场作为输入,实时重构了自由空间及半空间介质圆柱体的相对介电常数和电导率.数值结果显示了该方法的有效性及准确性,为目标的实时逆散射研究提供了一种高效的方法.     

介质散射的双线性高阶叠层基函数矩量法分析

提出了一种基于双线性思想的高阶基函数降低矩量法分析介质目标电磁散射问题时的未知量.论文给出了双线性高阶叠层基函数的构造过程, 并将其应用于介质的面积分方程分析中.数值算例比较了不同阶数时所需未知量以及计算精度, 表明该高阶叠层基函数在满足相同积分方程的计算精度时能比低阶基函数节省未知量.     

Boundary integrodifferential equations for electromagnetic scattering problems in three dimensions

On the basis of two vector representations of electromagnetic fields we introduce a new system of boundary integrodifferential equations for the solution of scattering problems in three dimensions. The unknowns of this system present two scalar functions, namely, the "" coefficients of Atkinson-Wilcox expansion; electromagnetic field being reconstructed with these functions by means of certain recursive-differential operators. We define an algebraic analog of the equations by expanding unknowns into Fourier series with respect to spherical harmonics. Verification of the approach is done on the basis of the solution of well-known canonical problems.     

Simultaneous thermal and electrical analysis of nonlinear microwavecircuits

A software tool for the simultaneous determination of the thermal and electrical steady-state regimes of nonlinear microwave circuits containing temperature-dependent active devices is introduced. The analysis technique is an extension of the classic piecewise harmonic-balance method, and is quite general-purpose. It can be applied to networks operating under multiple-tone excitation, including pulsed-RF regimes. The simulation problem is reduced to a nonlinear algebraic system whose unknowns are electrical and thermal state-variable harmonics. Advanced numerical techniques are used to overcome the difficulties arising from the high degree of nonlinearity and from the very large number of unknowns of the numerical problem. The program incorporates a facility for the evaluation of the thermal constants of multiple finger planar devices starting from geometrical data     

A theorem on the moment methods

The inner product involved in the moment methods is usually an integral, which is evaluated numerically by summing the integrand at certain discrete points. In connection with this inner product, a theorem is proved, which states that the overall number of points involved in the integration must not be smaller than the number of unknowns involved in the moment method. If these two numbers are equal, a point-matching solution is obtained, irrespective of whether one has started with Galerkin's method or the least squares method. If the number of points involved in the integration is larger than the number of the unknowns, a weighted point-matching solution is obtained.     

Fast and accurate solutions of extremely large integral-equation problems discretised with tens of millions of unknowns

The solution of extremely large scattering problems that are formulated by integral equations and discretised with tens of millions of unknowns is reported. Accurate and efficient solutions are performed by employing a parallel implementation of the multilevel fast multipole algorithm. The effectiveness of the implementation is demonstrated on a sphere problem containing more than 33 million unknowns, which is the largest integral-equation problem ever solved to our knowledge     

Mode-expansion method for calculating electromagnetic waves scattered by objects on rough ocean surfaces

A mode-expansion method that needs less than 6% the number of unknowns required by conventional method of moments is introduced in calculating two-dimensional electromagnetic wave scattering from perfectly conducting objects on rough ocean surfaces. Modes are selected for dominant propagation waves so that the number of unknowns in the matrix equation are minimized. In the numerical examples, ocean surfaces are modeled as perfectly conducting rough surfaces described by the Pierson-Moskowitz power spectrum. Bistatic radar cross-sections (RCS) of various objects, such as ship-like and low-observable targets, are calculated for a 1-GHz incident plane wave and are validated for accuracy against an iterative MoM solution.     

Improving Conditioning of Electromagnetic Surface Integral Equations Using Normalized Field Quantities

When the surface integral equation method is applied to study electromagnetic scattering by dielectric or composite metallic and dielectric objects, the unknowns, i.e., the electric and magnetic surface current densities, and the elements of the system matrix, are often of the very different scales. As a consequence, the system matrix may have a high (singular value) condition number. An efficient method is presented to balance the unknowns and the integral equations, and the elements of the system matrix, too. The method is based on the use of normalized field quantities and unknowns, and carefully chosen scaling factors. In the case of dielectric and composite objects the condition numbers of the SIE matrices can be reduced with several orders of magnitudes by the developed method. In the case of high contrast objects, or if the frequency is very low, the developed method leads also to a clear improvement on the convergence of iterative solutions     

A nonlinear estimation method in tomographic imaging

A nonlinear estimation approach to solving the inverse scattering problem, and reconstructing the space-varying complex permittivity of unknown objects is considered. The bilinear operator equations governing the scattering are approximated into finite dimensional spaces on the basis of the finite degrees of freedom of data, and on the simple concept that one cannot expect to reconstruct an arbitrary function from a finite number of independent equations. As a consequence, a discrete model, well suited to numerical inversion, is developed. The particular bilinear nature of the equations, and a suitable choice of contrast and field unknowns allows the functional adopted in the estimation to be minimized in an accurate and numerically efficient manner. Numerical experiments show how the method is capable, when a proper number of searched unknowns is adopted, to manage the possible convergence to local minima (which is a typical question in nonlinear inverse problems), and validate the effectiveness of the proposed approach     

A LINEARIZATION ATTACK ON MODIFIED E0 ALGORITHM

A linearization attack on the Key Stream Generator （KSG） of the modified Eo algorithm proposed by Hermelin [Proceedings of ICISC＇99, Springer LNCS 1787, 2000, 17-29] is given in this paper. The initial value can be recovered by a linearization attack with O（2^60.52） operations by solving a System of Linear Equations （SLE） with at most 2^20.538 unknowns. Frederik Armknecht [Cryptology ePrint Archive, 2002/191] proposed a linearization attack on the KSG olEo algorithm with O（2^70.341） operations by solving an SLE with at most 2^24.056 unknowns, so the modification proposed by Hermelin reduces the ability or E0 to resist the linearization attack by comparing with the results ofFrederik Armknecht.     

Application of the integral equation-asymptotic phase method totwo-dimensional scattering

A hybrid-procedure called the integral equation-asymptotic phase (IE-AP) method is investigated for scattering from perfectly conducting cylinders of arbitrary cross-section shape. The IE-AP approach employs an asymptotic solution to predict the relatively rapid phase dependence of the unknown current distribution, to leave a slowly varying residual function that can be represented by a coarse density of unknowns. In the present investigation, the current density appearing within the combined-field integral equation is replaced by the product of a rapidly varying phase function obtained from the physical optics current and a residual function. The resulting equation is discretized by the method of moments, using subsectional quadratic polynomial basis functions defined on curved cells to represent the residual function. Results show that the required density of unknowns can often be as few as one per wavelength on average without a significant loss of accuracy in the computed current density, even for scatterers with corners