玻恩近似下的无相位检测电磁成像方法

提出玻恩近似下用无相位检测数据重建复目标的方法.由电场积分方程推导散射场的实部和虚部之间的关系;在玻恩近似下基于散射场实部虚部关系,利用优化算法由无相位电磁检测数据反演目标散射场的实部和虚部;最后利用标准玻恩反演法对目标进行成像.数值实验显示该方法能有效地对弱散射目标成像.     

On the role of measurement configuration in contactless GPR data processing by means of linear inverse scattering

This paper deals with a linear two-dimensional inverse scattering problem within a half space geometry with data gathered far away from the interface, which is relevant in ground penetrating radar contactless data processing. In particular, the role of the measurement configuration is discussed with regard to the information retrievable from multifrequency data.     

Inverse scattering of two-dimensional dielectric objects buried ina lossy earth using the distorted Born iterative method

An efficient inverse-scattering algorithm is developed to reconstruct both the permittivity and conductivity profiles of two-dimensional (2D) dielectric objects buried in a lossy earth using the distorted Born iterative method (DBIM). In this algorithm, the measurement data are collected on (or over) the air-earth interface for multiple transmitter and receiver locations at single frequency. The nonlinearity due to the multiple scattering of pixels to pixels, and pixels to the air-earth interface has been taken into account in the iterative minimization scheme. At each iteration, a conjugate gradient (CG) method is chosen to solve the linearized problem, which takes the calling number of the forward solver to a minimum. To reduce the CPU time, the forward solver for buried dielectric objects is implemented by the CG method and fast Fourier transform (FFT). Numerous numerical examples are given to show the convergence, stability, and error tolerance of the algorithm     

High-frequency scattering by objects buried in lossy media

The uniform geometrical theory of diffraction (UTD) is extended so that it can be used to calculate the scattering from an object buried in a lossy medium. First, the accuracy of this high frequency method is examined by comparing numerical results for the scattering by a polygonal cylinder in a lossy medium of infinite extent with calculations based on a method of moments (MoM) solution. Next, the more difficult scattering problem of a polygonal cylinder in a lossy half space is treated. The UTD solution for the unbounded region is employed together with the fields of rays introduced by the interface between air and the lossy medium to obtain expressions for the scattered field in air and in the lossy medium     

地下目标散射的并行FDTD计算

着重讨论半空间FDTD并行计算方法,入射源的特殊处理方式,和如何调节负载不平衡三个问题,给出一种简单易行的地下目标雷达散射截面的并行时域有限差分(FDTD)方案,并在北京理工大学电磁仿真中心刘徽并行计算平台上,做了具体的程序实现和数值实验,实验不仅证实了算法的精度,而且还表明并行方案的高效,即当参与计算的处理器数量达到14个时,并行效率仍然可以保持在80%以上.在此基础上,用开发的程序计算了以往串行算法无法计算的电大地下典型目标的雷达散射截面.     

Electromagnetic scattering by buried objects of low contrast

The Born approximation is used to derive the plane-wave scattering matrix for objects of low dielectric contrast. For general shapes a numerical integration over the volume of the scatterer is required, but analytical expressions are derived for a sphere, a circular cylinder and a rectangular box (parallelepiped). The plane-wave scattering-matrix theory is used to account for the air-Earth interface. Numerical results are presented for the scattered field and far field for plane-wave excitation. The scattered field are weak for low-contrast objects, but the near-field results have application to electromagnetic detection of buried objects     

Electromagnetic scattering interference between two shallow objects buried under 2-D random rough surfaces

A rigorous electromagnetic model has been used to analyze the scattering from two dielectric shallow objects buried under the two-dimensional (2-D) random rough ground (3-D scattering problem) as a means of predicting false alarms. The method of moments (MoM) accelerated by the steepest descent fast multipole method (SDFMM) is used to compute the equivalent electric and magnetic surface currents on all scatterers (i.e., the rough ground and the two buried objects). The roughness parameters influence the scattering interference mechanism of the two objects, however, a large separation distance (e.g., several correlation lengths) showed stronger effect for small ground roughness.     

Polarimetric scattering from two-layered two-dimensional random rough surfaces with and without buried objects

A three-dimensional polarimetric analysis of the two-layered rough ground with and without buried objects is investigated here. A rigorous electromagnetic surface integral-equation-based model is used in this analysis. The statistical average of the polarimetric scattering matrix elements is computed based on the Monte Carlo simulations for both the vertically and horizontally polarized incident waves. The results show a significant impact on the scattered intensities due to the two-layer nature of the ground. However, these intensities show almost no difference between the ground signature with or without the object. On the other hand, the statistical average of the covariance matrix elements shows a distinct difference between these two signatures despite the small size of the buried object.     

Multidomain pseudospectral time-domain simulations of scattering by objects buried in lossy media

A multidomain pseudospectral time-domain (PSTD) method with a newly developed well-posed PML is introduced as an accurate and flexible tool for the modeling of electromagnetic scattering by 2-D objects buried in an inhomogeneous lossy medium. Compared with the previous single-domain Fourier PSTD method, this approach allows for an accurate treatment of curved geometries with subdomains, curvilinear mapping, and high-order Chebyshev polynomials. The effectiveness of the algorithm is confirmed by an excellent agreement between the numerical results and analytical solutions for perfectly conducting as well as permeable dielectric cylinders. The algorithm has been applied to model various ground-penetrating radar (GPR) applications involving curved objects in a lossy half space with an undulating surface. This multidomain PSTD algorithm is potentially a very useful tool for simulating antennas near complex objects and inhomogeneous media.     

Reconstruction of an embedded slab from multifrequency scattered field data under the distorted Born approximation

This paper deals with the inverse scattering problem regarding a loss-less dielectric slab embedded in a loss-less homogeneous half-space starting out from the knowledge of multifrequency scattered field data. The distorted Born approximation of the electromagnetic scattering is assumed and the features of two linear operators, accounting for the electromagnetic scattering and differing on account of the excitation law of the impinging plane wave, are discussed in terms of model error and class of retrievable unknowns by means of singular value decomposition (SVD) analysis. In particular, it is outlined how a suitable choice of the excitation of the impinging wave makes it possible to improve the reconstruction capabilities of the solution algorithm. In addition, the SVD-based solution approach makes it possible to handle the ill-posedness and to regularize the problem. Numerical results confirm the effectiveness of the approach and the theoretical expectations.     

Electromagnetic inverse scattering of two-dimensional perfectlyconducting objects by real-coded genetic algorithm

Shape reconstruction of two-dimensional perfectly conducting objects using noisy measured scattering data is considered. The contour of each conducting object is denoted by a shape function in the local polar coordinate which is approximated by a trigonometric series. A point-matching method is used to solve the scattering problem. The main idea of the inversion algorithm is to cast the inverse problem into a restrained minimization problem and to solve it by the real-coded genetic algorithm (RGA). The performance of this algorithm is demonstrated by numerically reconstructing arbitrarily shaped objects and by a detailed comparison with both the standard genetic algorithm (SGA) and the Newton-Kantorovitch method     

Electromagnetic inverse scattering of multiple two-dimensional perfectly conducting objects by the differential evolution strategy

Electromagnetic inverse scattering of multiple two-dimensional (2-D) perfectly conducting objects with transverse magnetic (TM) wave incidence by the differential evolution strategy (DES) is presented. The governing electric field integral equations for the scattering problem are expressed as surface integral over the cylinder contours. The cylinder contours are approximately represented by closed cubic B-splines local shape functions in local polar coordinate system. The inverse problem is to locate the cylinders and to reconstruct their shape with or without a priori knowledge of the number of cylinders. It is cast into an optimization problem and is solved using the DES. Both synthetic and real reconstructions are carried out. The reconstruction results agree with the true profiles very well. Comparison with the real-coded genetic algorithm has been carried out. It has been observed that the DES outperforms the real-coded genetic algorithm.     

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。     

Short-pulse plane-wave scattering from buried perfectly conductingbodies of revolution

The method of moments is used to analyze short-pulse plane-wave scattering from perfectly conducting bodies of revolution buried in a lossy, dispersive half space. The analysis is performed in the frequency domain, with the time-domain fields synthesized via Fourier transform. To make this analysis efficient, the method of complex images is used to compute the frequency-dependent components of the half-space dyadic Green's function. Results are presented for short-pulse scattering from buried spheres and cylinders, using measured frequency-dependent soil parameters (permittivity and conductivity)     

On the numerical computation of scattering from inhomogeneous penetrable objects

A formulation of electromagnetic scattering problems involving a penetrable inhomogeneous scatterer is presented. The formulation combines the concepts of invariant imbedding and surface equivalent currents with the method of moments. It effectively circumvents the problem of intractable matrix size which has been a major deterrent in the past.     

Unified theory of near-field analysis and measurement: Scattering and inverse scattering

The many scanning procedures of the author's unified theory of near-field analysis and measurement are applied to the determination of complex bistatic scattering patterns of scalar and electromagnetic systems, both with and without correction for the patterns of the probes, one of which may be a compact range. For high accuracy, both the incident and scattered fields are expressed as linear combinations of exact solutions of the differential equations involved (Maxwell's in the electromagnetic cases). For high efficiency, natural orthogonalities with respect to summation are used to decouple the simultaneous equations expressing the measurements in terms of the desired pattern coefficients, implemented by the highly efficient fast Fourier transform as an approximation-free symmetry decomposition. The scanning procedures include spherical, a new, more accurate, more efficient type of plane polar, and many types of plane rectangular, plane radial, and circular cylindrical. All these results are expressed with a single notation and generally applicable equations, based upon symmetry analysis and relativistic invariances. The full apparatus of group representations is applied to reducing the measurement and computational effort, determining symmetries from scattering patterns, and determining Garbacz and singularity expansion method (SEM) modes. Further, Snell's laws, Fresnel's law, and conservation of momenta (e.g., propagation constants) are explained in terms of relativistic invariances.     

电磁逆散射无相位检测的相位复原

利用入射场作为约束条件,用最优化泛函的方法由总场幅度信息重建散射场幅度和相位.分析了散射场自由度和噪声对重建结果的影响.讨论了成功实施相位复原和避免局部极小值的条件.首次用实验结果验证电磁逆散射无相位检测相位复原的可行性,证明了方法的有效性、高度稳定性和较强的抗噪声能力.该方法的计算成本低,实用性较强.研究结果是实施无相位检测电磁逆散射的基础.     

Improvement of the Rayleigh approximation for scattering from asmall scatterer

An improvement of the Rayleigh approximation for electromagnetic scattering from an arbitrary small scatterer is developed through solving the conventional integro-differential equation governing the electric field inside the scatterer. The improvement is represented by an equivalent polarizability tensor differing from that used in the Rayleigh approximation in two aspects: (i) it is frequency dependent, and (ii) it gives a scattering amplitude tensor satisfying energy conservation. The energy conservation is investigated through formulating the extinction, scattering, and absorption losses according to the equivalent polarizability tensor. Numerical calculations on spheres and spheroids indicate that the present extinction loss formulation has a wider validity range than the extinction loss formulation according to the Rayleigh approximation     

Reduction of forward scattering from cylindrical objects using hardsurfaces

We discuss how forward scattering can he characterized in terms of an equivalent blockage width, and a relation between this and the bistatic scattering width is derived. Then, we show how cylinders such as struts and masts can be constructed to reduce their blockage widths. Thereby, when the cylinders are mounted in front of an antenna, the sidelobes and losses caused by the blockage will be reduced. For thin metal cylinders the blockage width reduction is obtained by giving its cross section an oblong shape and, in addition (for the TM case), by coating the outer metal surface with dielectric material to obtain a hard boundary condition. For thick cylinders, the reduced scattering is obtained by designing them as dielectric-filled parallel plate waveguides with the outer surfaces of the plates coated in the same way as for the thin struts. Dual-polarized performance is obtained in both cases by strip loading the outer surfaces. The performance of both the thin and the thick struts have limited frequency bandwidth. Both computed and measured results are presented; the computations being done with the moment method. The designs are based on the concept of soft and hard surfaces in electromagnetics, and the results can be regarded as a proof of the existence of hard surfaces for electromagnetic waves. The study considers reduction of forward scattering which also will give a reduction of the total integrated power of the scattered field over all directions-even backward     

Edge-based FEM solution of scattering from inhomogeneous andanisotropic objects

This paper presents an application of the edge-based vector finite element method to scattering problems of anisotropic and inhomogeneous objects. Based on conventional FEM functional, a hybrid finite element-surface integral formulation is established by introducing permittivity and permeability tensors. The space domain is divided into interior and exterior regions by an imaginary surface conformal to the scatterer. Edge vector finite elements are used to model the anisotropic and inhomogeneous interior, and a surface integral equation is used to model the unbounded exterior. Compared to other hybrid techniques, the approach here retains the symmetry and sparsity of the FEM matrix and introduces only one type of unknown equivalent current in the moment matrix equation. To validate the theory, typical 2-D numerical results are first presented, which show excellent agreement with exact eigenmode expansion solutions or accurate MoM data