Sensitivity Analysis of Electromagnetic Scattering from Dielectric Targets with Polynomial Chaos Expansion and Method of Moments

In this paper,an adaptive polynomial chaos expansion method(PCE)based on the method of moments(MoM)is proposed to construct surrogate models for electromagnetic scattering and further sensitivity analysis.The MoM is applied to accurately solve the electric field integral equation(EFIE)of electromagnetic scattering from homogeneous dielectric targets.Within the bistatic radar cross section(RCS)as the research object,the adaptive PCE algorithm is devoted to selecting the appropriate order to construct the multivariate surrogate model.The corresponding sensitivity results are given by the further derivative operation,which is compared with those of the finite difference method(FDM).Several examples are provided to demonstrate the effectiveness of the proposed algorithm for sensitivity analysis of electromagnetic scattering from homogeneous dielectric targets.

Analytical solution for electromagnetic scattering from a sphere of uniaxial left-handed material

Based on the analytical solution of electromagnetic scattering by a uniaxial anisotropic sphere in the spectral domain, an analytical solution to the electromagnetic scattering by a uniaxial left-handed materials (LHMs) sphere is obtained in terms of spherical vector wave functions in a uniaxial anisotropic LHM medium. The expression of the analytical solution contains only some one-dimensional integral which can be calculated easily. Numerical results show that Mie series of plane wave scattering by an isotropic LHM sphere is a special case of the present method. Some numerical results of electromagnetic scattering of a uniaxial anisotropic sphere by a plane wave are given.

RESEARCH ON ELECTROMAGNETIC SCATTERING FROM PARTIALLY COATED CONDUCTING CYLINDERS

In this paper,moment methods are applied to solving the electromagnetic scatteringproblems(for both E-and H-polarizations)involving partially coated conducting cylinders.Thecomputer programs have been compiled for general use.They can be used to solve the electro-magnetic scattering from uncoated,partially coated or entirely coated cylinders of arbitrary crosssectional shape.Numerical examples are also presented to show the validity and versatility of themethod.

NEW APPROACH TO ELECTROMAGNETIC SCATTERING FROM A CONDUCTIVE DISC-RING STRUCTURE

A one-dimensional modified integral equation for electromagnetic scattering from a metallic concentric disc-ring structure is derived by using Fourier expansion and separating the contribution of charge density from conventional EFIE. In the case of normal incidence, the derived equation is very simple and easy to be solved numerically regardless of the electric dimension of the structure. The comparison between the data of RCS of a disc by using this method and analytical method shows the effectiveness of this approach. Numerical results are also given for current distributions and the scattered fields for a disc-ring structure.

MONOSTATIC RADAR CROSS SECTION FOR REFLECTOR ANTENNAS

The calculation formulas of monostatic radar cross-section (RCS) of arbitrary re-flectors with arbitrarily polarized plane-wave incidence are derived, where the spicular field isobtained by geometrical optics (GO) and the edge-diffracted field is calculated by the method ofequivalent currents (MEC). Some typical calculated results are given by means of RCS spatialgraphs. For both horizontal and vertical polarizations, the theoretical results obtained in thispaper agree very well with the experimental results as well as the results from uniform theory ofdiffraction.

Electromagnetic scattering characteristics of coaxial helicopter based on dynamic transformation method

With the development of coaxial rotors and high-speed helicopters, the electromagnetic scattering characteristics of coaxial helicopters have gradually become a research hotspot. In order to deal with the Radar Cross-Section(RCS) of high-speed rotating rotors or coaxial main rotors, a Dynamic Scattering Method(DSM) based on dynamic process simulation and grid coordinate transformation is presented. Instantaneous electromagnetic scattering from rotors and helicopters is solved using Physical Optics(PO) and Physical Theory of Diffraction(PTD). Important factors are analyzed and discussed in detail, including individual rotor rotation, azimuth, elevation angle,fuselage, pitch angle, and roll angle. The results show that the electromagnetic scattering characteristics of rotor-type components are dynamic and periodic. The dynamic RCS period of a single rotor is related to the dynamic RCS period of the coaxial main rotor. Choosing different observation angles and attitude angles has a great impact on the static and dynamic RCS of the helicopter.The presented DSM is effective and efficient to analyze and determine the dynamic electromagnetic scattering characteristics of conventional helicopters or coaxial helicopters.

Radar Cross Section Calculation for Wing-Body Blended Targets

A new method of calculating the radai cross section (RCS) for wing-body blended targets is presented and verified. The method utilizes a computer program for modeling targets' geometry in terms of small pieces. The calculation is based on physical optics approximation. Examples are given to show the validity of the method.

Electromagnetic backscattering from one-dimensional drifting fractal sea surface Ⅱ：Electromagnetic backscattering model

Sea surface current has a significant influence on electromagnetic（EM） backscattering signals and may constitute a dominant synthetic aperture radar（SAR） imaging mechanism. An effective EM backscattering model for a one-dimensional drifting fractal sea surface is presented in this paper. This model is used to simulate EM backscattering signals from the drifting sea surface. Numerical results show that ocean currents have a significant influence on EM backscattering signals from the sea surface. The normalized radar cross section（NRCS） discrepancies between the model for a coupled wavecurrent fractal sea surface and the model for an uncoupled fractal sea surface increase with the increase of incidence angle,as well as with increasing ocean currents. Ocean currents that are parallel to the direction of the wave can weaken the EM backscattering signal intensity, while the EM backscattering signal is intensified by ocean currents propagating oppositely to the wave direction. The model presented in this paper can be used to study the SAR imaging mechanism for a drifting sea surface.

CONTOUR DESIGN AND PRACTICE FOR A LOW RADAR CROSS SECTION VEHICLE

On the basis of the canard configuration a contour stealth design including chiefly the wing, the fuselage and their connection type is projected. The prime project of a blended wing body vehicle with canard is provided and through the change of the fuselage head form and the different fin disposals, the radar cross section (RCS) is optimized. The average value of RCS and the value of RCS in the ± 45 ° front sector for different designs are illustrated. The model measurement proves that the project having a sharp head fuselage and 30 ° angle double fin has the minimum value of RCS. The wind tunnel test to the model with RCS optimized proved that the vehicle project has excellent aerodynamic characteristics such as high lift curve slope, up to 26° stalling angle, high lift / drag ratio equal to 8, and also has low RCS value in the front sector and in the lateral sector.

Synthesis of Load Impedances That Reduce TE-Scattering from Conducting Cylinder with Arbitrary Cross Section

The reduction of TE-scattering by a conducting cylinder with multiple surface impedance loads is investigated. Synthesis procedures are developed to find the optimal loading which result in zero scattering in the desired directions or at several frequencies. Numerical results of these procedures are presented. The theoretical predictions are confirmed with an experiment. The proposed synthesis procedure is completely general and can be applied to arbitrarily shaped conducting bodies.

RADAR CROSS-SECTION COMPUTATIONS OF ARBITRARILY COMPLICATED TARGETS BY APPLYING THE PANEL METHOD

The present paper deals with the method for the radar cross-section (RCS)computations of arbitrarily complicated targets based on the work by D. Klement et al.(1988).This method is convenient in use, fast in operation and precise in calculating RCS of a complicatedtarget. With this method, the RCS of classic scatterers, for example, a cone and a cylinder, arecomputed with the result of good agreement with experimental data. Furthermore, the RCS’of an aircraft model at various attitudes are calculated with the result of good agreement withexperimental data also.

Scattering of Electromagnetic Waves by Drift Vortex in Plasma

In a quasi-two-dimensional model, the scattering of incident ordinary electromag- netic waves by a dipole-electrostatic drift vortex is studied with first-order Born approximation. The distribution of the scattering cross-section and total cross-section are evaluated analytically in different approximate conditions, and the physical interpretations are discussed. When the wavelength of incident wave is much longer than the vortex radius （kia〈〈1）, it is found that the angle at which the scattering cross-section reaches its maxim depends significantly on the approximation of the parameters of the vortex used. It is also found that the total scattering cross-section has an affinitive relation with the parameters of the plasma, while it is irrelevant to the frequency of the incident wave in a wide range of parameters of the vortex. In a totally different range of parameters when incident wave is in the radar-frequency range （then kia 〈〈 1, the wavelength of incident wave is much shorter than the vortex radius）, the numerical procedure is conducted with computer in order to obtain the distribution and the total expression of the scattering crosssection. Then it is found that the total scattering cross-section in the low frequency range is much larger than that in high frequency range, so the scattering is more effective in the low frequency range than in high frequency range.

FDTD Simulation of Bistatic Scattering by Conductive Cylinder Covered with Inhomogeneous Time-Varying Plasma

A finite-different time-domain （FDTD） algorithm is applied in this paper to study the bistatic electromagnetic（EM） scattering by a conductive cylinder covered with inhomogeneous, collision, cold, time-varying plasma. The collision frequency of plasma is a function of the electron density and the pressure of the background gas. The plasma density follows any prescribed distribution function of the rise time of plasma and the radius of the column. The bistatic radar cross section （RCS） of the conductive cylinder covered with inhomogeneous time-varying plasma and inhomogeneous steady-state plasma is calculated under different conditions. The results illustrate that a plasma cloaking system can successfully reduce the RCS of the conductive cylinder.

Bistatic RCS Prediction with Graphical Electromagnetic Computing (GRECO) Method for Moving Targets

Graphical Electromagnetic Computing (GRECO) is recognized as one of the most valuable methods of the RCS (Radar Cross Section) computation for the high frequency region. The method of GRECO and Monostatic bistatic Equivalence Theorem was used to calculate the bistatic RCS for moving targets in the high frequency region. Some computing examples are given to verify the validity of the method. Excellent agreement with the measured data indicates that the method has practical engineering value.

Exploitation of Electromagnetic Models for Sea Wind Speed Estimation from C-Band Sentinel-1 Images

Among the different available wind sources, i.e. in situ measurements, numeric weather models, the retrieval of wind speed from Synthetic Aperture Radar (SAR) data is one of the most widely used methods, since it can give high wind resolution cells. For this purpose, one can find two principal approaches: via electromagnetic (EM) models and empirical (EP) models. In both approaches, the Geophysical Model Functions (GMFs) are used to describe the relation of radar scattering, wind speed, and the geometry of observations. By knowing radar scattering and geometric parameters, it is possible to invert the GMFs to retrieve wind speed. It is very interesting to compare wind speed estimated by the EM models, general descriptions of radar scattering from sea surface, to the one estimated by the EP models, specific descriptions for the inverse problem. Based on the comparisons, some ideas are proposed to improve the performance of the EM models for wind speed retrieval.

基于共享孔径技术的低RCS电磁超构表面天线设计

提出一种基于共享孔径技术设计低雷达散射截面(radar cross section,RCS)电磁超构表面天线的新方法.该方法首先设计具有低RCS性能的超构表面,然后借鉴共享孔径技术思想,将超构表面和传统天线的辐射结构直接共享口径紧密排列,得到新型低RCS天线结构,并结合电流分析和局部结构修正,优化天线的辐射性能,最终同时实现天线的良好辐射和宽带低RCS性能.为了阐明该方法,基于极化旋转机理设计了一款低RCS超构表面,采用共享孔径思想和电流分析,得到了一款宽带低RCS超构表面天线,详细分析了该天线的工作机理与性能.结果表明:设计的天线在保证较好辐射的同时,实现了超宽带RCS减缩,提出的设计方法摒弃了从天线到低RCS天线的传统设计思路,通过逆向思维,将散射的优化问题转化为辐射的优化问题,不仅实现了天线与超构表面的一体化设计,还显著地降低了低RCS天线优化设计的难度,加速了天线优化进程.

人工等离子体云团与无人机群的散射研究

电离层中释放的金属蒸气产生人工等离子体云团,其可显著改变无线电波传播。本文利用几何绕射理论(geometrical theory of diffraction, GTD)和有限元法(finite element method, FEM)相结合的方法,给出了经由天线、人工等离子云团和无人机(unmanned aerial vehicle, UAV)群组成的传播链路中信号强度计算方法。利用30~70 MHz甚高频(very high frequency, VHF)信号研究人工等离子体云团与UAV群的复合散射特性,得出如下结论:接收功率随着信号频率增加呈下降趋势;当机群由N架UAV构成时,阵因子迭加使机群雷达散射截面(radar cross section, RCS)出现一定的起伏,同相迭加时,接收功率可比单个UAV高约20lg N dB;利用人工等离子体云团散射可实现VHF频段用于对米级尺度RCS目标进行超视距探测,有助于解决紧急情况下电离层扰动对高频探测的不利影响。

吸波涂层对直升机旋翼电磁散射特性影响研究

基于几何光学、物理光学和等效电磁流的雷达散射截面(RCS)高频近似算法,联合傅里叶变换提取和分析了涂覆雷达吸波材料旋翼的动态电磁散射特性,重点研究了涂层位置、涂层厚度以及电磁频率等因素变化对旋翼动态电磁散射特性的影响。结果表明:在旋翼表面添加吸波涂层能够有效减缩旋翼的时域动态RCS和频域分量幅值,并且吸波材料的涂覆位置决定了频域上分量幅值降低的频带范围,通过RCS的频谱特征能够判断出旋翼表面何处存在吸波涂层;吸波涂层的厚度和所处的电磁环境,均会影响涂层的吸波效果,根据重量指标、作战任务等需求合理制定旋翼的涂覆吸波材料方案,针对性降低旋翼特定部位的散射强度,能够干扰旋翼时域和频域的动态电磁散射特性,更好地发挥吸波涂层提升旋翼雷达隐身性的能力。

利用高阻抗表面缩减天线雷达截面的新方法

针对现有雷达截面(RCS)减缩技术会导致天线辐射性能退化,体积、重量和成本增加,减缩性能受安装精度影响而不稳定的缺点,提出了一种缩减天线RCS的新方法.将高阻抗表面(HIS)与天线制作于介质板的同一表面,使HIS的同相反射相位带隙的频率与天线工作频率重合,通过高阻抗表面与天线的散射场的对消来减小天线的雷达散射截面,从而实现对垂直照射的天线工作频带内的同极化威胁雷达波的隐身.实验结果表明:在天线工作频带内,其RCS得到了5.3dB以上的减缩,同时,带外RCS亦得到了有效的抑制,S11的-10dB并未展宽,天线间的互耦得到了有效抑制.