Spectral acceleration of the generalized forward-backward method

The generalized forward-backward (GFB) method was introduced as an iterative method of moments solution to compute the electromagnetic scattering from targets on ocean-like rough surfaces. In this paper, an existing spectral acceleration algorithm is adapted to the GFB. The spectral acceleration is presented for both polarizations and for perfect electrically conducting (PEC) and non-PEC surfaces. The accelerated scheme reduces the computational scaling cost of the GFB from O(N²) to O(N) per iteration as the length of the sea surface increases linearly with the number of unknowns N. The numerical results demonstrate that the spectral acceleration introduces negligible error while greatly improving the efficiency of the GFB method     

分形粗糙表面涂覆目标太赫兹散射特性

该文采用物理光学法方法研究了具有分形粗糙表面的涂覆目标太赫兹散射特性。基于分形粗糙面建立表面粗糙目标模型,根据菲涅尔反射系数得出表面电流分布进而得到涂覆粗糙目标的雷达散射截面。对比分析了具有粗糙表面和光滑目标的散射结果,详细讨论了不同频率、不同涂层厚度的表面粗糙钝锥目标模型的太赫兹散射特性,计算结果表明在太赫兹波段目标表面的粗糙度对散射有显著的影响。      

基于柱面扫描近场成像的RCS 测量方法研究

该文提出一种基于柱面扫描近场成像的RCS(Radar Cross Section)测量新方法:以理想的各向同性点散射中心模型为核心假设,通过详细的理论推导给出了一种具有通用性的基于柱面扫描近场成像的RCS 测量方法。该方法先得到目标的3 维雷达散射图像,再通过这些等效理想散射中心的散射场叠加获得远处散射场进而给出目标的远场RCS 值。该方法不仅能得到被测目标的3 维雷达散射图像,还能获得一定立体角域的目标远场RCS。相比只能得到2 维雷达散射图以及2 维平面角域RCS 结果的圆迹扫描测试相比,该文所提的柱面扫描测试能得到更多的目标散射信息,具有较强的实用性。仿真结果验证了新方法的可靠性。      

基于联合处理的复杂目标RCS估计方法

目标雷达散射截面积（Radar Cross Section, RCS）计算在隐身设计、电子对抗、目标探测、识别和成像等方面具有重要的研究价值,是目标电磁散射特性的重点研究方向。针对复杂目标RCS估计问题,基于属性散射中心模型的单一方法在估计大角度范围的目标RCS时会产生较大误差,而物理光学方法需要在每个观察角度对目标表面的面元进行遮挡判别才能准确得到目标RCS,计算量大。因此,本文提出一种联合属性散射中心模型和物理光学的处理方法,在部分观察角度通过物理光学方法分析确定目标的属性参数集,再通过属性散射中心模型分析快速估计任意观察角度、不同频率下的目标RCS,获得在大角度范围的结果更加准确、计算量更小。最后采用FEKO软件仿真验证了所提方法的有效性。     

粗糙金属和介质目标的太赫兹散射特性分析

太赫兹频段金属和介质粗糙目标的散射特性是研究太赫兹雷达目标特性的重要基础。当目标表面的主曲率半径远远大于入射波长,且粗糙表面高度起伏与斜率起伏远小于入射波长时,根据稳定相位法和标量近似法,可获得粗糙金属和介质目标的相干散射截面和非相干散射截面。基于稳定相位法,任意目标的相干散射截面可退化为粗糙导体、光滑介质和粗糙介质目标的相干散射。该文分析了电大尺寸光滑金属铝和介质白漆球的散射截面,与Mie理论计算的介质球的散射特性吻合,散射截面误差小于0.1 dBm2。采用朗伯定理,验证了粗糙介质球的太赫兹非相干散射精确解,当目标表面剖分精度越高,非相干散射的计算精度越高。该文数值计算了粗糙介质球的太赫兹相干和非相干散射特性,分析了表面粗糙度和表面材料对散射特性的影响,为电大尺寸空间目标太赫兹散射特性分析提供了理论基础。      

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.     

如何理解目标高分辨率雷达图像及其像素值

经过图像定标后的雷达图像及其像素值具有雷达散射截面（radar cross section，RCS）的量纲，但是对于该像素值是否能代表目标实际RCS电平一直存在不同的理解.本文通过引入与传统RCS定义相一致的目标散射函数和散射分布函数基本概念，结合经典目标的散射机理和雷达像分析，讨论复杂目标高分辨率雷达图像理解和对像素值的解释.研究表明:雷达图像的像素值不应直接解释为目标的RCS电平，但在空间频率域和图像域，两者数据之间满足帕萨瓦定理；在小孔径角成像条件下，空间频率域的RCS均值等于强度图像的全部像素值之和.     

海上舰船在平面波照射下的半空间散射研究

本文研究了海面舰艇在平面波照射下的散射特性,为在该类问题中有效计入海面背景的影响,舍弃了以往文献中采用自由空间格林函数为内核求解的模型,而是利用介质半空间格林函数在海面建立电场积分方程,并采用GFB(广义前向后向法)算法结合MOM(矩量法)计算了海面以及海面舰船的感应电流分布以及双站雷达散射截面,对入射波角度、海面粗糙度、以及有船无船对海面电流分布及双站雷达散射截面的影响进行了讨论.     

Time-domain synthesis of broad-band absorptive coatings fortwo-dimensional conducting targets

A novel time-domain synthesis approach for broadband absorptive coatings suitable for radar cross section (RCS) management is introduced. The algorithm involves a finite-difference-time-domain (FD-TD) forward-scattering representation of Maxwell's curl equations in a numerical feedback loop with the Levenberg-Marquardt (L-M) nonlinear optimization routine. L-M is used to adjust many geometric and constitutive parameters that characterize a target, while FD-TD is used to obtain the broadband bistatic RCS response for each target adjustment. A recursive improvement process is established to minimize the broadband RCS response over a selected range of bistatic angles using the available engineering degrees of freedom. The solution is valid over the potentially broad bandwidth (frequency decade or more) of the illuminating pulse used in the FD-TD computational model. Examples of this method are provided in the area of RCS management for canonical two-dimensional conducting targets     

The finite-element method with domain decomposition for electromagnetic bistatic scattering from the comprehensive model of a ship on and a target above a large-scale rough sea surface

The domain decomposition method (DDM) and finite-element method (FEM) are developed for numerical solution of bistatic scattering from the composite model of a ship on and a target above a two-dimensional (2-D) randomly rough sea surface under an electromagnetic (EM) wave incidence at low grazing angle. The coupling boundary conditions on the interface between two adjacent subdomains are derived when the conformal perfectly matched layer is used as the truncation boundary of the FEM, and the final coupling matrices are obtained by using the inward-looking approach. Because the computational domain with several millions of unknowns can be solved on a personal computer, our FEM-DDM method is powerful for scattering simulation of a very large-scale rough surface with targets presence. In addition to reduction of the memory storage, the superiority of this method in computing time over the conventional FEM is also demonstrated. Our codes are examined by the FEM without DDM, the forward-backward method (FBM), and generalized FBM for some simple cases. Numerical simulations of bistatic scattering from a comprehensive model of a ship on and a target above the 2-D randomly rough perfectly conducting sea surface in large electric scale are obtained, and its functional dependence on many physical parameters of the targets and oceanic status are discussed.     

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.     

Studies of the angular correlation function of scattering by randomrough surfaces with and without a buried object

The discrimination of the scattered wave from an object buried in shallow ground from that of the rough surface is a difficult task with present ground penetrating radar (GPR) systems. Recently, a new approach for this classical problem has been proposed and its effectiveness has been verified. This new method is based on the angular correlation function (ACF) of the scattered wave observed at two or more different incident and scattered angle combinations. It has been shown that the angular memory signatures of rough surfaces are substantially different from those of typical man-made targets and by choosing the appropriate incident and scattered angles, the surface scattering can be minimized whereas the scattering from the target is almost unchanged. The authors present detailed numerical studies of the ACF of the scattered wave from rough surfaces with and without a buried object. To obtain the ACF, the three averaging methods: realization, frequency and angular averaging, are tested numerically. It is shown that a single random rough surface of moderate extent can exhibit memory effect by using frequency averaging. Frequency averaging with a wide bandwidth is also effective for suppressing fluctuation in ACF and is most useful for practical applications. Numerical simulations indicate that even when the ratio of scattered intensities with and without the buried object is close to unity, the corresponding ratio of ACF magnitude can be more than 10 dB. Thus, using the ACF is superior to using the radar cross section (RCS) in the detection of buried objects     

岸基雷达模拟器的设计与实现

针对岸基雷达工作的海面环境和目标特点,在雷达模拟器中设置了目标的雷达截面积（RCS）起伏、检测概率计算及海杂波模型。对于海面舰船目标而言,其RCS通常受舰船的姿态、方位角等状态影响较大,在此通过计算舰船相对于雷达站的方位实现目标的RCS起伏。再根据目标的RCS利用Albersheim公式即可获得目标的检测概率。另外,雷达的工作状态不同,检测到的海杂波分布模型亦不同。这里介绍了2种最常用的海杂波分布模型,分别是Rayleigh分布和对数正态分布。改进后的雷达模拟器能够更加真实准确地模拟岸基雷达的实际工作环境。     

Low angle scattering from 2-D targets on a time-evolving sea surface

The results of a numerical investigation of the electromagnetic scattering from two-dimensional (2-D) targets on a time-evolving sea surface are presented. The 2-D radar cross section (RCS), or "echo width," is computed as a function of time as the sea surface evolves linearly using the spectrally accelerated generalized forward-backward method. It is shown that the RCS varies with time on the order of seconds, which is much longer than the typical pulse width or pulse repetition rate of search radars. It is also observed that for low-grazing angles of incidence the primary sea surface influence on the RCS comes from the long waves in the ocean spectrum.     

海面对低空目标短波散射特性的影响

海上目标散射特性的分析在短波超视雷达的应用中起着极其重要的作用。本文从求解半空间散射问题的矩量法出发，分析研究了海面对目标后向散射截面的影响。数值结果表明，海面与目标间的耦合导致后向散射截面围绕某一固定值上下波动，随高度的增加波动幅度逐渐减小，结合目标在自由空间中的双站散射特性，文中给出了分析和判定海面影响的近似方法。     

基于柱坐标系抛物方程的太赫兹目标RCS计算

针对太赫兹(THz)波段目标雷达散射截面(RCS)的计算问题,提出柱坐标系抛物方程模型的计算方法。基于柱坐标系中的电场通解式,利用三角函数的正交性分解各模式的激励系数,将抛物方程方法拓展到柱坐标系,得到柱坐标系中抛物方程的分步傅里叶求解形式。在此基础上,将目标等效为一系列的面元或线元,然后通过边界条件和场的迭代递推方法求解抛物方程,进而获得这一系列面元在传播方向某一截面上的散射场。数值算例表明,该方法能用于电大尺寸目标的RCS计算,相比于传统的抛物方程方法,克服了散射角度的限制,计算误差更小。     

基于AWE技术的宽角度与宽频带RCS的快速计算

目标的雷达散射截面（RCS）与照射角度和照射频率都有关系,采用渐近波形估计（AWE）技术在角度域和频率域上预测任意形状的理想导体的单站RCS,通过Pade逼近求出给定角度域内任意角度及给定频带内任意频点的表面电流密度分布,进而计算出给定目标的散射场及雷达散射截面。对数值结果与矩量法逐点求解的结果进行了比较,两者吻合较好,而且提高了计算效率。     

基于时域光谱系统的太赫兹圆柱RCS 测量

雷达散射截面（RCS）测量对于太赫兹雷达系统论证设计具有重要意义。详细描述了时域光谱（TDS）系统测量RCS 的原理,以及实验方案的设计。基于TDS 系统对太赫兹频段圆柱RCS 进行了测量实验,获得光滑和粗糙圆柱在太赫兹频段下的回波,对光滑圆柱回波进行傅里叶变换得到其宽带 RCS,与理想圆柱RCS 的物理光学解进行比较,发现RCS 理论值与测量值基本一致,验证了TDS 系统可用于太赫兹频段目标RCS 的测量;同时将光滑圆柱和粗糙圆柱的RCS 测量值进行对比分析,结果表明:太赫兹频率越高,粗糙面对RCS 的影响越大,粗糙度大于八分之一波长为粗糙面的传统定义须重新考虑。     

Target reflectivity and RCS interactions in integrated maritimesurveillance systems based on surface-wave high-frequency radars

Abnormal fluctuations have been observed in detected power levels of some of the targets during trials of the integrated maritime surveillance system (IMS) based on the Canadian east coast surface-wave high-frequency radar (HFSWR). The power level of most of the surface and air targets fluctuated within measurement-error limits (a few dB) during consecutive detections. These fluctuations have been observed to be more than 15 dB for a huge oil platform and nearby large tankers. These fluctuations are quite different than those observed in microwave radars, such as pulse-to-pulse or scan-to-scan fluctuations (which are modeled as different Swerling-type targets), and as are mentioned in most of the classical radar handbooks. In order to understand the reason behind these fluctuations, the behavior of the target reflectivity and radar cross section (RCS) of surface and air targets and their mutual RCS interaction were investigated. Powerful numerical techniques were used to model and understand the target reflectivity and RCS interactions, mostly in the resonance regime. Different scenarios were created, and the mutual RCS behavior of nearby large targets (such as oil tankers and/or fixed offshore oil platforms) as they were maneuvering were modeled. It was shown that 10 dB to 20 dB RCS fluctuations should be expected when targets interact, especially in the resonance regime