Detection and Analysis of Anisotropic Scattering in SAR Data

Scattering from man-made objects in SAR imagery exhibits aspect and frequency dependencies which are not well modeled by standard SAR imaging techniques. If ignored, these deviations will reduce recognition performance due to the model mismatch, but when appropriately accounted for, these deviations from the ideal point scattering model can be exploited as attributes to better distinguish scatterers and their respective targets. With this premise in mind, we have developed an efficient modeling framework that incorporates scatterer anisotropy. One of the products of our analysis is the assignment of an anisotropy label to each scatterer conveying the degree of anisotropy. Anisotropic behavior is commonly predicted for geometric scatterers (scatterers with a simple geometric structure), but it may also arise from volumetric scatterers (random arrangements of interfering point scatterers). Analysis of anisotropy arising from these two modalities shows a clear source-dependent relationship between the anisotropy classification and parameters of the scatterer. In particular, the degree of anisotropy is closely related to the size of the scatterer, and increasing the aperture size reduces the incidence of volumetric anisotropy but preserves the detection rate for geometric anisotropy. This result helps to address the question in the SAR community regarding the utility of wide-aperture SAR data for ATR since wide-aperture data reveals geometric anisotropy while resolving volumetric anisotropy into individual isotropic scatterers.     

Scattering by three-dimensional anisotropic scatterers

The coupled dipole approximation method has been extended to compute the scattering characteristics of three-dimensional, homogeneous, lossless, anisotropic objects. This method is simple, and does not require the solution of any integrodifferential equations. The method will work for any constitutive dyadic, provided it is lossless; furthermore, scatterers of any shape can be accommodated. Thus, this procedure will work well for any three-dimensional, lossless, anisotropic scatterer. Numerical results are given for uniaxial spheres made of titanium dioxide     

Moment method with isoparametric elements for three-dimensionalanisotropic scatterers

A new method for computing the frequency-domain electromagnetic fields scattered from, and penetrating into, arbitrarily shaped, three-dimensional, lossy, inhomogeneous anisotropic scatterers is presented. The method is based on a general volume integro-differential formulation of the scattering problem, and consists of the numerical solution of the coupled integral equations by the moment method and point matching. A particularly powerful feature of this method is that the numerical model of the scatterer is obtained by parametric volume elements and the basis functions used to represent the field within each element are the same used in the finite-element method. Element integration problems due to the singular kernel of the integral equations are treated in some detail. Numerical results for both the isotropic and the anisotropic spherical scatterer are presented, including comparisons with results obtained by different numerical methods for the isotropic cases considered. The capability of the numerical code presented here to deal with cases where the material parameters of the scatterer are given by singular matrices is discussed for two particular examples     

任意取向非球形群聚粒子极化散射及其数值模拟

利用Ｔ矩阵法，通过严格求解Ｎ个粒子的多次数散射方程，发展了任意大小、空间分布和取向的多个非球形粒子集合性相干散射的数值计算方法。     

A phase matrix for a dense discrete random medium: evaluation ofvolume scattering coefficient

In the derivation of the conventional scattering phase matrix of a discrete random medium, the far-field approximation is usually assumed. In this paper, the phase matrix of a dense discrete random medium is developed by relaxing the far-field approximation and accounting for the effect of volume fraction and randomness properties characterized by the variance and correlation function of scatterer positions within the medium. The final expression for the phase matrix differs from the conventional one in two major aspects: there is an amplitude and a phase correction. The concept used in the derivation is analogous to the antenna array theory. The phase matrix for a collection of scatterers is found to be the Stokes matrix of the single scatterer multiplied by a dense medium phase correction factor. The close spacing amplitude correction appears inside the Stokes matrix. When the scatterers are uncorrelated, the phase correction factor approaches unity. The phase matrix is used to calculate the volume scattering coefficients for a unit volume of spherical scatterers, and the results are compared with calculations from other theories, numerical simulations, and laboratory measurements. Results indicate that there should be a distinction between physically dense medium and electrically dense medium     

Design rules for chipless RFID tags based on multiple scatterers

In this paper, we present some design rules to create a chipless RFID tag that encodes the information in the frequency domain. Some criterions are introduced to make the best choice concerning the elementary scatterers that act like signal processing antennas. The performance of several scatterers will be compared before a study on the radiating properties of a versatile C-like scatterer. An electrical model as well as a transfer function model is presented to best understand the frequency response of both a single-layer and a grounded scatterer. An example of the design and the optimization of a chipless RFID tag based on the use of multiple scatterers are provided, and the frequency optimization step for each resonant peak will be discussed.     

改变散射体RCS方向特性的有源加载方法研究

散射体的雷达横截面积(RCS)随着观测角度的不同而发生变化。对高频区的散射体电磁散射特性进行了建模,根据各个散射中心的位置关系和信号入射角度,计算散射体在特定方向上的RCS。提出了集中式和分布式的有源加载方法改变散射体RCS方向特性,通过施放有源干扰信号使散射体在特定方向上的RCS方向图出现凹点。仿真结果表明,该方法可以有效改变散射体的RCS方向特性,在特定方向上实现散射体射频隐身。     

Conservative Polarimetric Scatterers and Their Role in Incorrect Extensions of the Cameron Decomposition

The properties of conservative symmetric polarimetric scatterer scattering matrices are examined. It is shown that the unambiguous rotation angle for "conservative" symmetric polarimetric scatterers is the interval (-pi/4,+pi/4] as compared with (-pi/2,+pi/2] for nonconservative symmetric scatterers. Errors relating to the Cameron decomposition, which are recently introduced into the literature, are discussed. It is demonstrated that these errors arise from a faulty symmetric scatterer scattering matrix distance measure. This, in turn, leads to an improper mapping of the symmetric scatterer unit disk to a hemisphere. The correct mapping of the symmetric scatterer unit disk to the symmetric scatterer unit sphere is described     

Radar scattering from intermittently contacting metal targets

Radar returns from moving multielement metal targets often exhibit an unexpected modulation that has both random (or noise-like) and semicoherent components. One possible mechanism for producing this effect is the modification of the current distribution on the target that results when electrical contacts between target elements are altered intermittently by the forces associated with target motion. We propose that this mechanism can be modeled in terms of an impedance-loaded scatterer, and we present preliminary experimental results obtained using resonant dipole scatterers.     

Tunneling electron transport through heterobarriers with nanometer heterogeneities

The effect of spherical quantum objects (scatterers) embedded into semiconductor barriers on the tunnel-current flow through them is studied. For this purpose, the problem of scattering on steplike spherically symmetric potential of the scatterer is solved for the incident and reflected electron wave functions damped (at the energy lower than the barrier potential). It is shown that the vortex current structures can arise inside the barrier in this case.     

Theoretical and experimental waveguide characterization of smallwire scatterers

A simple method is presented in this paper that allows us to verify numerically obtained polarizability tensors of electrically small scatterers by waveguide measurements. To this end, a model of the scattering process within the waveguide is developed. Measurements performed on a small helix in two different waveguide setups are compared to the theoretical data obtained from the model. A good agreement is demonstrated. Furthermore, the measured data are highly sensitive to the orientation of the scatterer within the waveguide. Thus, the polarizability tensors can be verified     

The null field approach to electromagnetic scattering fromcomposite objects: the case of concavo-convex constituents

Time-harmonic electromagnetic scattering from a composite body consisting of a (dielectric or metallic) core plus one or several dielectric coatings was studied using the null-field approach. Previously developed null-field approaches to scattering from composite bodies do not apply when these coatings are of concavo-convex shapes. The authors examine this case and develop alternative null-field approaches to such geometries. While the scattering problem is usually solved by determination of the total transition matrix, referring to spherical waves, for the composite scatterer, the authors' approaches lead to different algebraic expressions for the transition matrix. Two main alternatives are studied. One of these makes use of Q-matrices for open surfaces while the other is based on a limit procedure applied to a previously developed formalism for layered scatterers. The numerical accuracy of the results is less than that obtained for homogeneous scatterers of similar exterior shape and electrical size. The convergence of the numerical implementation of the equations is studied in terms of several indicators such as dependence on the truncation order, the accuracy with which general constraints such as symmetry and unitarity are fulfilled, and the influence of different choices of expansion functions obtained from a moment-method solution     

Near-field detection of buried dielectric objects

The plane-wave scattering-matrix method is used to compute the response of a detector to a buried dielectric scatterer. The Born approximation is used to derive the scattering matrix for scatterers of small dielectric contrast, but the general theory is not limited to such cases. Specific numerical results are generated for a UHF dipole detector swept over a buried dielectric cube. The maximum response is obtained when the detector is located at the soil interface, and the response decays rapidly with the detector height. The sweep curves are symmetrical in the horizontal direction and have a null when the detector is directly over the object. An experimental curve for a free-space environment has the same qualitative features     

Fading model for antenna array receiver for a ring-type cluster of scatterers

This correspondence presents an alternate method for simulating the time-varying flat fading wireless channels for the antenna array receivers, in which the discrete ring of scatterers is incorporated around the mobile transmitter to model the spreading of azimuth. The ring-type cluster of scatterers continuously changes due to the movement of the mobile unit. Under this time-varying environment, each scatterer, at successive stages of the ring, is correlated with the scatterers at preceding stages of the ring using the second-order Markov modelling. The correlation of fading waveforms generated with the proposed paradigm is compared with the expected analytical correlation, which clearly depicts that the simulation results are consistent with the findings based on Jakes’ model.     

Average dielectric properties of discrete random media using multiple scattering theory

Using rigorous multiple scattering theory in determining the average or bulk dielectric properties of discrete random media is the objective of this communication. The random medium is modeled as a random distribution of identical, spherical scatterers imbedded in an homogeneous unbounded background medium. At high scatterer concentration, the form of the radial distribution function becomes important; two forms are considered here, viz., virial series and the self-consistent form. The average loss tangent of the bulk medium is computed as a function of frequency and scatterer concentration, and compared with a frequently used mixture formula, e.g., Maxwell-Garnett. The results show that multiple scattering losses are significant at the higher concentrations and must be accounted for whenka gtrsim 0.05. The theory and the computational procedure can thus he used as a mixture formula forkain the range0 < ka lesssim 5.0and concentrations in the range0.01 < c lesssim 0.40.     

On modeling the tissue response from ultrasonic B-scan images

The authors model tissue as a collection of point scatterers embedded in a uniform media, and show that the higher-order statistics (HOS) of the scatterer spacing distribution can be estimated from digitized radio frequency (RF) scan line segments and be used in obtaining tissue signatures. The authors assume that RF echoes are non-Gaussian, on the grounds of empirical/theoretical justifications presented in the literature. Based on their model for tissue microstructure, the authors develop schemes for the estimation of reasonable periodicity as well as correlations among nonperiodic scatterers, Using HOS of the scattered signal, the authors define as tissue "color" a quantity that describes the scatterer spatial correlations, show how to evaluate it from the higher-order correlations of the digitized RF scan line segments, and investigate its potential as a tissue signature. The tools employed, i.e., HOS, were chosen as the most appropriate ones because they suppress Gaussian processes, such as the one arising from the diffused scatterers. HOS, unlike second-order statistics, also preserve the Fourier-phase of the signature, the color of the tissue response. Working on simulated and clinical data, the authors show that the proposed periodicity estimation technique is superior to the widely used power spectrum and cepstrum techniques in terms of the accuracy of estimations. The authors also show that even when there is no significant periodicity in data, they are still able to characterize tissues using signatures based on the higher-order cumulant structure of the scatterer spacing distribution.     

The differentiated on-surface poynting vector as a measure of radiation loss from wires

This article explores use of the on-surface Poynting vector to investigate the power flow at the surface of various thin wires, excited as antennas or scatterers. By differentiating the axial Poynting vector at the wire's surface, the rate of change of power flow in the current and charge along a perfectly electrically conducting (PEC) wire can be determined. The idea being explored is to see if a change in axial power flow can reveal anything about possible power loss due to radiation. The results from the differentiated Poynting vector are normalized with those obtained from an approach developed by the author called FARS (far-field analysis of radiation sources). Two antenna-excitation models are investigated for a straight-wire dipole: the usual tangential E-field model, and a two-wire transmission-line feed. Comparison of FARS and the differentiated Poynting vector results for a 10-wavelength dipole shows them to agree to within 10% relative to end peaks in the distribution of spatial radiation, except in the vicinity of the antenna's feed point. Both also show that radiation occurs not only near the feed point and ends of the antenna, but is distributed along its length, being associated with maxima of the current standing wave. Results presented for antennas such as a bent-wire dipole, circular and square loops, and for a straight-wire scatterer demonstrated the effects of shape and excitation on the distributed radiation     

A technique for organizing large moment calculations for use with iterative solution methods

Through a specific choice of evenly spaced basis and testing functions, one can construct a moment matrix formulation wherein significant redundancy is embodied in the matrix. A scheme that exploits this redundancy to effect substantial computer storage reduction in an iterative field solution, such as the conjugate-gradient method is described. The expense incurred is a modest increase in computation time. The redundancy results from translational similarity features of specific classes of geometries and leads to matrix equations that may be interpreted as discrete convolutions. For illustration, the analysis here is carried out on a planar scatterer, but the strategy can be applied to spherical or cylindrical scatterers.     

A new numerical approach to the calculation of electromagnetic scattering properties of two-dimensional bodies of arbitrary cross section

A new method is introduced for formulating the scattering problem in which the scattered fields (and the interior fields in the case of a dielectric scatterer) are represented in an expansion in terms of free-space modal wave functions in cylindrical coordinates, the coefficients of which are the unknowns. The boundary conditions are satisfied using either an analytic continuation procedure, in which the far-field pattern (in Fourier series form) is continued into the near field and the boundary conditions are applied at the surface of the scatterer; or the completeness of the modal wave functions, to approximately represent the fields in the interior and exterior regions of the scatterer directly. The methods were applied to the scattering of two-dimensional cylindrical scatterers of arbitrary cross section and only the TM polarization of the excitation is considered. The solution for the coefficients of the modal wave functions are obtained by inversion of a matrix which depends only on the shape and material of the scatterer. The methods are illustrated using perfectly conducting square and elliptic cylinders and elliptic dielectric cylinders. A solution to the problem of multiple scattering by two conducting scatterers is also obtained using only the matrices characterizing each of the single scatterers. As an example, the method is illustrated by application to a two-body configuration.