Inversion of Phaseless Total Field Data Using a Two-Step Strategy Based on the Iterative Multiscaling Approach

In this paper, a new approach for the quantitative electromagnetic imaging of unknown scatterers located in free space from amplitude-only measurements of the total field is proposed and discussed. The reconstruction procedure splits the problem into two steps. The method is based on the use of an inverse source algorithm to first complete the scattering data by estimating the distribution of the radiated field in the investigation domain. The object's function profile is then retrieved from the phaseless data via an iterative multiresolution procedure integrated with an effective minimization technique based on the particle swarm algorithm. Numerical examples are provided to assess the effectiveness of the whole two-step strategy in the presence of synthetic noise-corrupted data as well as in dealing with experimental data sets. Comparisons with full-data and "bare" approaches are reported as well     

Experimental validation of a PO-based shape reconstruction algorithm

The effectiveness of a linear inverse scattering algorithm for the shape reconstruction of perfectly conducting objects is experimentally validated by processing measured data. The data are collected via an automatic system for free-space measurements under reflection mode geometry. The amplitude and phase of the scattered field is measured in a multistatic and multifrequency configuration for different locations (views) of the transmitting antenna. The reliability of the measurement setup is shown by a comparison with simulated data. No a priori knowledge of the shape of the scatterers is assumed. The results of inversions of experimental data collected under single-view illumination agree well with those of inversions of synthetic data, so showing the robustness of the algorithm.     

Image reconstruction from TE scattering data using equation ofstrong permittivity fluctuation

Compared to the TM case, the inverse scattering problem for the TE incident field is more complicated due to its stronger nonlinearity. This work provides an effective method for the reconstruction of two-dimensional (2-D) inhomogeneous dielectric objects from TE scattering data. The algorithm applies the distorted Born iterative method to the integral equation of strong permittivity fluctuation to reconstruct scatterers with high-permittivity contrast. Numerical simulations are performed and the results show that the distorted Born iterative method (DBIM) for strong permittivity fluctuation (SPF-DBIM) converges faster and can obtain better reconstructions for objects with larger dimensions and higher contrasts in comparison with ordinary DBIM. A frequency hopping technique is also applied to further increase the contrast     

Parallel GA-based approach for microwave imaging applications

Genetic algorithms (GAs) are well-known optimization strategies able to deal with nonlinear functions as those arising in inverse scattering problems. However, they are computationally expensive, thus offering poor performances in terms of general efficiency when compared with inversion techniques based on deterministic optimization methods. In this paper, a parallel implementation of an inverse scattering procedure based on a suitable hybrid genetic algorithm is presented. The proposed strategy is aimed at reducing the overall clock time in order to make the approach competitive with gradient-based methods in terms of runtime, but preserving the capabilities of escaping from local minima. This result is achieved by exploiting the natural parallelism of evolutionary techniques and the searching capabilities of the hybrid approach . The effectiveness of the proposed implementation is demonstrated by considering a selected numerical benchmark related to two-dimensional scattering geometries.     

Microwave Imaging in Layered Media: 3-D Image Reconstruction From Experimental Data

A prototype microwave imaging system for imaging 3-D targets in layered media is developed to validate the capability of microwave imaging with experimental data and with 3-D nonlinear inverse scattering algorithms. In this experimental prototype, the transmitting and receiving antennas are placed in a rectangular tub containing a fluid. Two plastic slabs are placed in parallel in the fluid to form a five-layer medium. The microwave scattering data are acquired by mechanically scanning a single transmitting antenna and a single receiving antenna, thus avoiding the mutual coupling that occurs when an array is used. The collected 3-D experimental data in the fluid are processed by full 3-D nonlinear inverse scattering algorithms to unravel the complicated multiple scattering effects and produce 3-D digital images of the dielectric constant and conductivity of the imaging domain. The image reconstruction is focused on the position and dimensions of the unknown scatterers. Different dielectric and metallic objects have been imaged effectively at 1.64 GHz.      

幅度和相位估计的ISAR超分辨成像方法

传统距离多普勒(Range Doppler, RD)成像方法分辨率取决于发射信号的带宽和信号在方位向积累的多普勒带宽。超分辨成像可以在给定带宽条件下,获得比RD方法更优的分辨率。给出一种基于幅度和相位估计(Amplitude and Phase Estimation, APES)的逆合成孔径雷达(Inverse Synthetic Aperture Radar, ISAR)超分辨成像方法,该方法根据回波数据构造自适应滤波器对目标散射点进行重建,仿真和实测ISAR数据成像结果验证了基于APES的ISAR超分辨成像算法的有效性。相比其他超分辨成像方法,该方法重建的散射点幅度更为精确,副瓣更低,图像对比度和图像信噪比增加,整体成像效果较佳。     

Scattering-Based Tomography for HRR and SAR Prediction

This paper introduces a method for predicting HRR radar signatures and SAR images by creating a parametric three-dimensional scattering model from existing measured or model-based HRR signatures and/or SAR images. The method identifies potential three-dimensional persistent scatterers and estimates their scattering patterns. The results are parametric HRR signature and SAR image reconstruction functions of range, azimuth, and elevation.The modeling is accomplished through a scattering-based tomography technique. This technique localizes potential scatterers by using a filtered back-projection algorithm for the inverse radon transform. Once found, potential scatterers may then have their two-dimensional (azimuth and elevation) scattering patterns parameterized through the use of a truncated spherical harmonic series.Results using the reconstructions from HRR data are presented. A M109 model is reconstructed based on HRR signatures. The model allows us to predict what the vehicle would look like from any arbitrary orientation using SAR. Finally an M548 vehicle is modeled using 26 measured HRR signatures. The model is shown to be better than the synthetic model data. Additionally we show that the new model results can be combined with the synthetic data to provide a better target model for signature matching.     

An inverse scattering method based on contour deformations by means of a level set method using frequency hopping technique

A shape reconstruction method for electromagnetic tomography based on contour deformations by means of a level set method is proposed. The level set method adds quite valuable features to the inverse procedure such as natural regularization of the contours and automatic breaking and merging capability. The frequency hopping technique turns out to be a key point of the inverse scattering procedure. The numerical results clearly indicate that the use of this inversion algorithm yields to accurate reconstructions of one or several objects from one initial guess object, even with noise-contaminated data and limited coverage of the fields.     

计算电大尺寸目标RCS的外推技术与矩量法结合的混合法

本文提出了另一种将外推技术和矩量法结合起来解决电大尺寸目标的电磁散射问题的混合法,在提取目标的特征信息时应用了抗噪声性能较好的矩阵束方法,在外推时应用了一种简单有效的拟合方法。用本文的方法计算了介质条带和细直导线的双站散射截面,所得结果与一般分域基矩量法结果吻合得很好,表明了本文方法的有效性。     

基于DSM-CSI的非线性逆散射算法研究

由于反演问题中散射体所处区域和散射体个数信息的缺失,进而导致非线性逆散射方法的待重构空间维数高和运算代价高。基于线性逆散射方法在重构目标区域和目标个数信息的计算代价低,本文提出一种DSM（Di-rect Sampling Method）线性方法和CSI（Contrast Source Inversion）非线性方法相结合的非线性逆散射方法。该方法利用DSM方法的重构结果为CSI方法提供散射体所处区域和散射体个数的先验信息,进而降低待重构空间的维数以及非线性逆散射问题的计算量。实验计算结果证明了该方法的有效性。     

基于GTD模型的目标二维散射中心提取

为精确描述隐身目标的高频电磁散射特性,该文用基于几何绕射理论的GTD模型代替以镜面散射为主的指数和模型。同时,为克服传统矩阵束算法(MEMP)计算量大、距离坐标配对等问题,该文提出用修正矩阵束算法和二维旋转不变技术(2D-ESPRIT)提取基于GTD模型的目标二维散射中心参数。仿真实验表明,该文的两种方法均适用于以边缘绕射等为主要散射源的隐身目标。     

Equivalent current density reconstruction for microwave imagingpurposes

Numerical computer simulations are used to study the possibilities of reconstructing the distribution of the equivalent current density vector in a domain with a known volume, inside which dielectric scatterers stand at arbitrary locations. An integrodifferential formulation of the three-dimensional electromagnetic inverse scattering is transformed into matrix form through the application of the moment method. A pseudoinversion algorithm overcoming ill-conditioned problems is used to obtain the distribution of the equivalent current density also in the case where the input data (i.e. the simulated values of the scattered field vector to be obtained in an observation domain) are affected either by Gaussian noise or by uniformly distributed errors. The results furnish information that could be used to devise a possible imaging method for detecting the locations and surface shapes of scattering objects     

近场散射体对阵列超分辨波达方向估计性能的影响和补偿方法

测定电波到达方向（DOA）是无线电定位系统的基本任务之一,先进的阵列超分辨测向技术获得广泛关注。在船载和机载等平台应用中,阵列天线由于受安装空间所限,其旁边存在散射体,这时阵列接收到的信号既包括远场目标信号又有近场散射体散射出多径信号。本文分析了这种近场散射信号对DOA估计性能的影响,即散射信号会在DOA谱上多形成一个干扰性质的谱峰,并且若散射信号来波方向靠近直达波来波方向时,会造成模糊现象。最后文章在多重信号分类算法（MUSIC）的基础上提出一种补偿方法,能够很好的抑制散射信号所造成的这种影响,实验仿真结果验证该算法的有效性。      

基于迭代多尺度深度网络的非线性逆散射成像方法

传统的迭代多尺度方法(iterative multiscaling approach, IMA)在求解非线性电磁场逆散射问题时,可以自适应提高成像空间的分辨率,缓解逆问题的病态性,但容易陷入局部极小值且无法做到实时重构。文中提出了一种迭代多尺度深度网络,该网络结合传统IMA和深度网络的优势,将IMA展开成深度网络模型（命名为IMA-Net）. 该方法迭代地执行一种感兴趣区域(regions of interest, ROI)提取算法,在不同尺度的ROI内构建目标函数,并将目标函数分解成若干个优化子问题,子问题的迭代更新过程映射到深度网络结构中,交替更新相关分量,求解出目标函数的最优值. 实验结果验证了该方法的有效性和优越性,为目标实时重构提供了一个有效方案.     

Numerical scattering analysis for two-dimensional dense randommedia: characterization of effective permittivity

A new numerical method for determining effective permittivity of dense random media in two dimensions is presented. The core of the method is to compare the average scattered field of a random collection of scatterers confined within an imaginary boundary with the scattered field from a homogeneous dielectric of the same shape as the imaginary boundary. The two-dimensional (2-D) problem is aggressively studied to provide insight into the dependence of the method's convergence on particle size, boundary shape, and boundary dimension. A novel inverse scattering method is introduced based on the method of moments (MoM), which greatly reduces the computation time and increases the flexibility of the procedure to analyze a variety of geometries. Results from this 2-D method may be used directly to compare with theoretical methods for determining effective permittivity such as the Polder-Van Santen (1946) mixing formula or field techniques such as the quasi-crystalline approximation     

Linear tomographic inversion of stepped-frequency GPR data: experimental results on two test-sites

This paper deals with the tomographic inversion of experimental data acquired by a stepped-frequency ground penetrating radar (SFGPR).The experimental SFGPR has been designed for archaeological prospecting and makes it possible to exploit a multi-view/multi-static/multi-frequency configuration. The tomographic algorithm is based on a linear model of the electromagnetic scattering to reconstruct the shape of strongly scattering targets. It has been already validated against experimental data for objects in free-space and synthetic data for buried scatterers.Here, we present experimental results for buried objects starting from measurements collected at two test-sites. The first one has been a dedicated outdoor test facility which has allowed to test the experimental setup and the inversion algorithm in controlled conditions; the second one has been an archeological test-site.     

Solution of combined-field integral equation using multilevel fastmultipole algorithm for scattering by homogeneous bodies

We present an accurate method of moments (MoM) solution of the combined field integral equation (CFIE) using the multilevel fast multipole algorithm (MLFMA) for scattering by large, three-dimensional (3-D), arbitrarily shaped, homogeneous objects. We first investigate several different MoM formulations of the CFIE and propose a new formulation, which is both accurate and free of interior resonances. We then employ the MLFMA to significantly reduce the memory requirement and computational complexity of the MoM solution. Numerical results are presented to demonstrate the accuracy and capability of the proposed method. The method can be extended in a straightforward manner to scatterers composed of different homogeneous dielectric and conducting objects     

A new approximate solution for scattering by thin dielectric disks of arbitrary size and shape

In this paper, an approximate solution for electromagnetic scattering by a very thin planar homogeneous dielectric object is presented. This solution is obtained from a volumetric integral equation using Fourier transform and is shown to be uniformly valid from low to high frequencies at all incidence angles including edge-on incidence. Validity of the solution is demonstrated through a comparison with canonical objects such as an infinite dielectric slab, and a number of two-dimensional (2-D) and three-dimensional (3-D) dielectric scatterers. For 2-D, and 3-D scatterers, the approximate solution is compared with a method of moments solution. In all cases examined the approximate formulation provides very accurate results except for situations where the dielectric constant is very high.     

Three-Dimensional Through-Wall Imaging Under Ambiguous Wall Parameters

A through-wall imaging problem for a 3-D geometry is considered. Scatterers are located beyond a wall represented by a dielectric slab whose features are unknown or known with some degree of uncertainty. A two-step imaging procedure is presented. First, the thickness and the dielectric permittivity of the wall are estimated by a simple procedure which takes into account that actual measurements concern the total scattered field (i.e., the field reflected by the wall plus the one scattered by the obscured scatterers). Then, the problem is cast as a linear inverse scattering problem and solved by means of a truncated-singular value decomposition algorithm. In particular, a 2-D sliced approach is employed to obtain the 3-D scene. Numerical examples are shown to assess the effectiveness of the reconstruction procedure.     

表面辐射条件在非光滑凸散射体上的应用

Kriegsman等人(1987年)提出了一种计算凸体电磁散射的新技术表面辐射条件(OSRC)法。但当散射体存在奇异边缘时,用这种新技术求得的散射场与实际相差很远,大大地限制了OSRC法的应用范围。本文考虑到奇异边缘的作用,提出了有效解决这一问题的一种奇异电流冲激法。用这一方法求解了金属带、多棱柱的散射,表明奇异电流的贡献将大大改善散射场预测的精度,并得到奇异电流的影响程度是散射体不光滑点处内角的函数。