Through-wall Imaging Research Articles

Propagation Models and Inversion Approaches for Subsurface and Through-Wall Imaging

1Department of Electrical and Computer Engineering, University of Illinois at Chicago, 1020 SEO, MC 154, 851 South Morgan Street, Chicago, IL 60607-7053, USA 2 Istituto per il Rilevamento Elettromagnetico dell’Ambiente, Consiglio Nazionale delle Ricerche, Via Diocleziano 328, 80124 Napoli, Italy 3Department of Electrical and Computer Engineering, University of Illinois at Urbana Champaign, 379 Everitt Lab, MC 702, 1406 W. Green Street, Urbana, Illinois 61801, USA

Through-Wall Image Enhancement Based on Singular Value Decomposition

Singular value decomposition and information theoretic criterion-based image enhancement is proposed for through-wall imaging. The scheme is capable of discriminating target, clutter, and noise subspaces. Information theoretic criterion is used with conventional singular value decomposition to find number of target singular values. Furthermore, wavelet transform-based denoising is performed (to further suppress noise signals) by estimating noise variance. Proposed scheme works also for extracting multiple targets in heavy cluttered through-wall images. Simulation results are compared on the basis of mean square error, peak signal to noise ratio, and visual inspection.

An Automatic Framework Using Space-Time Processing and TR-MUSIC for Subsurface and Through-Wall Multitarget Imaging

We present an automatic framework combined space-time signal processing with Time Reversal electromagnetic (EM) inversion for subsurface and through-wall multitarget imaging using electromagnetic waves. This framework is composed of a frequency-wavenumber (FK) filter to suppress direct wave and medium bounce, a FK migration algorithm to automatically estimate the number of targets and identify target regions, which can be used to reduce the computational complexity of the following imaging algorithm, and a EM inversion algorithm using Time Reversal Multiple Signal Classification (TR-MUSIC) to reconstruct hidden objects. The feasibility of the framework is demonstrated with simulated data generated by GPRMAX.

Estimation of wall parameters based on range profiles

Compensation of wall effects is of main concern in SAR through-wall sensing. Wall parameters are essential for the compensation, which are normally not known a priori. A model of SAR working in side-looking strip mode for through-wall imaging is established, with the path of signal due to refraction taken into consideration. Through analyzing the echo configuration in time domain, a new procedure for estimating wall parameters using high range resolution profiles is provided. By extracting time delay of target counterpart, the thickness and permittivity of wall are figured out exactly by employing numerical equation method and intersecting of searching curves. With the estimated parameters, focused image of targets in a room is acquired after wall effects are compensated in imaging processing with back projection algorithm. Simulation results verify the validity of the theoretical analysis and imaging algorithm.

See-Through Walls: Motion Tracking Using Variance-Based Radio Tomography Networks

This paper presents a new method for imaging, localizing, and tracking motion behind walls in real time. The method takes advantage of the motion-induced variance of received signal strength measurements made in a wireless peer-to-peer network. Using a multipath channel model, we show that the signal strength on a wireless link is largely dependent on the power contained in multipath components that travel through space containing moving objects. A statistical model relating variance to spatial locations of movement is presented and used as a framework for the estimation of a motion image. From the motion image, the Kalman filter is applied to recursively track the coordinates of a moving target. Experimental results for a 34-node through-wall imaging and tracking system over a 780 square foot area are presented.

Time delay estimation of UWB radar signals backscattered from a wall

Estimating the basic characteristics of a wall is especially important in the field of through-wall radar imaging. In this article, we focus on the time delay estimation of signals measured by an ultra-wideband short-pulse radar. We describe the essential signal preprocessing steps, which are applied before a subspace super-resolution method is used to estimate the time delays of overlapping pulses backscattered from the wall. The signal correlation matrix is estimated using only one snapshot of the measured data, and the algorithm can operate in real time. In addition, to resolve closely spaced correlated echoes, we apply an improved spatial smoothing method. The performance of our algorithm is verified by simulations, and we also present the measurement results for two real brick walls. © 2011 Wiley Periodicals, Inc. Microwave Opt Technol Lett 53:1444–1450, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.25985

The Application of Time Domain Beamforming to Ultra-wideband Through-wall Imaging Radar

摘要: 超宽带穿墙成像雷达作为一种能够隔墙探测和定位的新型雷达系统,在很多领域得到应用。该文提出利用时域波束形成对穿墙雷达进行成像的方法。电磁波穿墙传播时产生速度变化、衰减和折射等现象。该文在电磁波穿墙传播的折射模型上用解析的方法计算电磁波的传播时间,在时域波束形成成像算法中考虑了墙体对电磁波的影响,并用FDTD模拟对成像方法进行验证。最后定性分析了墙体厚度、介电常数估计误差对成像的影响。 关键词: 成像雷达; 超宽带; 穿墙成像; 时域波束形成

Through-Wall Opportunistic Sensing System Utilizing a Low-Cost Flat-Panel Array

A UWB through-wall imaging system is proposed based on a planar low profile aperture array operating from 0.9 GHz to 2.3 GHz. The goal is to provide a lightweight, fixed array to serve as an alternative to synthetic aperture radars (SAR) that require continuous array movement while collecting data. The proposed system consists of 12 dual-linear printed elements arranged within a triangular lattice, each forming a “flower” shape and backed by a ground plane. The array delivers half-space radiation with wideband performance, necessary for imaging applications. UWB capability is realized by suppressing grating lobes via the introduction of virtual phase centers interwoven within the actual array feeds. The proposed system is demonstrated for through-wall imaging via a non-coherent process. Distinctively, several coherent images are forged from various fixed aperture locations (referred to as “snapshot” locations) and appropriately combined to create a composite scene image. In addition to providing a unique wideband imaging capability (as an alternative to SAR), the system is portable and inexpensive for collecting/storing scattering data. The array design and data collection system is described, and several through-wall images are presented to demonstrate functionality.

Experimental Validation of a Simple System for Through-the-Wall Inverse Scattering

In this letter, we propose a system for through-wall imaging based on a bistatic setup and a two-step inversion procedure. Since the adopted configuration is simple and low cost but only delivers a limited amount of data, the data-processing stage is tackled in a stepwise fashion. First, the targets are detected from the estimation of the surface impedance along the wall and then the area behind the identified portion of the wall is imaged by means of a linearized microwave-tomographic algorithm. The overall device is described, and the results of a proof-of-concept experimental validation are reported.

A Real-Time Through-Wall Detection Based on Support Vector Machine

In this paper, a real-time approach for solving the throughwall imaging is proposed based on support vector machine. The proposed technique avoids the consideration and computation of the effect of the walls, such as refraction and speed change, which is a primary difficulty and must be computed using the conventional methods. Numerical results show that once the training phase is completed, this technique only needs computational time in an order of seconds to locate the target with small errors. Noiseless and noisy measurements are discussed, making the proposed technique more realistic. Numerical experiments also show how the method is capable, when noise contamination is considered, to locate targets in throughwall scenes. This validates the effectiveness of the proposed approach.

Subspace-based optimization method for inverse scattering problems with an inhomogeneous background medium

This paper proposes a version of the subspace-based optimization method to solve the inverse scattering problem with an inhomogeneous background medium where the known inhomogeneities are bounded in a finite domain. Although the background Green's function at each discrete point in the computational domain is not directly available in an inhomogeneous background scenario, the paper uses the finite element method to simultaneously obtain the Green's function at all discrete points. The essence of the subspace-based optimization method is that part of the contrast source is determined from the spectrum analysis without using any optimization, whereas the orthogonally complementary part is determined by solving a lower dimension optimization problem. This feature significantly speeds up the convergence of the algorithm and at the same time makes it robust against noise. Numerical simulations illustrate the efficacy of the proposed algorithm. The algorithm presented in this paper finds wide applications in nondestructive evaluation, such as through-wall imaging.

UWB Through-Wall Imaging Based on Compressive Sensing

To achieve high-resolution 2-D images, through-wall imaging (TWI) radar with ultra-wideband and long antenna arrays faces considerable technical challenges such as a prolonged data collection time, a huge amount of data, and a high hardware complexity. This paper presents a novel data acquisition scheme and an imaging algorithm for TWI radar based on compressive sensing (CS), which states that a signal having a sparse representation can be reconstructed from a small number of nonadaptive randomized projections by solving a tractable convex program. Instead of measuring all spatial-frequency data, a few samples, by employing an overcomplete dictionary, are sufficient to obtain reliable target space images even at high noise levels. Preliminary simulated and experimental results show that the proposed algorithm outperforms the conventional delay-and-sum beamforming method even though many fewer CS measurements are used.

Corrections to “UWB Through-Wall Imaging Based on Compressive Sensing” [Mar 10 1408-1415

In the above titled paper (ibid., vol. 48, no. 3, pp. 1408-1415, Mar. 2010), there are two errors in the top line of (13) which we correct here.

Autofocusing imaging through the unknown building wall with EM inverse scattering algorithm

AbstractThe through‐wall imaging (TWI) problem is studied within the context of EM inverse scattering in this article.A novel real time TWI algorithm is proposed by employing the spectrum Green's function and exploding source model under Born approximation. The complex scattering process because of the presence of the wall is well modeled in the imaging formulation through the layered medium Green's function. Inspired by the concept of time reversal imaging, a novel autofocusing strategy is incorporated in the proposed algorithm. The autofocusing is achieved by introducing a time factor in the TWI formulation to get a dynamic image at different focusing time. The image at the time instant, when the defined entropy is minimized, is stored as the output of the imaging result. Numerical and experimental results are presented to show that the proposed method can provide high quality focused image in a short computation time regardless of the estimated value of the wall parameters. © 2010 Wiley Periodicals, Inc. Microwave Opt Technol Lett 52: 732–736, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.25013

A Novel Autofocusing Approach for Real-Time Through-Wall Imaging Under Unknown Wall Characteristics

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A novel real-time through-wall imaging (TWI) algorithm with autofocusing ability in the presence of wall ambiguities is proposed in this paper. The spectrum Green's function is employed to formulate the TWI algorithm, where the fast Fourier transform can be used to reconstruct the image in a very short computation time. The complex scattering process due to the presence of the wall is automatically included in the imaging formulation through the multilayer Green's function. The autofocusing is achieved by introducing a time factor in the TWI formulation to get a dynamic image at different focusing time. The image at the time instant when the defined entropy is minimized is stored as the output of the TWI result. Simulation results show that the proposed method can provide high-quality focused image in a short computation time regardless of the estimated value of the wall parameters. </para>

Experimental Validation of a Linear Inverse Scattering TWI Algorithm by a SF-CW Radar

We present the experimental validation of a through-wall imaging (TWI) algorithm based on a sliced linearized inverse scattering approach. The experimental validation is performed by processing measurements collected by a stepped frequency ultrawideband radar in a realistic TWI scenario.

THROUGH-WALL IMAGING: APPLICATION OF SUBSPACE-BASED OPTIMIZATION METHOD

A model for two-dimensional layered medium is proposed for through-wall imaging problem. It is integrated with the subspace- based optimization method for reconstructing the relative permittivity proflle in a fast and robust manner. Numerical results have shown that the concealed targets within walls can be reconstructed well using the proposed model even though full aperture of targets is not available due to the presence of walls. The model has also been employed for studying the efiect of the presence of walls on imaging.

Adaptive CLEAN With Target Refocusing for Through-Wall Image Improvement

Signal processing techniques to improve building interior images are introduced. Specifically, an adaptive CLEAN algorithm is introduced in conjunction with target refocusing techniques to de-emphasize undesired signal effects from external walls, and thus more accurately image the lesser contrast objects. Instead of using the point spread function as in the conventional CLEAN algorithm, adaptive CLEAN incorporates a delay spread model found more appropriate for subtracting sidelobes that could obscure interior objects. Refocusing also allows for correcting target distortion and displacement due to through-wall propagation. It is shown that by combining adaptive CLEAN with target refocusing techniques, we are able to reconstruct interior targets (with minimal exterior wall distortion). The introduced processing techniques are demonstrated via simulations generated with a well validated high frequency ray tracing code, and with measured data.

Coherence Factor Enhancement of Through-Wall Radar Images

The coherence factor (CF), defined as the ratio of coherent power to incoherent power received by an array of sensors for a given point in an image scene, is applied to through-wall image processing. It is shown that CF filtering reduces clutter and increases clarity and uniformity in the image by suppressing the low-coherence features. It is also shown that through-wall propagation degrades the CF, thereby enabling its use as a quantitative metric for autofocus and image correction algorithms.

Adaptive Polarization Contrast Techniques for Through-Wall Microwave Imaging Applications

In this paper, we describe and utilize polarization contrast techniques of the adaptive polarization difference imaging algorithm and its transient modification for through-wall microwave imaging (TWMI) applications. Originally developed for optical imaging and sensing of polarization information in nature, this algorithm is modified to serve for target detection purposes in a through-wall environment. The proposed techniques exploit the polarization statistics of the observed scene for the detection and identification of changes within the scene and are not only capable of mitigating and substantially removing the wall effects but also useful in detecting motion, when conventional Doppler techniques are not applicable. Applications of the techniques to several TWMI scenarios including both homogeneous and periodic wall cases are presented.