Through-the-Wall Radar Imaging

Through-the-Wall Radar Imaging

During the past decade, the technology associated with “seeing” through walls has witnessed a growing interest. The objectives of sensing through walls and inside enclosed structures range from determining building layouts, discerning the nature of activities inside buildings, and imaging building interiors to detect, identify, classify, and track the whereabouts of humans and moving objects. These attributes are highly desirable for a range of organizations, including police, fire and rescue personnel, first responders, and defense forces.

To achieve these objectives, electromagnetic waves are considered very effective due to their ability to penetrate man-made building materials and to image targets behind opaque structures. Through-the-wall radar imaging (TWRI) is a multifaceted technology. It requires a blending of several disciplines in the field of electrical engineering, especially those that involve signal, array, and image processing as well as radars, antennas, and electromagnetic (EM) waves. Solutions of TWRI problems must, however, take phenomenological issues into consideration and should be based on a solid understanding of the intricacies of EM wave interactions with exterior and interior building objects and structures. Furthermore, urban sensing operations put various demands and constraints on the imaging system, which may limit its application. Many factors must be considered, namely, size, weight, mobility, acquisition time, aperture distribution, power, bandwidth, standoff distance, and, most importantly, reliable performance and delivery of accurate information.

This book provides a broad overview of TWRI and discusses possible applications. It provides comprehensive coverage of all the important aspects from the algorithmic, modeling, experimentation, and system design perspectives. The book is motivated by the fact that there are important differences in the physics and objectives between TWRI and other typical radar imaging and remote sensing technologies. The phenomenology of wave propagations and interactions with and within exterior and interior walls, floors, and ceiling, and in between multiple targets, combined with the underlying nature and peculiar demands of indoor settings on system requirements make TWRI rather unique.

The recent developments in TWRI are captured in the 15 chapters of this book. The content and organization of the chapters allow a smooth transition from topic to topic.

Chapter 1 examines the electromagnetic properties of walls and building materials, which is of paramount importance to study and model the effects of walls on signal delay time, amplitude, and pulse shape. These effects must be properly incorporated when dealing with target images, detection, and localization. Various types of lossy and lossless walls are considered, including homogeneous material composition and heterogeneous air hole structure organization. The chapter also discusses wall attenuation and dispersion effects in view of angles of incidence and antenna polarization.

Chapter 2 covers important techniques in the design of antenna elements and array configurations. It discusses key antenna-related integration issues for consideration in TWRI systems and describes an implemented prototype mobile imaging system along with its onboard antennas. It also covers wideband, low-profile, and printed antennas, as well as planar/conformal ultrawideband antennas. Finally, it provides a detailed account of array developments for portable systems using slotted microstrip and printed Vivaldi arrays, together with system control mechanisms.

Chapter 3 discusses a number of beamforming concepts and issues that arise in TWRI. It presents the notion of point spread function (PSF), which is a standard measure of beamforming performance, with special emphasis on the PSF characteristics that result from the use of ultrawideband waveforms. The effect of various wall types on beamforming is also examined. Various array configurations and beamforming algorithms that may be employed for imaging are described and evaluated, and the issues associated with array implementation are addressed.

Chapter 4 describes technologies to image, localize, and track behind-the-wall targets using an antenna array with collocated and distributed apertures. It discusses coherent and noncoherent imaging techniques and provides practical examples. It also describes successful experiments for detecting moving targets behind walls using change-detection techniques. The chapter advocates the use and applicability of a dual-frequency radar in TWRI, which uses two CW signals with separate frequencies to provide range information of a single target or a small number of targets.

Chapter 5 discusses several suitable waveforms for use in TWRI applications. In addition to image resolution, it presents other key factors in waveform selection and design that are related to wall characteristics, building structure materials, electromagnetic interference, and covertness, and that are pertinent to system specifications, such as size and weight constraints. The design of emerging waveforms to optimize target detection by increasing target-to-noise and clutter ratio is discussed, with special emphasis on matched illumination-based signature exploitation techniques.

Chapter 6 deals with inverse scattering approaches and revolves around the relevance of physical-based model approaches in TWRI. It studies EM wave propagation through walls and shows the dispersion and blurring effects on behind-the-wall target images. Refocusing, in which wall electric properties are estimated from the early wall returns and then taken into account to produce a focused image, is shown to achieve impressive deblurring results. A class of linear and effective inverse scattering approaches based on simplified models of EM scattering is also presented.

Chapter 7 presents theoretical and experimental research in 3D building tomography using microwave remote sensing. It addresses the highly nonlinear inverse problem with the potentially huge number of degrees of freedom associated with building elements. The inverse problem uses prior information and performs a hierarchy of local inversions, isolations, and subtractions to achieve higher levels of building details. This chapter describes the physics of ray interaction with various types of buildings, especially those that have a periodic transverse internal wall structure, causing Bragg conditions.

Chapter 8 presents high-frequency asymptotic modeling methods that have been successfully used to carry out building imagery within the framework of the uniform geometric theory of diffraction (UTD), with an emphasis on multiple wall transmission, reflection, and diffraction. The methods discussed are based on generalizations of the UTD coefficients, and include both edge and corner diffraction coefficient generalizations that are modified to account for penetrable dielectric walls. This chapter demonstrates that the UTD with ray tracing technologies can create accurate 3D building images.

Chapter 9 describes synthetic aperture radar (SAR) techniques for TWRI. It discusses SAR image formulation methods based on strip-map and spotlight SAR techniques, and applies these methods to both modeling and experimental data. It demonstrates the benefits of using EM modeling to obtain high-fidelity results, permitting the separation of the scattering phenomenology and the artifacts introduced by image formation processing. This chapter presents radar signatures of the human body and shows a variety of simple and complex SAR images.

Chapter 10 covers impulse radars, which emit carrierless and very short pulses and implement SAR techniques. It demonstrates that ImpSAR can produce high-fidelity images, which assist in weapon and human detection and classification. Both inanimate objects and static people inside buildings at reasonably large standoff distances are considered in the analyses and experimentations. This chapter introduces the properties of an ImpSAR system, including phenomenology, hardware, and signal processing, and touches upon the ImpSAR system design principles.

Chapter 11 tackles the issue of airborne radar imaging of multi-floor buildings. It describes a technique for estimating the scattering primitives, such as dihedral and trihedral, from the radar returns, and uses the results to infer information about the building interiors. A maximum likelihood estimation technique is applied and efficiently implemented to obtain the location of primitive features and wall propagation/reflection parameters. The imaging and parameter estimation technique presented is successfully validated using EM simulations of a two-story building.

Chapter 12 presents strategies for target detection in TWRI applications. Different approaches based on centralized and decentralized detection schemes are discussed. Behind-the-wall target detection is addressed for a single view and also for multiple views, when available. The latter is possible when dealing with buildings at street corner locations or when reimaging with different system positions along the same side of the building. The focus of this chapter is on stationary targets, and the detection schemes are applied to the images generated using wideband synthetic aperture beamforming.

Chapter 13 deals with the detection of concealed targets, such as weapons and explosives, in TWRI applications. It addresses the unique challenges of distinguishing the signal returns arising from the sought target inside walls in which the target is concealed. It describes an imaging technique based on the inverse scattering and tomography approaches. This technique does not depend on high- or low-frequency approximations and is specifically designed to detect and possibly identify a target or a repository of targets concealed behind a wall.

Chapter 14 considers fast imaging acquisition schemes based on reduced data volume. The schemes presented include compressive sensing (CS), which allows a radar image of almost the same resolution and quality to be generated as that produced when employing all data samples. This chapter shows how targets behind walls can be accurately located using a fraction of the data measurements by using model and experimental data. Finally, data reduction methods and CS strategies under both frequency-domain and time-domain pulsing are delineated.

Chapter 15 focuses on the detection of indoor moving targets and discusses how the Doppler principle can be used to measure motion at a very fine level of detail. The material in this chapter describes the biometric radar in which micro-Doppler signatures, associated with human gait, breathing, and heartbeat, are used for the remote monitoring of human motion pertinent to TWRI applications. Moving target indicator techniques implemented in the raw signal domain and the final image domain are discussed and used for target detection and identification.

I would like to express my sincere gratitude to all book contributors. They have applied their expertise in modeling, experimentation, algorithm development, and system design to deliver high-quality documentations of the state of the art in the area of TWRI and urban sensing. The results is a comprehensive coverage of the subject, supported by relevant references. The breadth and diversity of the topics covered will allow students, engineers, academicians, technical group leaders, and program managers to gain a deep understanding of the challenges facing this technology, and to appreciate the various contributions that have been made in this area.

Moeness Amin

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