Millimeter-wave imaging techniques and systems have been developed at the Pacific Northwest National Laboratory (PNNL), Richland, WA, for the detection of concealed weapons and contraband at airports and other secure locations. These techniques were derived from microwave holography techniques that utilize phase and amplitude information recorded over a two-dimensional aperture to reconstruct a focused image of the target. Millimeter-wave imaging is well suited for the detection of concealed weapons or other contraband carried on personnel since millimeter-waves are nonionizing, readily penetrate common clothing material, and are reflected from the human body and any concealed items. In this paper, a wide-bandwidth three-dimensional holographic microwave imaging technique is described. Practical weapon detection systems for airport or other high-throughput applications require high-speed scanning on the order of 3 to 10 s. To achieve this goal, a prototype imaging system utilizing a 27-33 GHz linear sequentially switched array and a high-speed linear scanner has been developed and tested. This system is described in detail along with numerous imaging results.

Three-dimensional millimeter-wave imaging for concealed weapon detection

The physical optics approximation is applied to the acoustic and electromagnetic direct scattering integral representation, yielding an inverse scattering identity which relates the characteristic function of a scatterer to the three-dimensional spatial Fourier transform of the augmented far-field scattering amplitude. This identity requires full scattering information for all frequencies and aspect angles. An integral equation for incomplete scattering information for this identity is developed. This integral equation is for the unknown characteristic function of the scatterer in terms of the known incomplete scattering information. The kernel of this integral equation is the three-dimensional spatial Fourier transform of the known characteristic function of the scattering information aperture. A regularized analytic closed-form solution to this integral equation is obtained. Synthesized numerico-experimental results verifying this solution are presented. The details of some special cases, consisting of a priori knowledge about the scatterer or the scattering information aperture, are presented.

A survey of the physical optics inverse scattering identity

The problem of all-weather noncooperative target recognition is of considerable interest to both defense and civil aviation agencies. Furthermore, the discipline of radar inverse scattering spans a set of real-world problems whose complexity can be as simple as perfectly conducting objects in uniform, isotropic, and clutter-free environments but also includes problems that are progressively more difficult. Consequently, this topic is attractive as a practical starting point for general object characterization schemes. But traditional radar target models—upon which most current radar systems are based—are nearing the end of their usefulness. Unfortunately, most traditional research programmes have emphasized instrumentation over image and model analysis and, consequently, the discipline is unnecessarily jargon-laden and device-specific. The result is that recent contributions in advanced imaging, inverse scattering and model fitting methods have often been 'excluded' from mainstream radar efforts. This topical review is intended to serve as a survey of current and proposed schemes and an overview and discussion of roadblocks to successful implementation of some of the more popular approaches. The presentation has been constructed in a manner that (it is hoped) will appeal to those physicists and applied mathematicians who are not approaching the subject of radar imaging from a formal radar background.

Mathematical problems in radar inverse scattering

Radar targets can be identified by either forming images with sufficient resolution to be recognized by the human observer or by forming signatures or representations of the target for automated machine recognition. Tomographic microwave diversity imaging techniques that combine angular (aspect), spectral, and polarization degrees of freedom have been shown, as summarized in the first part of this paper, to be capable of producing images of the scattering centers of a target with near optical resolution. In the second part of the paper the author shows that collective nonlinear signal processing based on models of neural networks combined with the use of suitable target signatures (here sinogram representations) offer the promise of robust super-resolved target identification from partial information. Results presented are of numerical simulations for a neuromorphic processor where the neural net performs simultaneously the functions of data storage, processing, and recognition by automatically generating an identifying object label, and fast optoelectronic architectures and hardware implementations are briefly mentioned. Practical considerations and extensions to real systems are briefly discussed. >

Microwave diversity imaging and automated target identification based on models of neural networks

Results of a study demonstrating the feasibility of broadband and speckle-free projective imaging of a complex shaped scattering object in the 6-17 GHz range with centimeter resolution are presented. It is shown how angular, spectral, and polarization diversities can be combined in the data acquisition process with a unique target-derived reference technique to access the three-dimensional Fourier space of the scatterer cost-effectively. Imagea retrieval algorithms, based on the projection slice theorem and knowledge of object symmetry, are utilized in obtaining images of a model aircraft with near optical resolution. The implications for high-resolution three-dimensional tomographic imaging radar networks are discussed.

Prospects for three‐dimensional projective and tomographic imaging radar networks

The reconstruction problem of perfectly conducting bodies from far-field scattering data is discussed within the context of physical optics. The general bistatic scattering configuration, discussed in this investigation, adds inherent complexity to the problem but also introduces possibilities not present in the backscatter limit.

S. Rosenbaum-Raz

Papers

On scatterer reconstruction from far-field data