This paper describes a study performed at the Pacific Northwest National Laboratory which investigated the use of active millimeter-wave radar imaging to perform threat detection in non-divested shoes. The purpose of this study was to determine the optimal imaging system configuration for performing this type of task. While active millimeter-wave imaging systems have proven to be effective for personnel screening, the phenomenology associated with imaging within a heterogeneous medium, such as a shoe, dictates limits for imaging system parameters. Scattering, defocusing, and multipath artifacts are significantly exaggerated due to the high contrast index of refraction associated with the boundary at the air and shoe interface. Where higher center-frequency and bandwidth result in much improved lateral and range resolution in the body scanning application, smaller wavelengths are significantly defocused after penetrating the sole of the shoe. Increased bandwidth, however, is essential for the shoe scanning application as well. Obtaining fine enough depth resolution is critical in separating the scattering contribution of each layer of the shoes in range to isolate possible threats embedded within the sole. In this paper, the results of a study to optimize the following imaging system parameters are presented: antenna illumination beamwidth, antenna polarization, transceiver bandwidth, and physical scanning geometry.
Active millimeter-wave imaging is currently being used for personnel screening at airports and other high-security facilities. The cylindrical imaging techniques used in the deployed systems are based on licensed technology developed at the Pacific Northwest National Laboratory. The cylindrical and a related planar imaging technique form three-dimensional images by scanning a diverging beam swept frequency transceiver over a two-dimensional aperture and mathematically focusing or reconstructing the data into three-dimensional images of the person being screened. The resolution, clothing penetration, and image illumination quality obtained with these techniques can be significantly enhanced through the selection of the aperture size, antenna beamwidth, center frequency, and bandwidth. The lateral resolution can be improved by increasing the center frequency, or it can be increased with a larger antenna beamwidth. The wide beamwidth approach can significantly improve illumination quality relative to a higher frequency system. Additionally, a wide antenna beamwidth allows for operation at a lower center frequency resulting in less scattering and attenuation from the clothing. The depth resolution of the system can be improved by increasing the bandwidth. Utilization of extremely wide bandwidths of up to 30 GHz can result in depth resolution as fine as 5 mm. This wider bandwidth operation may allow for improved detection techniques based on high range resolution. In this paper, the results of an extensive imaging study that explored the advantages of using extremely wide beamwidth and bandwidth are presented, primarily for 10-40 GHz frequency band.
The cylindrical millimeter-wave imaging technique, developed at Pacific Northwest National Laboratory (PNNL) and
commercialized by L-3 Communications/Safeview in the ProVision system, is currently being deployed in airports and
other high-security locations to meet person-borne weapon and explosive detection requirements. While this system is
efficient and effective in its current form, there are a number of areas in which the detection performance may be
improved through the use of other reconstruction algorithms and sensing configurations. PNNL and Northeastern
University (NEU) have teamed together to investigate higher-order imaging artifacts produced by the current cylindrical
millimeter-wave imaging technique using full-wave forward modeling and laboratory experimentation. Based on
imaging results and scattered-field visualizations using the full-wave forward model, a new imaging system is proposed.
The new system combines a multistatic sensor configuration with the generalized synthetic aperture focusing technique
(GSAFT). Initial results show an improved ability to image in areas of the body where target shading, specular
reflections, and higher-order reflections occur.
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