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 sub-millimeter (sub-mm) wave frequency band from 300 - 1000 GHz is currently being developed for standoff
concealed weapon detection imaging applications. This frequency band is of interest due to the unique combination of
high resolution and clothing penetration. The Pacific Northwest National Laboratory (PNNL) is currently developing a
350 GHz, active, wideband, three-dimensional, radar imaging system to evaluate the feasibility of active sub-mm
imaging for standoff detection. Standoff concealed weapon and explosive detection is a pressing national and
international need for both civilian and military security, as it may allow screening at safer distances than portal
screening techniques. PNNL has developed a prototype active wideband 350 GHz radar imaging system based on a
wideband, heterodyne, frequency-multiplier-based transceiver system coupled to a quasi-optical focusing system and
high-speed rotating conical scanner. This prototype system operates at ranges up to 10+ meters, and can acquire an
image in 10 - 20 seconds, which is fast enough to scan cooperative personnel for concealed weapons. The wideband
operation of this system provides accurate ranging information, and the images obtained are fully three-dimensional.
During the past year, several improvements to the system have been designed and implemented, including increased
imaging speed using improved balancing techniques, wider bandwidth, and improved image processing techniques. In
this paper, the imaging system is described in detail and numerous imaging results are presented.
A prototype active wideband 350 GHz imaging system has been developed to address the urgent need for standoff
concealed-weapon detection. This system is based on a wideband, heterodyne, frequency-multiplier-based transceiver
system coupled to a quasi-optical focusing system and high-speed conical scanner. This system is able to quickly scan
personnel for concealed weapons. Additionally, due to the wideband operation, this system provides accurate ranging
information, and the images obtained are fully three-dimensional. Waves in the microwave, millimeter-wave, and
terahertz (3 GHz to 1 THz) frequency bands are able to penetrate many optical obscurants, and can be used to form the
basis of high-resolution imaging systems. Waves in the sub-millimeter-wave band (300 GHz to 1 THz) are particularly
interesting for standoff concealed-weapon detection at ranges of 5 - 20+ meters, due to their unique combination of high
resolution and clothing penetration. The Pacific Northwest National Laboratory (PNNL) has previously developed
portal screening systems that operate at the lower end of the millimeter-wave frequency range around 30 GHz. These
systems are well suited for screening within portals; however, increasing the range of these systems would dramatically
reduce the resolution due to diffraction at their relatively long wavelength. In this paper, the standoff 350 GHz imaging
system is described in detail and numerous imaging results are presented.
Imaging in the sub-millimeter wave range of 300 - 1000 GHz is useful for a variety of applications including security
screening, imaging through obscurations, and non-destructive evaluation. Waves in this frequency range have
wavelengths ranging from 0.3 to 1.0 mm and are able to penetrate many optical obscurants. The ability to form high-resolution
images that penetrate clothing makes imaging in this frequency range particularly interesting for personnel
security screening at standoff distances. The Pacific Northwest National Laboratory (PNNL) has previously developed
portal screening systems that operate at the lower end of the millimeter-wave frequency range around 30 GHz. These
systems are well suited for screening within portals and can achieve resolution of about 5 mm at ranges of less than 1
meter. However, increasing the range of these systems would dramatically reduce the resolution due to diffraction at
their relatively long wavelength. Operation at much higher frequencies, for example 350 GHz, will enable an order of
magnitude improvement of the resolution at a given range, while still achieving adequate clothing penetration. PNNL's
portal imaging systems have relied on wavefront reconstruction, or holographic, imaging techniques to mathematically
focus the imagery. In the sub-millimeter-wave, this may not always be practical due to sensitivity of the system to slight
changes in the position of the imaging target during data collection. In this case, physical focusing using lenses or
reflectors may be more practical. In this paper, we examine the effectiveness of imaging near 350 GHz for security
screening applications. Imaging results are presented using the holographic wavefront reconstruction technique, as well
as a focused reflector-based imaging system.
A novel personnel surveillance system has been developed to rapidly obtain 360 degree, full-body images of humans for the detection and identification of concealed threats. Detectable threats include weapons fabricated with metal, plastic, and ceramic, as well as explosive solids and liquids. This new system uses a cylindrical mechanical scanner to move a seven-foot, 384 element, Ka band (26 - 30 GHz) array circumferentially around a person in four to seven seconds. Low power millimeter-waves, which are nonionizing and not harmful to humans, are employed because they readily penetrate clothing barriers and reflect from concealed threats. The reflected waves provide information that is reconstructed into 3-D cylindrical holographic images with high-speed, digital signal processing (DSP) boards. This system is capable of displaying in an animation format eight, sixteen, thirty-two or sixty-four image frames at various aspect angles around the person under surveillance. This new prototype surveillance system is operational and is presently under laboratory testing and evaluation.
A novel wideband millimeter-wave imaging system is presently being developed at Pacific Northwest National Laboratory (PNNL) that will allow rapid inspection of personnel for concealed explosives, handguns, or other threats. Millimeter-wavelength electromagnetic waves are effective for this application since they readily penetrate common clothing materials, while being partially reflected from the person under surveillance as well as any concealed items. To form an image rapidly, a linear array of 128 antennas is used to electronically scan over a horizontal aperture of 0.75 meters, while the linear array is mechanically swept over a vertical aperture of 2 meters. At each point over this 2-D aperture, coherent wideband data reflected from the target is gathered using wide-beamwidth antennas. The data is recorded coherently, and reconstructed (focused) using an efficient image reconstruction algorithm developed at PNNL. This algorithm works in the near-field of both the target and the scanned aperture and preserves the diffraction limited resolution of less than one-wavelength. The wide frequency bandwidth is used to provide depth resolution, which allows the image to be fully focused over a wide range of depths, resulting in a full 3-D image. This is not possible in a normal optical (or quasi-optical) imaging system. This system has been extensively tested using concealed metal and plastic weapons, and has recently been tested using real plastic explosives (C-4 and RDX) and simulated liquid explosives concealed on personnel. Millimeter-waves do not penetrate the human body, so it is necessary to view the subject from several angles in order to fully inspect for concealed weapons. Full animations containing 36 - 72 frames recorded from subjects rotated by 5 - 10 degrees, have been found to be extremely useful for rapid, effective inspection of personnel.
KEYWORDS: Holography, Imaging systems, Weapons, Extremely high frequency, Transceivers, Antennas, 3D image processing, Surveillance systems, Ka band, Ku band
A new wideband millimeter wave holographic imaging technqiue is under developement for use in concealed weapons detection system. This new wideband technique provides far superior images than single frequency holographic techniques on thick objects such as the human body. The wideband technique obtains fully focused images over a designated volume and provides excellent lateral and depth resolution. Using this method, a 3D volumetric hologram is gathered with a millimeter wave linear array, a mechanical scanner, and a sweep frequency tranceiver. The 3D volumetric hologram is then processed by high-speed computational processors to reconstruct the fully focused image. Two prototype wide band millimeter wave holographic arrays have been developed at the Pacific Northwest Laboratory. The two arrays consist of sequentially switched 2 by 37 Ku band (12.5-18 GHz) and 2 by 64 Ka band (26.5-40 GHz) systems which are coupled to high-speed sweep frequency heterodyne transceivers. The arrays are used to obtain volumetric imaging data at high speeds by electronically sequencing and frequency sweeping the array antennas along 1D while performing a mechanical scan along the other dimension. The current prototype system scans an aperture the size of a large human body in about one second. Extensive laboratory testing has been performed with people carrying various concealed weapons and innocuous items with both imaging arrays during the first quarter of 1995.
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