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Near-field millimeter-wave imaging for weapons detection

Various millimeter-wave imaging systems capable of imaging through clothing for the detection of contraband metal, plastic, or ceramic weapons, have been developed at PNL. Two dimensional scanned holographic systems, developed at 35, 90, and 350 GHz, are used to obtain high resolution images of metal and plastic targets concealed by clothing. Coherent single-frequency amplitude and phase data, which is gathered over a two-dimensional scanned aperture, is reconstructed to the target plane using a holographic wavefront reconstruction technique. Practical weapon detection systems require high-speed scanning. To achieve this goal, a 35 GHz linear sequentially switched array has been built and integrated into a high speed linear scanner. This system poses special challenges on calibration/signal processing of the holographic system. Further, significant improvements in speed are required to achieve real time operation. Toward this goal, a wideband scanned system which allows for a two- dimensional image formation from a one-dimensional scanned (or array) system has been developed. Signal/image processing techniques developed and implemented for this technique are a variation on conventional synthetic aperture radar (SAR) techniques which eliminate far- field and narrow-bandwidth requirements. Performance of this technique is demonstrated with imaging results obtained from a Ka-band system.