Privacy Preserving Contraband Detection Using a Millimeter-Wave Dynamic Antenna Array

Abstract

We present a privacy preserving technique for contraband detection using a real-time 75 GHz rotational dynamic antenna array that samples only a reduced set of Fourier-domain information to prevent image reconstruction of the screened subject. Complex objects such as handguns have sharp edges that correspond to sharp spatial frequency features in the Fourier domain. These features are dependent on the orientation of the object and manifest at millimeter-wave frequencies through conditions such as being concealed under clothing. The technique combines a dynamically rotated active interferometer and simple arithmetic feature extraction approach supporting a threshold-based classifier to identify when a handgun is concealed under clothing. The active interferometer technique utilizes noise transmitters to illuminate the test subject to produce thermal-like radiation, enabling Fourier-domain signal sampling, while the rotational dynamics support sampling that spans a circle in the 2-D Fourier domain to retrieve sharp-edge responses induced by concealed objects. We experimentally demonstrate the detection of concealed objects under clothing in a laboratory environment using a 75 GHz dynamic antenna array based on simple heuristic features extracted from statistics of the measured responses. The classifier is evaluated based on processing one or more subsequent measurements, showing a classification accuracy of 0.908 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$F$</tex-math> </inline-formula> 1-score of 0.916 using four consecutive measurements.