Passive cylindrical scan by unphased diffraction-limited antennas for low-cost concealed weapon detection

Abstract

This paper analyzes a simple low-cost scan system for concealed weapon detection (CWD) on a cooperative subject. The passive imaging system is based on a cylindrical sensing geometry, realized by mechanical vertical scan of a horizontal circle, filled with many diffraction-limited antennas surrounding the subject, over the whole body height. This system is dimensioned to scan an ideally coaxial cylindrical subject of known radius with a fixed spatial resolution. Several system parameters influence the capability of anomaly detection: horizontal spatial resolution (constrained by diffraction limitations on the sensing circle), vertical spatial resolution and radiometric sensitivity (both related to vertical scan settings). Spatial resolution calculations are carried out in function of the working frequency, and achievable resolutions according to diffraction limitations are discussed. A qualitative and quantitative study is done to determine how high radiometric sensitivity (achievable with well-established commercial components) could overcome the poor spatial resolution related to low working frequencies, in view of dielectric anomaly detection; the optimal dwell time (giving a good radiometric/spatial resolution trade-off) is evaluated. Sub-pixel resolution capabilities are briefly considered, together with a least square matching criterium. Performance of an alternative configuration, consisting of a rotating vertical array, is derived from the circular system. Finally, the data fusion from both configurations is suggested.