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.
This paper analyzes simple imaging configurations to scan a human body, suitable as passive or active millimetre-wave imaging systems for concealed weapon detection (CWD). The first cylindrical configuration allows a 360 degrees scan: N unphased diffraction-limited antennas each of size L are placed on a circular support surrounding the subject (allowing scanning in the horizontal plane with N non-overlapping independent beams), and this circle is mechanically displaced over the whole body height. An analytical formula gives the maximum obtainable spatial resolution for different dimensions of the circular scanning device and operating frequencies, and the number of receivers achieving this optimal resolution.
Constraints to be taken into account are diffraction, the usable total length of the circle, and the full coverage by the N beams over the subject, which is modelled as a cylinder with variable radius, coaxial with the scanning circle. Numerical calculations of system resolution are shown for different operating microwave (MW) and millimetre-wave (MMW) frequencies; in order to study off-axis performances, situations where the subject is not coaxial with the scanning device are also considered.
For the case of a parallelepiped to be imaged instead of a cylinder, a linear array configuration is analyzed similarly to the circular one. A theoretical study is carried out to design other curved arrays, filled with unphased diffraction-limited antennas, for the imaging of linear subjects with finer resolution.
Finally, the application of such configurations is considered for the design of active imaging systems, and different system architectures are discussed.