Background and Theory
Active sensors partially or entirely illuminate the person being screened with microwave-or millimetre-wave radiation. The radar cross section (RCS) of easily concealed weapons, such as handguns and knives, depends strongly on the orientation of the object with respect to the transmitter and receiver as well as the frequency band of the illuminating radiation. The RCS plays an important role in active millimetre-wave sensors, determining the ability to discriminate concealed objects against the background body by their different scattering characteristics. There are three important properties of any real-world objects which must be taken into account when considering the operation of millimetre-wave sensors: reflectance, transmittance and emissivity. By measuring the thermally radiated and reflected power at millimetre-wave frequencies, it is possible to ascertain information on concealed objects by a variety of methods. The concept of RCS is important for understanding the physical nature of scattering of electromagnetic signals and their analysis for the purpose of object detection.