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This paper describes how information about the electro- magnetic structure of targets can be obtained from direct detection radar techniques, where the relative phase of the transmitted and re- ceived signals is not measured. A comparison is made between the re- solved structure of a simple test target from an ultra wide band, pulse synthesis direct detection radar system at 14{40GHz and an equiv- alent heterodyne radar receiver where phase information is recorded. The test targets employed are wax sheet of thickness 20mm and 80mm which are illuminated alone and in contact with the human body. A vector network analyser is used as the radar system. The simplicity of constructing ultra wide band direct detection radar systems combined with their cost makes the use of such radar systems appealing for appli- cations such as concealed threat detection and non-destructive testing, where absolute range to the target, if required, can be determined by other methods.

The authors present transmission data, taken at Ka (36GHz) and W (95 GHz) bands in\ud the millimetre-wave region of the electromagnetic spectrum, for various dressing materials used in the\ud treatment and management of burn wounds. The results show that such materials are highly transparent\ud (typically > 90% transmission) and, in their dry state, will permit the sensing of the surface of the skin\ud through the thick layers (> 2 cm) of different dressings typically applied in medical treatment of burn\ud wounds. Furthermore, the authors present emissivity data, taken at the same frequency bands, for\ud different regions of human skin on the arm and for samples of chicken flesh with and without skin\ud and before and after localised heat treatment. In vivo human skin has a lower emissivity than chicken\ud flesh samples, 0.3–0.5 compared to 0.6–0.7. However, changes in surface emissivity of chicken samples\ud caused by the short-term application of heat are observable through dressing materials, indicating the\ud feasibility of a millimetre-wave imaging to map changes in tissue emissivity for monitoring the state of\ud burn wounds (and possibly other wounds) non-invasively and without necessitating the removal of the\ud wound dressings.

The detection and identiflcation of metal items and, in particular weapons, of linear size ‚ 10cm, concealed upon the human body, is demonstrated as being entirely feasible by using a phased array of suitably ultra wide band transceivers. The complex natural resonances and especially the fundamental resonance, are excited by ultra wide band, stepped frequency continuous wave illumination of the target, using a phased array of antennae to focus the radiation. Broadband illumination of the target with microwave radiation of suitable frequency range (Typically 0.3{3GHz for handgun sized objects) excites low order complex natural resonances and the late time response of the concealed item can be spatially located using phased array imaging techniques. Further processing of the late time response enables classiflcation of the concealed object, based on the complex natural resonant frequencies of the object, so that threat items such as handguns and knives can be difierentiated from benign items such as mobile phone handsets and cameras.

The detection and identiflcation of conducting objects using electromagnetic pulses to excite circulating eddy currents within the object is demonstrated by numerical simulation using a flnite element time domain electromagnetic solver. The ability to discriminate between objects is based on the decay rate of the induced currents in the object, typically » 100"S. The decay rates are difierent for a wide variety of everyday objects, allowing threat objects such as handguns, grenades and knives to be discriminated from benign objects such as mobile phones handsets, watches, keys, etc.. Crucially, the time constant characterising an object depends only upon the electrical properties of the object (conductivity) and the shape and size of the object; the orientation of the object is irrelevant. This aspect independence of temporal current decay rate forms the basis of a potential object detection and identiflcation system. By application of an algorithm based on the generalized pencil of function method, the authors demonstrate the ability to efiectively count and indentify multiple objects carried in close proximity providing that the objects do not have very similar time constants and that signal to noise ratio is high.

The authors discuss and demonstrate the feasibility of using ultra wide band microwave radar to detect and identify small arms flre. Detection and tracking is by standard radar techniques, but identiflcation is carried out by exciting the projectiles Complex Natural Resonances and using this aspect independent information to assign a caliber to the incoming projectiles. The typical sizes of small arms projectiles (calibers 5.56mm through to 13mm) imply that ultra wide band illumination in the microwave region of the spectrum between 1.5{5.5GHz is required to excite these object's fundamental resonances. The authors give a discussion of the efiects of motion on the quality of the complex natural resonance data obtainable and present both simulated and laboratory data for the radar cross section of three difierent caliber projectiles (5.56mm, 7.62mm and 13mm).