Nick Bowring

Active millimeter wave detection of concealed layers of dielectric material

Extensive work has been published on millimetre wave active and passive detection and imaging of metallic objects concealed under clothing. We propose and demonstrate a technique for revealing the depth as well as the outline of partially transparent objects, which is especially suited to imaging layer materials such as explosives and drugs. The technique uses a focussed and scanned FMCW source, swept through many GHz to reveal this structure. The principle involved is that a parallel sided dielectric slab produces reflections at both its upper and lower surfaces, acting as a Fabry-Perot interferometer. This produces a pattern of alternating reflected peaks and troughs in frequency space. Fourier or Burg transforming this pattern into z-space generates a peak at the thickness of the irradiated sample. It could be argued that though such a technique may work for single uniform slabs of dielectric material, it will give results of little or no significance when the sample both scatters the incident radiation and gives erratic reflectivities due to its non-uniform thickness and permittivity . We show results for a variety of materials such as explosive simulants, powder and drugs, both alone and concealed under clothing or in a rucksack, which display strongly directional reflectivities at millimeter wavelengths, and whose location is well displayed by a varying thickness parameter as the millimetre beam is scanned across the target. With this system we find that samples can easily be detected at standoff distances of at least 4.6m.

Near-field and three-dimensional aperture synthesis imaging

This paper investigates by simulation some of the capabilities of near-field and three-dimensional imaging which are made possible by accessing phase and amplitude of electric fields from radiometric emission using aperture synthesis systems. The aperture synthesis technique is the main stay of high resolution radio astronomy and is investigated here for the near-field application of personnel security screening in the millimetre wave band. The limitations of the standard radio astronomy visibility-function technique and a matrix method for image generation are investigated for this purpose. It is concluded that several hundred receivers are required for high pixel count (> few thousand) and good quality images and that a new and more efficient algorithms is required to process such numbers of channels from non-planar imaging arrays in the near-field. Investigating the resolution limits of three-dimensional imaging in the near-field region with this technique indicates sub-wavelength resolution may be possible

Simulations of three-dimensional radiometric imaging of extended sources in a security screening portal

This paper investigates by simulation the use of the three-dimensional aperture synthesis imaging technique to image three-dimensional extended sources. Software was written to access the three-dimensional information from computer graphics models in the formats of *.dxf and *.3ds and use these to generate synthetic cross-correlations, as if they would have been generated by an aperture synthesis antenna/receiver array measuring the radiometric emission from the three-dimensional object. A three-dimensional (near-field) aperture synthesis imaging algorithm generates [1] a voxel image of the three-dimensional object. Images created from a sphere indicate faithful reproduction about a single phase centre when the radius of the sphere is less than the Fresnel scale. However, for larger spheres, definition in the threedimensional imagery suffers and a phenomenon, referred to in this paper as Fresnel noise, appears in the image. Images of objects larger than the Fresnel scale can be created by having multiple smaller images, each having a size approximately of the Fresnel scale and centred on separate phase centres. Using the software to generate threedimensional imagery of a person, to demonstrate capabilities for portal security screening, indicates the technique works to first order. Improvements are needed in the software to improve the spatial sampling of the radiometric fields from the three-dimensional objects and implement a volumetric image mosaicking technique to remove the Fresnel noise.

Three-dimensional radiometric aperture synthesis microscopy for security screening

The three dimensional (3D) aperture synthesis imaging technique investigated here is a generalisation of the classic twodimensional radio astronomy technique with refinements for the near-field so it can be applied a personnel security screening portal. This technique can be viewed as a novel form of diffraction emission tomography and extends previous 3D aperture synthesis imaging research using matrix inversion techniques [1]. Simulations using three-dimensional Fourier transforms to create three-dimensional images from simulated three-dimensional visibility functions illustrate the Abbe microscopy resolution should be achievable in three dimensions simultaneously in a single sensor. The field-of-view is demonstrated to be limited by Fresnel scale effects and a means to over coming this by processing sub-sets of local visibility functions with different phase centres throughout the imaging volume is presented. The applications of this technique to a full 3D imaging security screening portal is explored and a route to extending simulation software for market driven imaging scenarios is discussed.

An aviation security (AVSEC) screening demonstrator for the detection of non-metallic threats at 28-33 GHz

The unique selling proposition of millimetre wave technology for security screening is that it provides a stand-off or portal scenario sensing capability for non-metallic threats. The capabilities to detect some non-metallic threats are investigated in this paper, whilst recommissioning the AVSEC portal screening system at the Manchester Metropolitan University. The AVSEC system is a large aperture (1.6 m) portal screening imager which uses spatially incoherent illumination at 28-33 GHz from mode scrambling cavities to illuminate the subject. The imaging capability is critically analysed in terms of this illumination. A novel technique for the measurement of reflectance, refractive index and extinction coefficient is investigated and this then use to characterise the signatures of nitromethane, hexane, methanol, bees wax and baking flour. Millimetre wave images are shown how these liquids in polycarbonate bottles and the other materials appear against the human body.

Screening vehicles for stowaways using aperture synthesis passive millimetre wave imaging

This paper presents part of a feasibility study into the use of the aperture synthesis passive imaging technique to screen vehicles for persons. The aperture synthesis technique is introduced and shown how in the near-field regime of a vehicle screening scenario that a three-dimensional imaging capability is possible. A suggested antenna receiver array is presented and the three-dimensional point spread function which this enables is calculated by simulation. This shows that over the majority of the inside of the vehicle the spatial resolution in all three spatial dimensions is of or less than the radiation wavelength, which at the suggested operational radiation frequency of 20 GHz is 1.5 cm. A radiation transport model that estimates the radiation temperatures of persons and backgrounds when viewing the vehicle either from the side or the top is presented, such a model being useful in the design of vehicle screening systems and as a basis for interpretation codes to assist operators in recognising persons in vehicles.

Comparative modelling and parameter extraction of a single- and two-diode model of a solar cell

In this paper, a parameter extraction technique of a single- and a two-diode photovoltaic (PV) solar cell models using MATLAB/Simulink are presented. To reduce the computational time, the proposed method uses an accurate iterational technique and limits the inputs to four parameters, which are normally provided by the manufacturers datasheets. Derived models are validated by plotting their I-V characteristics and compared to a photovoltaic (PV) cell under variable conditions of solar irradiance and temperature. The main contribution of this work is to validate the utilization of the two-diode model as a better representative model of a PV cell. This is important particularly at low levels of solar irradiance and temperature.

A microwave/millimetre radar approach to shoe screening for airline passenger security

Security screening of airline passengers is commonplace and very often requires that, in addition to screening of baggage and body, the passengers shoes should be examined. The process of shoe screening is currently carried out by use of x-ray imaging, whereby the passengers shoes are placed into an x-ray machine and inspected by a skilled operator for signs of any concealments, for example explosive materials; weapons or narcotics. Exposure to x-ray radiation is tightly regulated for health and safety reasons and passengers must remove their shoes prior to the screening process and then replace them after; this comes at the cost of reduced passenger throughput and is one of the most cited inconveniences by air passengers. Additionally, screening of 100% of passengers shoes is not always acceptable with the current x-ray approach and therefore shoe screening is not necessarily encompassing.