Neural networks are made up of inputs and outputs with interconnections which can be tuned or ‘trained’ to produce a desired output, they are similar to those in a biological nervous system. The connections between inputs and outputs determine the function of the network. By adjusting the values of the connections (weights) between elements the network can be trained. They are usually trained to match an input to a particular output (target) within a specified tolerance. A network usually consists of many input- target pairs (Math-Works, 2009).
Matlab’s Signal Processing Toolbox contains a function called ‘xcorr’, this function performs cross-correlation. Xcorr estimates the cross-correlation sequence of a random
process (Math-Works, 2009). This function was used in processing data to re-order data which was similar to its neighbour in an array of data, by aligning data sets containing more than one peak in the time domain.
Finite Element Analysis is a numerical technique which gives approximate solutions to differential equations. The problem geometry is drawn and then divided into a finite number of smaller regions, triangles can be used in 2-D or tetrahedrons in 3-D. The equations are then solved for each finite element and summed for the whole problem (Pepper, 1992). COMSOL (2009) was used to model and solve lens and horn designs.
Chapter 3
Literature Review
3.1
Introduction
Concealed weapons detection is an active research topic. In 2002 the US Transport Se- curity Administration estimated its funding requirements would be$6.8 billion, mostly for aviation security (Mead, 2002). Passive emission in the millimetre wave band can be used to form images of people which can be used for security screening. Differences in emitted brightness between the human body and a concealed object provides the contrast in the image. The body and the object must have different emissivities, sur- face temperature or reflectance for the required contrast to be present. The contrast image can then be viewed by an operator or processed autonomously by a computer and a decision as to the potential threat made as shown in figure 3.1.
Figure 3.1: This figure shows a concealed metal object on the person when outdoor (cold sky) contrast is used to illuminate the subject (Doyle et al., 2004).
Active imaging uses radiation transmitted from a millimetre wave source and the subsequent reflections to form an image of a target. The difference in reflectivity of the body and any concealed weapon are used to form an image from which the presence of the weapon can be seen. The black body radiation emitted by people and objects can be used, but passive techniques tend to suffer from poor contrast unless used outdoors where the sky provides a cold source which improves contrast; the sky can be used as a reference temperature (Appleby, 2004).
Creating images of people causes privacy issues (Agurto et al., 2007), because of this non-imaging techniques have been investigated. These include active and passive techniques; active techniques produce a stronger signal from the target but passive techniques may be preferred if radiating microwaves would prevent covert use of a detection system. In the work described here, active techniques are used to measure the radar return from a target and the reflections from the concealed weapons.
In 1995 imaging systems that can operate through clothing were reviewed by Currie et al. (1995). As part of this work, a passive millimetre wave radiometer detection system was reviewed. The system looks for small variations in temperature between a concealed weapon and the background. The body and the weapon will be at different temperatures unless the weapon is very close to the body or artificially heated; the system operates in the 94GHz band. The radiometer was scanned across a scene and produced an image showing metallic and ceramic handguns through clothing.
Dallinger et al. (2005) present an active system which uses a wide-band lens and scans a bandwidth of 10GHz. This gives a spatial resolution of 1.5cm at a range of 1.5m. The sweep time is 15 seconds. The image produced covers an area which is 35 by 40 cm. The image contains a lot of clutter, these are bright reflections from the body which are not from the concealed object, they could be from the movement of the arms for example. The concealed object was detected but it is difficult to distinguish it from the background. Imaging systems can be broadly divided into two types. Systems using a lens have a low computer processing requirement, but the physical aperture needs to be large. Synthetic aperture systems require greater processing time, but can
have small apertures which can be scanned. Synthetic aperature systems can have the disadvantage of higher cost due to the multiple receivers needed. The skin effect of electromagnetic waves means that millimetre wavelengths do not penetrate human skin to more than a millimetre, supported by measurements of the permittivity of skin. The human body is a fairly good reflector. In this paper, they are using a PVC dummy to replace the human target. Illuminating the target from different angles would reduce specular reflections which result in bright spots or clutter in the image.
Terrorist threats and gun crime have become an increasing problem for govern- ments and law enforcement agencies over recent years. Agurto et al. (2007) present an overview of different techniques which can be applied to concealed weapons detection. Power levels used in active systems are typically less than that emitted by a mobile phone, so operating at these low power levels should not cause concern to regulatory bodies for EM power emissions or the public.
At infrared wavelengths (which are defined as 10−5m or about 3000 GHz) it is difficult to detect weapons on the body because the temperature of the weapon is close to the body temperature and the infrared radiation is significantly attenuated in the clothing. The sensor simply sees the surface temperature of the clothing and not what lies underneath. Infrared sensors have much, much better noise-equivalent temperature than millimetre wavelength sensors and therefore better sensitivity and they also have far superior spatial resolution, so operate well as night vision cameras (Agurto et al., 2007). Millimetre wavelengths are defined as 1cm to 1mm or 30 GHz to 300 GHz.
Appleby & Wallace (2007) notes that explosives carried close to the body are par- tially transparent and partially reflective at an operating frequency of 100 GHz which produces a characteristic radar return which can be used to detect concealed explosives, also the reflectivity of the skin closely matches that of water. In their paper Appleby & Wallace (2007) conclude that a passive system required to penetrate all types of clothing will probably need to operate below 500 GHz.