One of the biggest threats to modern society is the increasing use by criminals and
terrorists of concealed weapons and person born improvised explosive devices
(PBIED).
Current highly mature security screening technologies using x-ray and metal
detectors have limited deployment scenarios based on health and safety issues and
operational range, respectively. Given that most clothing is greater than 90%
transmissive in the microwave region, this spectral band is ideal for screening
people for concealed threats. However, due to diffraction, imagery to screen
subjects is limited due to the small number of pixels. In this regime, the exploitation
of microwave polarimetry from the field of remote sensing has particular benefits, as
it extracts maximum information content from a single pixel.
The work presented in this thesis has assembled a full polarimetric frequency
stepped radar from a vector network analyser (VNA), a linear orthogonal mode
transducer (OMT) of the turnstile type and a conical corrugated horn antenna. The
system’s characterisation by antenna pattern measurements, the measuring of
canonical targets of the plane, dihedral, dipole and helical reflectors showed the
system to be capable of making localised Sinclair matrix measurements of targets
at ranges of two to three metres.
The work presents a calibration procedure comprising the VNA’s internal calibration
and an external calibration to compensate for dispersion and cross-polar leakage of
system components. Static target measurements (canonical and various surrogate
items) were analysed, using range gating for clutter rejection. Calibrated Sinclair
parameter measurements compared with those from simple simulations, all
software being programmed in Matlab.
Measurements of moving targets revealed the phenomenon of speckle, this
introducing rapid changes in the Sinclair Parameters. Data analysis performed using
the coherency matrix and the Cloude/Pottier decomposition minimised the effects
of speckle in the processed data. Measurements show movement from particularly
rough surfaces increased the parameter of the Cloude/Pottier entropy, the level of
this being directly linked to the degree of speckle.
Application of the Huynen polarisation fork technique (a type of decomposition) has
proved to aid the identification of static and moving targets. A detailed analysis of
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the Huynen fork responses is made of the human torso on its own, weapons on their
own and then weapons positioned against the human torso. Responses of nondangerous objects such as keys and a smartphone are additionally presented