This paper examines the suitability and potential of reducing the acquisition requirements of a novel radiation mapper
through the application of the non-linear deconvolution technique, CLEAN. The radiation mapper generates a threshold
image of the target scene, at a user defined distance, using a single pixel detector manually scanned across the scene .
This paper provides a discussion of the factors involved and merits of incorporating CLEAN into the system. In this
paper we describe the modifications to the system for the generation of an intensity map and the relationship between
resolution and acquisition time for a target scene. The factors influencing image fidelity for a scene are identified and
discussed with the impact on fill-factor of the intensity image, which in turn determines the ability of the operator to
accurately identify features of the radiation source within a target scene. The CLEAN algorithm and its variants have
been extensively developed by the radio astronomy community to improve the image fidelity of data collected by sparse
interferometric arrays. However, the algorithm has demonstrated surprising adaptability including terrestrial imagery, as
detailed in Taylor et al. SPIE 9078-19 and Bose et al., IEEE 2002. CLEAN can be applied directly to raw data via a
bespoke algorithm. However, this investigation is a proof-of-concept and thus requires a well tested verification method.
We have opted to use the public ally available implementation of CLEAN found in the Common Astronomy Software
Applications (CASA) package. The use of CASA for this purpose dictates the use of simulated input data and radio
astronomy standard parameters. Finally, this paper presents the results of applying CLEAN to our simulated target scene,
with a discussion of the potential merits a bespoke implementation would yield.
This paper investigates the application of the CLEAN non–linear deconvolution method to Late Time Response (LTR)
analysis for detecting multiple objects in Concealed Threat Detection (CTD). When an Ultra-Wide Band (UWB)
frequency radar signal is used to illuminate a conductive target, surface currents are induced upon the object which in
turn give rise to LTR signals. These signals are re-radiated from the target and the results from a number of targets are
presented.
The experiment was performed using double ridged horn antenna in a pseudo-monostatic arrangement. A Vector
network analyser (VNA) has been used to provide the UWB Frequency Modulated Continuous Wave (FMCW) radar
signal. The distance between the transmitting antenna and the target objects has been kept at 1 metre for all the
experiments performed and the power level at the VNA was set to 0dBm. The targets in the experimental setup are
suspended in air in a laboratory environment.
Matlab has been used in post processing to perform linear and non-linear deconvolution of the signal. The Wiener filter,
Fast Fourier Transform (FFT) and Continuous Wavelet Transform (CWT) are used to process the return signals and
extract the LTR features from the noise clutter. A Generalized Pencil-of-Function (GPOF) method was then used to
extract the complex poles of the signal. Artificial Neural Networks (ANN) and Linear Discriminant Analysis (LDA)
have been used to classify the data.
KEYWORDS: Image processing, Antennas, Radio astronomy, Point spread functions, Interferometry, Imaging systems, Chemical elements, Deconvolution, Scene simulation, Astronomy
Image processing techniques can be used to improve the cost-effectiveness of future interferometric Passive MilliMetre Wave (PMMW) imagers. The implementation of such techniques will allow for a reduction in the number of collecting elements whilst ensuring adequate image fidelity is maintained. Various techniques have been developed by the radio astronomy community to enhance the imaging capability of sparse interferometric arrays. The most prominent are Multi- Frequency Synthesis (MFS) and non-linear deconvolution algorithms, such as the Maximum Entropy Method (MEM) and variations of the CLEAN algorithm. This investigation focuses on the implementation of these methods in the defacto standard for radio astronomy image processing, the Common Astronomy Software Applications (CASA) package, building upon the discussion presented in Taylor et al., SPIE 8362-0F. We describe the image conversion process into a CASA suitable format, followed by a series of simulations that exploit the highlighted deconvolution and MFS algorithms assuming far-field imagery. The primary target application used for this investigation is an outdoor security scanner for soft-sided Heavy Goods Vehicles. A quantitative analysis of the effectiveness of the aforementioned image processing techniques is presented, with thoughts on the potential cost-savings such an approach could yield. Consideration is also given to how the implementation of these techniques in CASA might be adapted to operate in a near-field target environment. This may enable a much wider usability by the imaging community outside of radio astronomy and thus would be directly relevant to portal screening security systems in the microwave and millimetre wave bands.
KEYWORDS: Radio astronomy, Interferometry, Receivers, Image processing, Synthetic apertures, Astronomy, Detection and tracking algorithms, Point spread functions, Imaging systems, Digital signal processing
This PhD programme is contributing to the development of Passive Millimetre-Wave Imagers (PMMWI) using the
principles of interferometric aperture synthesis and digital signal processing. The principal applications are security
screening, all-weather flight aids and earth observation. To enhance the cost-effectiveness of PMMWI systems the
number of collecting elements must be minimised whilst maintaining adequate image fidelity. A wide range of
techniques have been developed by the radio astronomy community for improving the fidelity of sparse interferometric
array imagery. This paper brings to the attention of readers these techniques and discusses how they may be applied to
imaging using software packages publicly available from the radio astronomy community. The intention of future work
is to adapt these algorithms to process experimental data from a range of realistic simulations and real-world targets.
This paper examines the sourcing of low cost components for next generation passive millimetre wave (PMMW)
aperture synthesis imagers. Splitting the elements of the imager into antennas/receivers, analogue to digital converters
(ADCs), digital signal processors (DSP) and a host computer, technologies are identified that can minimise the cost of
these in future systems. It is established that the follow-on aperture PMMW imagers can be constructed at relatively low
cost, using a combination of low frequency (< 30 GHz) satellite receiver technology, high-speed clocked comparators,
DSP (both Field Programmable Gate Arrays (FPGAs) and Graphical Processor Units (GPUs)) and the latest personal
computers that use high-speed high lane count PCI Express Bus technology.
The first video rate imagery from a proof-of-concept 32-channel 22 GHz aperture synthesis imager is reported. This
imager has been brought into operation over the first half of year 2011. Receiver noise temperatures have been measured
to be ~453 K, close to original specifications, and the measured radiometric sensitivity agrees with the theoretical
predictions for aperture synthesis imagers (2 K for a 40 ms integration time). The short term (few seconds) magnitude
stability in the cross-correlations expressed as a fraction was measured to have a mean of 3.45×10-4 with a standard
deviation of ~2.30×10-4, whilst the figure for the phase was found to have a mean of essentially zero with a standard
deviation of 0.0181°. The susceptibility of the system to aliasing for point sources in the scene was examined and found
to be well understood. The system was calibrated and
security-relevant indoor near-field and out-door far-field imagery
was created, at frame rates ranging from 1 to 200 frames per second. The results prove that an aperture synthesis imager
can generate imagery in the near-field regime, successfully coping with the curved wave-fronts. The original objective of
the project, to deliver a Technology Readiness Level (TRL) 4 laboratory demonstrator for aperture synthesis passive
millimetre wave (PMMW) imaging, has been achieved. The project was co-funded by the Technology Strategy Board
and the Royal Society of the United Kingdom.
This paper discusses a practical and affordable approach to the accurate calibration of electronic beam-forming passive
millimetre-wave imagers. With the aim of calibrating imagers with radiometric sensitivities ΔT < 1 K, we have
constructed a thermal radiation source at ambient temperature that fills the imager field-of-view at close range and can
support several controllable thermal radiation sources to provide absolute and differential radiation temperature
standards. Using a variety of temperature sensors, which have been extensively cross-calibrated against each other and a
commercially provided calibration standard that is accurate to < 0.1 K, we have achieved absolute and relative
calibration temperature uncertainties of less than 0.25 K.
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