Abstracts of MSc Theses: 1995


List of MSc Theses: 1995

  1. Norman Ballard Electrode Length Measurement in Electric Arc Furnaces.
  2. Mark Gebhardt Speckle Reduction in SAR Imagery.
  3. Marten H Kabutz RF Hardware Design of a Stepped Frequency Continuous Wave Ground Penetrating Radar.
  4. Thomas W Kusel On the Waveform Fidelity of Broadband Digital Storage Architectures

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Ballard N

Electrode Length Measurement in Electric Arc Furnaces


Accurate measurement of electrode length in electric arc furnaces will result in decreased maintenance time and improved plant productivity. This thesis describes the development of a microwave-based Soderberg electrode length-measurement system.

Various methods of electrode-length measurement were investigated and it was found that a microwave measurement system based on a conventional frequency modulated continuous wave (FMCW) radar presented most feasible technique. In this system, microwaves are propagated down a waveguide placed in the electrode. As the waveguide melts, they continue propagating in the resulting cavity until they are reflected by the discontinuity at the bottom of the electrode. The time taken for the return journey to the bottom of the electrode and back is measured, and the electrode length calculated.

FMCW radar measures return time by mixing a transmitted linear frequency sweep with the reflection to produce the difference frequency. The resulting beat frequency is then prorportional to the distance from the reflecting object. An investigation into the required linearity of the microwave source showed that the linearity is crucial to obtaining high signal-to-noise ratios in the beat frequency and therefore also to achieving good measurement accuracy. Although the designed radar had built-in linearisation capabilities, it was found to be more cost-effective to use a temperature-stabilised linear voltage controlled oscillator.

Ultimately, the accuracy with which electrode length can be determined depends on how accurately the peaks in the beat frequency spectrum can be determined. Various spectral estimation techniques were investigated, and it was found that the fast Fourier transform in conjunction with zero padding and weighted averages provided a good combination of fast, robust and accurate results.

A steel pipe with the same dimensions as the one to be placed in the electrode was used to test the measurement system. Electrode tip erosion was modelled by moving a reflective plunger up the waveguide. Length measurements were performed between 4.5m and 6.9m at one-centimetre intervals. The RMS error associated with the measurement was found to be 1.37cm, and the linearity of the system was excellent. Further experiments up to 9m long confirmed the accuracy and linearity. Finally, a 40cm long section of Soderberg electrode was placed at the end of the electrode to confirm that the waves would continue to propagate after the waveguide had melted. The length-measurement system performed as well as in previous experiments with the electrode section having no adverse effects on the measurements. Various other experiments to determine the effects of waveguide joints, temperature changes and electrode terminations also had favourable results.

A radar-based system was thus designed to measure the length of the Soderberg electrodes in electric arc furnaces. The system was then built and tested, and it was found that it provided excellent measurement accuracy under a range of conditions. The system must now be implemented in a real furnace in order to evaluate the effect of the environment further.

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Gebhardt M

Speckle Reduction in SAR Imagery


Synthetic Aperture Radar (SAR) is a popular tool for airborne and spaceborne remote sensing. Inherent to SAR imagery is a type of multiplicative noise known as speckle. There are a number of different approaches which may be taken in order to reduce the amount of speckle noise in SAR imagery. One of the approaches is termed post image formation processing and this is the main concern of this thesis.

Background theory relevant to the speckle reduction problem is presented. The physical processes which lead to the formation of speckle are investigated in order to understand the nature of speckle noise. Various statistical properties of speckle noise in different types of SAR images are presented. These include Probability Distribution Functions as well as means and standard deviations. Speckle is considered as a multiplicative noise and a general model is discussed. The last section of this chapter deals with the various approaches to speckle reduction.

Chapter three contains a review of the literature pertainign to speckle reduction. Multiple look methods are covered briefly and then the various classes of post image formation processing are reviewed. A number of non-adaptive, adaptive and segmentation-based techniques are reviewed. Other classes of techniques which are reviewed include Morphological filtering, Homomorphic processing and Transform domain methods. From this review, insights can be gained as to the advantages and disadvantages of various methods. A number of filtering algorithms which are either promising or are representative of a class of techniques, are chosen for implementation and analysis.

The chosen filters are implemented and a discussion of their algorithms is presented. The theory and operation of each of the filters is explained. The filters which are presented are the Mean, Median, Lorentzian, K Nearest Neighbour (KNN), Hirosawa, Maxium a Posteriori, Frost and Maximum Homogeneous Region filters. The filters all operate on the principle of a two dimensional window which is shifted across the image, one pixel at a time. The pixels covered by the window are used to determine the new value of the pixel at the centre of the window. For certain of the filters the local mean and standard deviation (the local statistics) are used to modify the filter response in the presence of edges or point targets. Detailed listings of the source code for all of the filters is given in the appendices.

The chosen filters are used to filter three test images, i.e. one and four look ESAR images and a simulated four look image. After filtering, both qualitative and quantitative assessments of filter performance are made. In order to measure the trade-off between geomertic and radiometric resolutions two quantities are calculated from the filtered and unfiltered images. These quantities are the Equivalent Number of Looks (an indication of radiometric resolution) and an edge measure, which represents the geometric resolution of the image. The local statistics filters (Frost and MAP) are found to produce the best geometric resolution, but only a slight reduction in the amount of speckle. Good edge preservation is also provided by the Median filter. The Mean filter is found to provide the best speckle reduction, but it causes degradation of the geometric resolution. Two filters which achieve a compromise between speckle reduction and edge preservation are the Hirosawa and KNN filters. The point is made that all filters are dependent on the selection of parameters. It is possible to change the performance of the filters by changing the number of iterations, the window size, or other parameters. The results presented in this section are therefore not absolute and merely serve to provide information on the typical performance of different filters.

The choice of filtering algorithm and its parameters is seen to be closely related to the purpose for which the final image will be used. Filters should be chosen according to whether large or small scale features are of interest. The work presented in this thesis provides valuable insights into the potential of post image formation speckle reduction methods. These algorithms can be used in addition to, or in lieu of, multiple look methods in order to reduce the speckle in SAR images.

Further research into post image formation techniques, as well as multiple frequency and multiple polarization methods, is suggested. This further comparison would provide valuable information about the potential for further reducing speckle in SAR imagery.

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Kabutz M H

RF Hardware Design of a Stepped Frequency Continuous Wave Ground Penetrating Radar


Research into stepped frequency continuous wave ground penetrating radar (SFCW GPR) at UCT has been carried out since 1990. A first generation system comprising of Hewlett Packard test equipment controlled by a PC was assembled. Cavity-backed log spiral antennas were designed and built by the University of Stellenbosch for the specific use of GPR. Measurements were the first generation system proved the concept of SFCW GPR and thus a dedicated second generation sytem was planned.

A SFCW GPR system was designed to replace the first generation system. Various designs for transmitter and receiver configuration were investigated and those found most suitable were used for the implementation.

The SFCW radar consists of a wideband CW transmitter and a coherent receiver. A 300-1000 MHz transmitter was constructed using varactor-tuned oscillators as frequency sources. A double-sideband, low-IF receiver was constructed for the 300-1000 MHz signal, to mix it to an IF of 10.7 MHz and I-Q demodulate it. The transmitter was found to operate according to specifications. The receiver was found to operate satisfactorily, but the dynamic range was less than expected.

A limiting problem encountered in the first generation GPR was a large direct coupling signal from the transmitter into the receiver. This large signal reduced the effective receiver dynamic range. A method of cancelling this large direct coupling signal was implemented, usnig a bi-phase modulator to generate the cancelling signal in antiphase to the coupling signal. A 20 dB reduction in coupling was shown.

The system was used to measure cable lengths to within the inherent accuracy of the system. A metal plate target was detected by the system feeding two antenna and a concrete floor was detected under 1m of asnd. It was thus shown that the SFCW system could be used as a second generation GPR.

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Kusel T W

On the Waveform Fidelity of Broadband Digital Storage Architectures


This thesis is aimed at comparing different techniques to store and recall broadband signals digitally. Different signal properties defining this fidelity are thus investigated in detail, and the different storage techniques are then compared on the basis of these signal properties.

Recent advances in digital technology resulted in the advent of high-density, very large-scale and very high-speed integrated circuit devices. For the first time it has become viable to store and/or process digitally sufficient lengths of very broadband signals. The digital capturing, storage and processing of a signal has some obvious advantages over traditional analogue methods. Given such a digital receiver technology, the system designer has at his disposal complete, real-time, receiver and processor reconfigurability.

In this thesis, a number of architectures for the digital capture, storage and reconstruction of broadband signals are investigated. A comparison of these architectures is made, based on the fidelity of the reconstructed signal. Techniques for improving the quality of the reconstructed signal are also investigated. The results are aimed at aiding the system designer in choosing a receiver architecture for a specific system requirement.

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