Abstracts of MSc Theses: 1994


List of MSc Theses: 1994

  1. Leon Alexander The Planning of a South African Airborne Synthetic Aperture Radar Measuring Campaign.
  2. C E Alston Standardising Modules for Tactical Command and Control Man-Machine Interfaces.
  3. Rebecca Eatock A Spatially-variable Fertiliser Applicator System.
  4. Richard M Fullalove Failsafe Display System.

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Alexander L

The Planning of a South African Airborne Synthetic Aperture Radar Measuring Campaign


This thesis sets out the results of work done in preparation for a South African Airborne Synthetic Aperture Radar (SAR) measuring campaign envisaged for 1994/5. At present both airborne and spaceborne SARs have found a niche in remote sensing with applications in subsurface mapping, surface moisture mapping, vegetation mapping, rock type discrimination and Digital Elevation Modelling. Since these applications have considerable scientific and economic benefits, the Radar Remote Sensing Group at the University of Cape Town committed themselves to an airborne SAR campaign. The prime objective of the campaign is to provide the South African users with airborne SAR data and enable the RRRSG to evaluate the usefulness of SAR as a remote sensing tool in South Africa.

In this thesis the details of how SAR images are formed are not important. Instead, the emphasis is on how the electromagnetic wave interacts with the surface because this determines how and what a SAR can measure. To this end, theoretical models, namely, physical optics, small perturbation and geometric optics models were investigated since these models enable the backscatter from a randomly rough surface to be determined. The Bragg resonance phenomena, which accounts for the relatively high backscatter from periodic surfaces was also investigated sinec this model is important in many sea surface imaging applications. Because the theoretical models are difficult to use, empirical and self-empirical models are popular. In subsurface applications, the penetration depth is determined by the microwave attenuation in the sand medium. The factors affecting the microwave attenuation in sand are the frequency of the radar signal, and the real and imaginary part of the permittivity. The lower the frequency, and the lower the loss tangent (defined as the ratio of imaginary part of the permittivity to the real part of the permittivity), the lower the microwave attenuation in the sand medium.

Both attenuation and scattering models are functions of the complex dielectric constant of the medium. This necessitated the need for an investigation into the factors influencing the complex permittivity of soil. These factors, listed in order of greatest influence are soil moisture, soil water salinity, soil type, soil density and soil temperature. The relative permittivity of dry soil ranges from 3 to 4, independent of frequency. As water is added, the relative dielectric permittivity increases rapidly as pure water has a value of approximately 80 for frequencies below 1 GHz. It is therefore obvious that the dielectric constant of a soil is greatly influenced by the volumetric water content. The water content of soil generally consists of *free* and *bound* water, where the amount of *bound* water is determined by the surface area of the soil particles. The smaller the individual soil particles the greater the total surface area. Thus, the amount of *bound* water in the soil is dependent on the soil textural composition. The real part of the complex dielectric constant of soil water is unaffected by salt content, whereas the imaginary part of the complex dielectric constant is affected, especially at low frequencies where ionic conductivity dominates. The remaining factors have negligible influence on the permittivity. Theoretical and empirical models for soil permittivity are available but the latter is more popular because the theoretical models are complicated and require too many input variables. The empirical models are not without problems. The real part of the permittivity can be successfully modelled from about 1 MHz to 18 GHz, whereas the imaginary part of the permittivity can only be successfully modelled from about 1 GHz to 18 GHz because the effects of ionic conductivity, which cannot easily be nicluded in the model, dominate at frequencies lower than 1 GHz.

Polarimetric imagery is a relatively new development in radar imagery and is at present an active research area. Polarimetric radars have the capability of identifying a scatterer from its polarization signature, which is obtained by a technique known as polarization synthesis. For example, a dihedral corner reflector signature can be used to identify buildings in an image, or a large smooth dielectric surface signature indicates the locations of clear-cut areas in an image of forest vegetation.

Next, the intended applications, namely, subsurface geological maping, surface rock type discrimination, near-surface soil moisture content mapping, vegetation mapping, and Digital Elevation Modelling are discussed. Since the prime objective of this thesis is the planning of an airborne SAR campaign, the most important part of each application discussed is the optimum parameters for that application.

There are numerous geological applications of SAR which generally fall into one of two groups, namely surface and subsurface applications. The latter, which is a unique feature of radar imaging will be discussed first. Low loss material, such as dry sand, which covers most of the surface of arid regions can easily be penetrated with low frequency radar. Thus, the images formed reveal the subsurface geology. The opimum parameters for subsurface imaging are chosen to minimize the microwave attenuation or maximize the microwave penetration depth in the medium. For a given frequency ad soil moisture content, sand consistently has the lowest attenuation. However, the greatest influence on microwave attenuation is the moisture content of the soil. Greater microwave penetration is observed for soils with lower moisture content. In addition, lower radar frequencies yield greater microwave penetration of the obscuring medium. The optimum incidence angle for subsurface imaging is a compromise between minimizing the path length in the sand medium and avoid incidence angles at which specular reflections at the air-sand interface dominates the radar backscatter. Thus the optimum incidence angle for subsurface applications is between 10 and 20.

SAR has been used very successfully in rock type discrimination since it is uniquely sensitive to the roughness or texture of the surface being imaged. The growing trend to use multifrequency and multipolarization radars has enabled considerably more information about the surface to be extracted from the radar image. The most powerful means of distinguishing between surfaces with different degrees of roughness is to exploit the wavelength dependence of the Rayleigh criterion. In this way roughnesses can be used to aid in the mapping of different lithologies and superficial deposits in terrain that is bare of vegetation. Also, the use of multipolarization has enabled rock types to be mapped with greater accuracy since each rock type can be matched to a particular polarization signature which is obtainable via a technique known as polarization synthesis.

SAR has the ability to measure the near-surface soil moisture content remotely. Radar backscatter is influenced by physical parameters (surface roughness and surface permittivity) and radar parameters (incidence angle, frequency and polarization). The soil moisture is inferred from the surface permittivity, which is in turn deduced from the measured radar backscatter. To determine soil moisture content from radar backscatter successfully requires the effects of vegetation, surface roughness and local incidence angle on the observed backscatter to be minimized or removed completely. This is accomplished by choosing an optimum frequency, incidence angle and polarization for imagery from which the soil moisture content will be extracted. Several researches have shown that the optimum parameters for soil moisture measurement is an incidence angle of 10, a frequency of 4 GHz and HH or VV polarization.

The feasibility of vegetation type discrimination, particularly for crop type distribution mapping was also investigated. To distinguish between two crops in a radar image requires that the backscatter from these crops differ in intensity. This backscatter intensity is influenced by vegetation parameters such as the canopy density and crop height. Unfortunately, the measured backscatter is also greatly influenced by the soil moisture and roughness. Hence, successful vegetation mapping depends on the degree to which the effect of the soil moisture acn be minimized. Researchers have shown that the soil moisture influence is minimized by using incidence angles of approximately 40 and frequencies in the vicinity of 8 GHz.

Two sea surface imaging applications were investigated namely, oil pollution and fish shoal monitoring. Detecting, monitoring and mapping of oil spills on the sea surface are applications based on the Bragg scattering phenomenon which accounts for the observed sensitivity of imaging radars to the amplitude of ocean capillary waves. The presence of oil on the ocean surface significantly reduces the amplitude of capillary waves which in turn significantly reduces the radar backscatter from that surface. Thus, oil on the sea surface corresponds to dark patches. Fish shoal monitoring is also based on the Bragg scattering phenomenon. Researchers have shown that certain fish such as bluefin tuna break the water surface by repeatedly jumping out of the water. This behaviour, which is associated with feeding, produces a rough surface which results in greater backscatter and thus appears as a bright feature in the radar image. Similarly, net floats cause a roughening of the surface which enables the location of the nets to be mapped. The optimum parameters for sea surface imaging applications are difficult to determine because of insufficient data on the subject. Nevertheless, an estimate of the optimum frequency and nicidence angles is gleaned from the limited literature on the topic. Since Bragg scattering is the theoretiacl basis for both air pollution and shoal monitoring, these applications are probably optimum at high frequencies (X-band), large incidence angles (40) and VV polarization.

Digital Elevation Models (DEM) can be generated by extracting topographic information from SAR data using Shape-from-shading, Stereoscopic imaging and Interferometric SAR techniques. In Shape-from-shading, the radar backscatter is assumed to be proportional to the local incidence angle which is a function of the terrain slope and incidence angle. Since the incidence angle is fixed by the sensor hardware, the slope of the terrain can be inferred from the backscatter. The DEM is then constructed from the terrain slope information. The accuracy of the DEM is extremely sensitive to the accuracy with which the backscatter nitensity versus local incidence angle can be modelled. The accuracy of the model is in turn dependent on the roughness of the terrain because rougher surfaces yield better models. Since roughness is frequency dependent, higher frequencies yield better models. An exact frequency can only be determined if the surface roughness is known but C-band frequencies will probably be high enough. The optimum incidence angle is chosen to minimize the likelihood of layover and shadowing. Without a priori knowledge of the topography, the optimum incidence angle is 45.

Stereoscopy is another technique whereby topographic information can be extracted from two overlapping images. The method is based on the apparent movement or parallax of features in the stereo image pair. The elevation of a feature is proportional to the observed parallax of the feature. The method requires two images of similar image quality, tone and texture, but different imaging geometry to present parallaxes for the height perception. The optimum stereo geometry is a compromise between two equally important criteria. Firstly, the two images forming the stereo pair must be very similar in image quality, terrain illumination, tone and texture so that the stereo pair correlates well. Secondly, the two images forming the stereo pair must be sufficienctly different in geometry to present parallaxes for height perception. Since radar actively illuminates the terrain, significant differences in viewing geometry also imply illuminations differences. In addition, the optimum incidence angles and intersection angles are also dependent on the terrain relief. For relatively flat terrain, the incidence angles should range from about 20 to 60 with an intersection angle greater than about 30. For relatively high relief terrain, the incidence angles should range from 40 to 70 with intersection angles ranging from 15 to 30.

Due to the coherent nature of SAR, interferometric principles can be used to extract height information from raw SAR data. If two receiving antenna are placed in the range-height plane, then the phase difference of the echoed power can be determined. This means that differences in height, modulo the radar wavelength, can be detected. In this way three dimensional terrain maps can be constructed. The accuracy of the DEM is maximized through minimization of phase errors, wavelength, and slant range and maximization of the baseline distance. However, the level of phase noise in the system increases with increasing baseline distance up to a critical baseline distance where the signals are no longer correlated. It can be shown that the optimum baseline distance ranges from 0.2 to 0.8 of the critical baseline distance. These baseline distances are limited by aircratf dimensions when both antennas are mounted on the same aircraft which implies that the frequency can be chosen so that the baseline distance is optimum.

Ideally, one imaging radar could be configured to have frequency and incidence angle ranges that would contain the optimum parameters for all the envisaged applications. Unfortunately, a combination of financial and technical limitations made this ideal imaging radar unrealistic. Hence, some compromises had to be made which automatically implied implementing some applications with less than optimum imaging parameters.

The details of the proposed South African airborne SAR campaign are contained in the appendix.

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Alston C E

Standardising Modules for Tactical Command and Control Man-Machine Interfaces


The operating environment of a Tactical Command and Control system is a highly tense one in which the operator needs to perform certain complex tasks with minimum confusion, and be able to obtain an instant response from the system. Since many of the systems designed for these types of environment are similar in nature with regard to the user-interface, a need has arisen to try and standardise certain elements of the systems.

This report looks specifically at standardising certain graphical display element and operator input device interfaces. It investigates the problem from a systems design level, identifying the elements required and their associated functions, discussing the results of work already undertaken in the field, and making recommendations on the use of the elements.

The main objective to standardising the Man-Machine Interface (MMI) design element is to make the code easily transferable between different hardware platforms. To transfer the code, one would ideally like to chaneg only the interface code to the new platform, in particular, the interface to a different set of operator input devices and a different type of graphics card.

Various topics related to the standardisation process are discussed, including a description of MMI design, some definitions of tactical command and control environment subjects, and a look at code reusability, rapid prototyping of systems, and object-oriented design.

The first major requirement of the standardisation process is the design of the graphical MMI display elements that are identified as being common to the various systems. This design includes the definition of co-ordinate systems, and an investigation of the use of colour to encode different meanings for groups or types of display elements, and the colours used for the different system modes of operation. The graphical MMI elements identified as being common to the various systems under investigation include: soft-buttons, totes, message bars, Plan Position Indicators (PPIs) and information boxes.

The second major requirement of the standardisation process is the identification of the operator input devices and the design of their interfaces. The input devices identified as being common to the various systems under investigation include: a keyboard, softkeys, a joystick, and a touchpanel. Related to the design of the input device interfaces, is the design of input device event queues that handle all the inputs from the various devices, and convert these inputs into system events that can be handled by the system state-machine. Configuration files are used to enable th ecode to run on various hardware platforms that have either different types of input devices, or a different method of interfacing to the devices. Various hardware configurations and interfaces have been used to date. One of these configurations is a PC prototyping environment with a mouse and an IBM keyboard as input devices, using a PC serial port and the keyboard input. Another is a Multibus II (MBII) environment with an alpha-numeric STORM keyboard, STORM keypad softkeys, a joystick and a touchpanel as input devices, using serial input ports and MBII message passing.

The system operation as a whole is based on a scheduler, input device interrupts, and state-machines. The two main system state-machines are the logic-and-control state-machine, which interfaces with the system sensors, and the user-interface state-machine, which is discussed in this report. The user-interface state-machine uses displayed menus as the state-machine states, and decodes operator inputs into events for the state-machine, each of which has its own action function.

To illustrate the success of the standardisation design process, examples are given of systems already designed, and of work in progress. Photographs are used to illustrate the display screens and operator input devices used, and togive an indication of screen layouts and colour codings used.

From system design work already undertaken, the transportability of the software between different hardware platforms and system configurations has been successfully performed a number of times. This has made possible the use of low-cost PC-platform prototyping to speed up the design process, and has allowed frequent meetings with the users to obtain their comments during this design process. Recommendations arising from this work include the investigation of using other programming languages and packages that may speed up the design process even further. It is also recommended that the system designed be observed under actual operating conditions.

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Eatock R

A Spatially-variable Fertiliser Applicator System


The aim of this study was to investigate the design and implementation of a Spatially Variable Fertilizer Application System (SVFAS) as an element of Spacially Variable Agricultural Production Systems (SVAPS). This entails the research and experimental testing of suitable systems, techniques and strategies with the aim of its possible implementation as a commercial venture.

There are two main motivations for developing agricultural systems that operate spatially witin fields. The first is due to the expected increase in crop yield due to the more efficient use of agricultural chemicals. The major advantage however, is the environmental benefits due to the reduction of residual nitrogen and other harmful chemicals leaching into the ground awter.

There is very little work that has been done locally. These systems have been justified and isolated topics implemented successfully overseas. A fair amount of success has been achieved within SVAPS in Germany, Britain, and the USA. The purpose of developing the system locally was to facilitate maintenance, equipment and personnel requirements and to develop a system that is locally understood and maintainable.

This document depicts the research into the design and the implementation of a SVFAS as an element of SVPAS. The comprehensive testing of a suitable positioning technique for a SVFAS and the experimental implementation of a typical phase of the SVFAS will also be described.

The document will begin with a more comprehensive description of a SVFAS and is followed by the literature review. The literature review is about the greater field of SVPAS and all the phases of a SVFAS. Tnferences gleaned from this literature review will follow, outlining some elements of the design and strategy of a SVFAS. Both the literature review and the related references are discussed under the topics of: basic agronomic considerations, the theory and feasibility of positioning techniques, progress already made in relation to SVPAS systems and the constraints of the project in the local context.

The next chapter begins the description of the experimental work. The purpose and procedures required for the testing of a suitable positioning technique and for the implementation of a typical phase of the SVFAS is described. The findings and analyses of the field test data is discussed and conclusions are drawn. Finally, recommendations for fuure project developments are made.

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Fullalove R M

Failsafe Display System


A requirement exists to reduce signalling costs on branch lines throughout the South African rail network, by replacing expensive track side signalling with a Radio Token System. The Radio Token System uses a non-failsafe radio data link and non-failsafe printer in the cab. Spoornet intend to replace the printer with a failsafe display in order to eliminate lengthy verbal message protocols and increase safety. This thesis investigates the various concepts of failsafe display, and the means to achieve them. It also investigates the wide range of display technologies available and their suitability for use in a failsafe display. The LED, together with monitoring circuitry, emerges as the most suitable for the application.

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