This thesis describes the investigation carried out on the propagation of radio pulses of frequency 4.73 Mc/s between Grahamstown and Durban. The thesis is divided into two sections - A and B. Section A consists of two chapters. The introductory chapter gives a brief account of how the existence of the ionosphere came to be known. Then follows a description of the different layers of ionization and a review of the theories that have been propounded on the formation of these layers. Chapter 2 deals with the apparatus which includes the transmitter in Grahamstown and the receiving apparatus in Durban. The receiving apparatus comprises: i) a superheterodyne receiver whose gain was high (between 130 and 140 dbs); (ii) a time delay calibrator which could measure time differences of 100 μsec fairly accurately; (iii) a 310 A Tektronix oscilloscope; (iv) a continuously running 35 mm recording camera. Section B is made up of three chapters and is concerned with the actual analysis of the data recorded. The theory of propagation of radio waves in the ionosphere is discussed in Chapter 3. The effects of the magnetic field are neglected since it is found that the error introduced would not make the results unacceptable. Chapter 4 contains the analysis of the data recorded. One summer day and one winter day are discussed in detail in order to obtain the pattern of the diurnal variations for both summer and winter. Some interesting phenomena are also dealt with. An attempt to do ray tracing was successful and the paths followed by a Pedersen and a lower ray from Grahamstown to Durban have been drawn. New topics for further research are discussed in Chapter 5. There are two appendices. Appendix I gives the time delays of all the pulses recorded and their possible identifications. An overall picture of the propagation via the various layers throughout the day (both for summer and for winter is presented in Appendix II.

The forced vibration of a partially delaminated structure such as an aircraft wing can result in catastrophic crack growth. In order to look at the underlying mechanism of the dynamics and failure of the material, a simplified model of a cantilever beam with a single delamination at its free end is considered. We investigate a number of aspects of this system, using mathematical models to gain insight into its behaviour.

In a telemetry system, it has been frequently proved that multipath channels and transmission noise are the most critical sources of distortion. While equalization allows a strong limitation of the multipath effects, the noise impact can be efficiently reduced if forward error correction is used. This paper proves that the combination of blind equalization and a powerful FEC like LDPC strongly improves bit error rates for the SOQPSK modulation. We also prove that a LDPC code is able to fully correct the residual errors that may persist at the equalizer output. In other terms, the combination of equalization and LDPC code enables quasi-error free transmissions in various channel scenarios that represent the various phases of a telemetry mission.

Worms have caused significant destruction over the last few years. Network security elements such as firewalls, IDS, etc have been ineffective against worms. Some worms are so fast that a manual intervention is not possible. This brings in the need for a stronger security architecture which can automatically react to stop worm propagation. The method has to be signature independent so that it can stop new worms. In this thesis, an automated defense system (ADS) is developed to automate defense against worms and contain the worm to a level where manual intervention is possible. This is accomplished with a two level architecture with feedback at each level. The inner loop is based on control system theory and uses the properties of PID (proportional, integral and differential controller). The outer loop works at the network level and stops the worm to reach its spread saturation point. In our lab setup, we verified that with only inner loop active the worm was delayed, and with both loops active we were able to restrict the propagation to 10% of the targeted hosts. One concern for deployment of a worm containment mechanism was degradation of throughput for legitimate traffic. We found that with proper intelligent algorithm we can minimize the degradation to an acceptable level.

Phytoremediation is a tool that uses plants that can absorb and accumulate metals in harvestable portions of the plant to cleanse contaminated soils. Most metals are more soluble in soils with an acidic pH. Nitrogen fertilizers acidify pH by different reactions in the soil. Goals of this research were: (1) develop a zinc-contaminated soil; (2) determine effects of nitrogen fertilizers on soil-zinc availability; (3) determine Zn-phytoextraction potential of Brassica juncea Czern. and Festuca arundinacea Schreb.; and (4) determine concentrations of nitrogen fertilizers needed to maximize Zn solubility in soils. After a 14-day incubation period, very little Zn in the soil was water-extractable, which suggested that Zn was reacting with the soil; therefore, an incubation time of 14 days was utilized. Morgan's solution, extracted higher concentrations of Zn than water. If soils were sequentially extracted with water, Morgan's solution, and Mechlich-3 solution, water extracted the least amount of Zn, Morgan's solution extracted higher concentrations than water or Mehlich 3, but Mehlich 3 extracted higher concentrations than water. Lowest pH values occurred with additions of urea (pH 5.18), sludge (pH 4.89), or calcium nitrate (5.26) than with compost (pH 5.33), manure (pH 5.50), or no fertilizer (pH 5.40) or if N was supplied at 400 mg/kg (pH 4.91). Brassica did not germinate well or survive in soil-Zn concentrations greater than 125 mg/kg. Soil-Zn concentrations utilized with brassica were 0 to 100 mg/kg. Highest accumulation of Zn was 0.29% of the dry mass, which occurred at 100 mg Zn/kg or in soils with urea added. Water-extractable Zn at this level averaged 1.1 mg/kg and Morgan's extractable Zn averaged 18 mg/kg. Fescue germinated well in soil-Zn concentrations ranging from 0 to 2000 mg/kg. The soil-Zn concentrations utilized with fescue were 0 to 1000 mg/kg. Highest accumulation of Zn by fescue was 0.33%, which occurred at 1000 mg Zn/kg or in soils with urea or sludge added. Water-extractable concentrations of Zn at this level averaged 11 mg/kg and Morgan's extractable Zn concentrations averaged 290 mg/kg. This research showed that fescue has phytoremediation potential that is as good or better than that of brassica.

Field, greenhouse, and laboratory studies were conducted with the following objectives: (1) develop an efficient extraction and analytical method for determining concentration of major flavonoids in apple (Malus domestica Borkh.) leaves, (2) determine if differences exist in levels of these components in leaves of ten apple cultivars and if these differences explain variation in cultivar susceptibility to spider mites ( Acari: Tetranychidae), (3) understand the effects of varying light and nutritional regimes on apple leaf flavonoid levels, (4) assess if the flavonoid phloridzin, when applied to bush bean plants, affects Two Spotted Spider Mite population density, and (5) Determine if summer pruning of mature, field-grown apple trees influences levels of foliar phloridzin. An efficient extraction and HPLC analytical method is presented using commonly available and least toxic solvents and an isocratic HPLC run of short duration allowing calculation of unknown concentrations with a high degree of accuracy. Different cultivars showed significant differences in mite susceptibility and in phloridzin concentration in leaves. However, phloridzin differences do not appear to explain variation in cultivar susceptibility to spider mites. Greenhouse trials indicated that phloridzin concentration was significantly lower in leaves of potted trees of the four cultivars tested under 70% shade cloth than under full sun. All cultivars responded similarly to sun or shade in regard to percent dry weight phloridzin. Nutrition treatments appeared to have an effect on phloridzin levels only in 1995. Horticultural bush bean plants (Phaeseolus vulgaris) were either sprayed or injected through a cotton wick with 0.01 M phloridzin or distilled water, and seeded with 5 gravid female Tetranychus urticae mites. Phloridzin, either when applied topically to bean plants, or when introduced into the transpirational stream via a cotton wick, resulted in smaller numbers of T. urticae compared to plants which were not treated with phloridzin. Summer pruning of mature, field grown “McIntosh” apple had no effect on phloridzin concentration, likely indicating that summer pruning is not a stimulus of induced changes in flavonoid content.

The mechanism of cave mining propagation still requires a better understanding to be attained outside the industry-accepted Duplancic conceptual model. While this model suggests a continuous damage profile to be followed when an orebody is undercut in cave mining operations, the research of Cumming-Potvin (2018) describes an extended conceptual model to cave propagation which highlights a different failure mechanism whereby discontinuous damage occurs through the advancing events of parallel fracturing termed `fracture banding'. In this dissertation, a physical modelling approach was adopted in an attempt to simulate the process of cave mining propagation at various stress states in order to observe the resulting failure mechanism. Four centrifuge tests were conducted utilising manufactured artificial rock material sample panels that were subjected to various ratios of horizontal to vertical stress. Before the applicability of this material to represent actual rock found in cave mines could be deemed adequate, a full characterisation of the properties of this artificial rock material was performed. The material testing included uniaxial compression strength tests, triaxial tests and Brazilian disc tests. These values were compared to typical corresponding parameters of various rock types in order to establish a suitable range of scale factors. Particle Image Velocimetry (PIV) was integrated into determining the critical extensional strain for the material which was used to establish a strain-based failure criteria for the artificial rock material using the model developed by Stacey (1981). Once the artificial rock material was manufactured to replicate the characteristics rock by a standard deemed acceptable; a scale factor range of 12 - 9 291 was achieved using an absolute critical extensional strain value of 0.014 %. The following set of conditions were achieved when conducting physical modelling: negligible horizontal confinement using sand with zero overburden pressure, minimal horizontal confinement with zero overburden pressure, maximum horizontal confinement with minimal overburden pressure, and lastly maximum horizontal confinement with maximum overburden pressure following the same proportion in lateral earth pressure (K ratio) as the previous test. Even though the results, with regard to the geometry of cave formation, in each of the tests were different; all four tests displayed an indication of `fracture banding'. In terms of the different geometries that had formed during cave progression, it was found that models with lower K ratios showed a higher development of the caving mechanism (at the same vertical stress), whilst models with higher K ratios suppresses this. Moreover, models conditioned with larger vertical stresses saw cave formation forming in a longer time period, but ultimately exhibited caves with a larger perimeter and area once the full undercut width had been reached. Total collapse occurred when the cave advanced vertically reaching the top surface of the sample through the formation of a `chimney'. On further investigation of mapping strains in models throughout common time-steps, the results showed that regions of high minor principal strains from PIV analysis of cave propagation correlated well with identified fractures. In all test cases, cracks propagated at the point of the material matching or exceeding the absolute critical extensional strain value in either new cracks opening via cave-back progression or reaching pre-existing fractures. In most cases, regions of these tensile strain values were observed to have been bisected by visual cracks in models which suggest that the perceived behaviour of parallel fracturing is tensile in nature. / Dissertation (MEng (Structural Engineering))--University of Pretoria, 2020. / Prof J. Wesseloo, Australian Centre for Geomechanics / Prof E.P. Kearsley, University of Pretoria / Civil Engineering / MEng (Structural Engineering) / Unrestricted

This work deals with the fundamental principles of wireless communications in mobile robotics. It introduces overview of the use of different frequency bands for wireless communications needs and control of mobile devices in Czech Republic. It also contains a brief overview of all applicable technologies and available modules that can be used for wireless communication. The theoretical part deals with some of the protocols that were used in the practical part of this work and the manner of wave propagation in buildings. The practical part consisted of a design of our own application to monitor and control the module Mikrotik Routerboard. With this application we have tested and compared two wireless cards of this module.

Although Essential Tremor (ET) is the most common type of tremor, many patients are left without satisfactory treatment options. One potential alternative treatment to medication or surgery is a wearable tremor-suppressing device. However, optimizing the effectiveness of such a device requires knowledge of which muscles/joints are most responsible for tremor. To answer this question, current efforts simulate tremor propagation using a model of the neuromusculoskeletal dynamics of the upper limb. To guide efforts to identify realistic model parameters and use the model to determine the mechanical origin of tremor, we performed preliminary parameter estimation work and a thorough sensitivity analysis of this tremor propagation model. The tremor propagation model included muscle activation inputs to the 15 major superficial muscles and joint displacement outputs in the 7 main degrees of freedom (DOF) from the shoulder to the wrist, resulting in 105 input-output relationships. We calculated the mean normalized sensitivities of all outputs to all 107 model parameters over the tremor band (4 to 8 Hz), resulting in approximately 12,000 sensitivities. We found that sensitivities were relatively constant in the tremor band, except for shoulder adduction-abduction, which exhibited a large peak in sensitivity between 4 and 5 Hz. Averaged across the tremor band, the system was most sensitive to select elements of inertia, muscle force, muscle moment arm, damping, muscle time constants, and stiffness (in that order). The 19 highest all-input-excitation sensitivities were between 1.2 and 4.57, meaning a 100% change in parameter value produces 120-457% change in tremor. Conversely, the model includes many parameters to which the outputs are relatively insensitive. For example, the sensitivities to almost one third of the 107 parameters are below 0.1, meaning a 100% change in parameter value produces only a 10% change in tremor. To gain additional insight, we compared the sensitivities of the full model to those of a simpler model including only two inputs and two outputs. Analyzing the two-input two-output model revealed patterns in sensitivity which persist in the full model. The sensitivities of the full model were further compared to past studies that performed rudimentary sensitivity analyses and were found to match while adding significantly more parameter-specific sensitivity information. Future work will extend this sensitivity analysis to tremor at the hand, where it matters most.

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