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Variability analysis of a sample of potential southern calibration sources

- Hungwe, Faith

A feasibility study into total electron content prediction using neural networks

- Habarulema, John Bosco

  • Authors: Habarulema, John Bosco
  • Date: 2008
  • Subjects: Electrons , Neural networks (Computer science) , Global Positioning System , Ionosphere , Ionospheric electron density
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5466 , http://hdl.handle.net/10962/d1005251 , Electrons , Neural networks (Computer science) , Global Positioning System , Ionosphere , Ionospheric electron density
  • Description: Global Positioning System (GPS) networks provide an opportunity to study the dynamics and continuous changes in the ionosphere by supplementing ionospheric measurements which are usually obtained by various techniques such as ionosondes, incoherent scatter radars and satellites. Total electron content (TEC) is one of the physical quantities that can be derived from GPS data, and provides an indication of ionospheric variability. This thesis presents a feasibility study for the development of a Neural Network (NN) based model for the prediction of South African GPS derived TEC. The South African GPS receiver network is operated and maintained by the Chief Directorate Surveys and Mapping (CDSM) in Cape Town, South Africa. Three South African locations were identified and used in the development of an input space and NN architecture for the model. The input space includes the day number (seasonal variation), hour (diurnal variation), sunspot number (measure of the solar activity), and magnetic index(measure of the magnetic activity). An attempt to study the effects of solar wind on TEC variability was carried out using the Advanced Composition Explorer (ACE) data and it is recommended that more study be done using low altitude satellite data. An analysis was done by comparing predicted NN TEC with TEC values from the IRI2001 version of the International Reference Ionosphere (IRI), validating GPS TEC with ionosonde TEC (ITEC) and assessing the performance of the NN model during equinoxes and solstices. Results show that NNs predict GPS TEC more accurately than the IRI at South African GPS locations, but that more good quality GPS data is required before a truly representative empirical GPS TEC model can be released.
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  • Date Issued: 2008

A VLBI study of OH masers in a proto-planetary nebula OH 0.9+1.3

- McAlpine, Kim

  • Authors: McAlpine, Kim
  • Date: 2008
  • Subjects: Very long baseline interferometry , Proto-planetary nebulae , Masers
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5499 , http://hdl.handle.net/10962/d1005285 , Very long baseline interferometry , Proto-planetary nebulae , Masers
  • Description: This thesis reports the calibration, imaging and analysis of one epoch of VLBA observations of the 1612 MHz OH maser emission from the protoplanetary nebula OH 0.9+1.3. These are the first polarisation VLBI observations of this source and the spatial morphology of the OH emission is resolved on this scale. Proto-planetary nebulae represent the transition phase in the evolution of stars between the asymptotic giant branch (AGB) phase and their emergence as planetary nebulae. A long-standing astronomical question is how the predominantly spherical circumstellar envelopes of AGB stars evolve into the bipolar and axisymmetric structures that are commonly observed in planetary nebula. Proto-planetary nebulae offer a unique opportunity to study this transformation process. The high-resolution VLBI maps produced in this thesis were used to investigate the morphology and kinematics of OH 0.9+1.3 with a view to gaining insight into the development of asymmetries in the circumstellar material. The OH maser emission of OH 0.9+1.3 has a double-peaked profile with one peak blue-shifted and the other red-shifted with respect to the stellar velocity. The total intensity maser maps demonstrate a considerable degree of asymmetry with the blue- and red-shifted emission located in spatially distinct regions of the envelope. The blue-shifted emission is distributed preferentially along an axis at a projected position angle of » 135± ( North through East). The morphology of this source is not consistent with the standard symmetric thin-shell model and an attempt to fit the traditional OH/IR kinematic model of a simple expanding shell to the maser components was found to be unsatisfactory. No definitive evidence of a bipolar outflow was observed either. The high degree of asymmetry observed in the source is consistent with its status as a proto-planetary nebula. The source was imaged in all four Stokes parameters and the fractional linear and circular polarisations of the maser components were derived from the Stokes parameter maps. In all except one of the components the total fractional polarisation was found to be low ( < 15%). The mean fractional linear and circular polarisation were calculated to be 5.54% and 7.11% respectively. The absence of an identifiable Zeeman pair in the Stokes V map prohibited the estimation of the magnetic field in the circumstellar envelope of this source.
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  • Date Issued: 2008

Designing and implementing a new pulsar timer for the Hartebeesthoek Radio Astronomy Observatory

- Youthed, Andrew David

  • Authors: Youthed, Andrew David
  • Date: 2008
  • Subjects: Astronomical observatories , Radio astronomy , Pulsars , Astronomical instruments , Reduced instruction set computers , Random access memory
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5458 , http://hdl.handle.net/10962/d1005243 , Astronomical observatories , Radio astronomy , Pulsars , Astronomical instruments , Reduced instruction set computers , Random access memory
  • Description: This thesis outlines the design and implementation of a single channel, dual polarization pulsar timing instrument for the Hartebeesthoek Radio Astronomy Observatory (HartRAO). The new timer is designed to be an improved, temporary replacement for the existing device which has been in operation for over 20 years. The existing device is no longer reliable and is di±cult to maintain. The new pulsar timer is designed to provide improved functional- ity, higher sampling speed, greater pulse resolution, more °exibility and easier maintenance over the existing device. The new device is also designed to keeping changes to the observation system to a minimum until a full de-dispersion timer can be implemented at theobservatory. The design makes use of an 8-bit Reduced Instruction Set Computer (RISC) micro-processor with external Random Access Memory (RAM). The instrument includes an IEEE-488 subsystem for interfacing the pulsar timer to the observation computer system. The microcontroller software is written in assembler code to ensure optimal loop execution speed and deterministic code execution for the system. The design path is discussed and problems encountered during the design process are highlighted. Final testing of the new instrument indicates an improvement in the sam- pling rate of 13.6 times and a significant reduction in 60Hz interference over the existing instrument.
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  • Date Issued: 2008

Reconstructing ionospheric TEC over South Africa using signals from a regional GPS network

- Opperman, B D L

  • Authors: Opperman, B D L
  • Date: 2008
  • Subjects: Global Positioning System Global Positioning System -- Data processing Electrons -- South Africa Ionosphere -- South Africa Ionospheric radio wave propagation -- South Africa
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5487 , http://hdl.handle.net/10962/d1005273
  • Description: Radio signals transmitted by GPS satellites orbiting the Earth are modulated as they propagate through the electrically charged plasmasphere and ionosphere in the near-Earth space environment. Through a linear combination of GPS range and phase measurements observed on two carrier frequencies by terrestrial-based GPS receivers, the ionospheric total electron content (TEC) along oblique GPS signal paths may be quantified. Simultaneous observations of signals transmitted by multiple GPS satellites and observed from a network of South African dual frequency GPS receivers, constitute a spatially dense ionospheric measurement source over the region. A new methodology, based on an adjusted spherical harmonic (ASHA) expansion, was developed to estimate diurnal vertical TEC over the region using GPS observations over the region. The performance of the ASHA methodology to estimate diurnal TEC and satellite and receiver differential clock biases (DCBs) for a single GPS receiver was first tested with simulation data and subsequently applied to observed GPS data. The resulting diurnal TEC profiles estimated from GPS observations compared favourably to measurements from three South African ionosondes and two other GPS-based methodologies for 2006 solstice and equinox dates. The ASHA methodology was applied to calculating diurnal two-dimensional TEC maps from multiple receivers in the South African GPS network. The space physics application of the newly developed methodology was demonstrated by investigating the ionosphere’s behaviour during a severe geomagnetic storm and investigating the long-term ionospheric stability in support of the proposed Square Kilometre Array (SKA) radio astronomy project. The feasibility of employing the newly developed technique in an operational near real-time system for estimating and dissimenating TEC values over Southern Africa using observations from a regional GPS receiver network, was investigated.
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  • Date Issued: 2008

Solar cycle effects on GNSS-derived ionospheric total electron content observed over Southern Africa

- Moeketsi, Daniel Mojalefa

  • Authors: Moeketsi, Daniel Mojalefa
  • Date: 2008
  • Subjects: Ionosphere -- Africa, Southern Electrons Ionospheric electron density -- Africa, Southern Ionosondes -- Africa, Southern Electromagnetic waves
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5489 , http://hdl.handle.net/10962/d1005275
  • Description: The South African Global Navigation Satellite System (GNSS) network of dual frequency receivers provide an opportunity to investigate solar cycle effects on ionospheric Total Electron Content (TEC) over the South Africa region by taking advantage of the dispersive nature of the ionospheric medium. For this task, the global University of New Brunswick Ionospheric Modelling Technique (UNB-IMT) was adopted, modified and applied to compute TEC using data from the southern African GNSS Network. TEC values were compared with CODE International GNSS services TEC predictions and Ionosonde-derived TEC (ITEC) measurements to test and validate the UNB-IMT results over South Africa. It was found that the variation trends of GTEC and ITEC over all stations are in good agreement and show pronounced seasonal variations with high TEC values around equinoxes for a year near solar maximum and less pronounced around solar minimum. Signature TEC depletions and enhanced spikes were prevalently evident around equinoxes, particularly for a year near solar maximum. These observations were investigated and further discussed with an analysis of the midday Disturbance Storm Time (DST) index of geomagnetic activity. The residual GTEC – ITEC corresponding to plasmaspheric electron content and equivalent ionospheric foF2 and total slab thickness parameters were computed and comprehensively discussed. The results verified the use of UNB-IMT as one of the tools for ionospheric research over South Africa. The UNB-IMT algorithm was applied to investigate TEC variability during different epochs of solar cycle 23. The results were investigated and further discussed by analyzing the GOES 8 and 10 satellites X-ray flux (0.1 – 0.8 nm) and SOHO Solar Extreme Ultraviolet Monitor higher resolution data. Comparison of UNB-IMT TEC derived from collocated HRAO and HARB GNSS receivers was undertaken for the solar X17 and X9 flare events, which occurred on day 301, 2003 and day 339, 2006. It was found that there exist considerable TEC differences between the two collocated receivers with some evidence of solar cycle dependence. Furthermore, the daytime UNB TEC compared with the International Reference Ionosphere 2001 predicted TEC found both models to show a good agreement. The UNB-IMT TEC was further applied to investigate the capabilities of geodetic Very Long Baseline Interferometry (VLBI) derived TEC using the Vienna TEC Model for space weather monitoring over HartRAO during the CONT02 and CONT05 campaigns conducted during the years 2002 (near solar maximum) and 005 (near solar minimum). The results verified the use of geodetic VLBI as one of the possible instruments for monitoring space weather impacts on the ionosphere over South Africa.
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  • Date Issued: 2008

Temperature dependence of the HartRAO pointing model

- Copley, Charles Judd

  • Authors: Copley, Charles Judd
  • Date: 2008
  • Subjects: Astronomical instruments , Observatories -- South Africa , Telescopes , Astronomical observatories , Astronomy -- Data processing , Antennas (Electronics)
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5491 , http://hdl.handle.net/10962/d1005277 , Astronomical instruments , Observatories -- South Africa , Telescopes , Astronomical observatories , Astronomy -- Data processing , Antennas (Electronics)
  • Description: This thesis investigates control aspects of the Hartebeeshoek Radio Astronomy Observatory (HartRAO) antenna. The installation of a new 22 GHz receiver has required the pointing accuracy to be improved to less than 4 mdeg. The effect of thermal conditions on the the HartRAO antenna pointing offset is investigated using a variety of modelling techniques including simple geometric modelling, neural networks and Principal Component Analysis (PCA). Convincing results were obtained for the Declination pointing offset, where applying certain model predictions to observations resulted in an improvement in Declination pointing offset from 5.5 mdeg to 3.2 mdeg (≈50%). The Right Ascension pointing model was considerably less convincing with an improvement of approximately from 5.5 mdeg to 4.5 mdeg (≈20%) in the Right Ascension pointing offset. The Declination pointing offset can be modelled sufficiently well to reduce the pointing offset to less than 4 mdeg, however further investigation of the underlying causes is required for the Right Ascension pointing offset.
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  • Date Issued: 2008

The variability and predictability of the IRI shape parameters over Grahamstown, South Africa

- Chimidza, Oyapo

  • Authors: Chimidza, Oyapo
  • Date: 2008
  • Subjects: Ionosphere -- Mathematical models -- South Africa , Atmosphere, Upper -- South Africa
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5496 , http://hdl.handle.net/10962/d1005282 , Ionosphere -- Mathematical models -- South Africa , Atmosphere, Upper -- South Africa
  • Description: The International Reference Ionosphere (IRI) shape parameters B0, B1, and D1 provide a representation of the shape of the F2 layer, the thickness of the F2 layer and the shape of the F1 layer of the ionosphere respectively. The aim of this study was to examine the variability of these parameters using Grahamstown, South Africa (33.3±S, 26.5±E) ionosonde data and determine their predictability by the IRI-2001 model. A further aim of this study was to investigate developing an alternative model for predicting these parameters. These parameters can be determined from electron density profiles that are inverted from ionograms recorded with an ionosonde. Data representing the B0, B1 and D1 parameters, with half hourly or hourly intervals, were scaled and deduced from the digital pulse sounder (DPS) ionosonde for the period April 1996 to December 2006. An analysis of the diurnal, seasonal, and solar variations of the behaviour of these parameters was undertaken for the years 2000, 2004 and 2005 using monthly medians. Comparisons between the observational results and that of the IRI model (IRI 2001 version) indicate that the IRI-2001 model does not accurately represent the diurnal and seasonal variation of the parameters. A preliminary model was thus developed using the technique of Neural Networks (NNs). All available data from the Grahamstown ionosonde from 1996 to 2006 were used in the training of the NNs and the prediction of the variation of the shape parameters. Inputs to the model were the day number, the hour of day, the solar activity and the magnetic index. Comparisons between the preliminary NN model and the IRI-2001 model indicated that the preliminary model was more accurate at the prediction of the parameters than the IRI-2001 model. This analysis showed the need to improve the existing IRI model or develop a new model for the South African region. This thesis describes the results from this feasibility study which show the variability and predictability of the IRI shape parameters.
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  • Date Issued: 2008

Verification of Ionospheric tomography using MIDAS over Grahamstown, South Africa

- Katamzi, Zama Thobeka

  • Authors: Katamzi, Zama Thobeka
  • Date: 2008
  • Subjects: Ionosphere -- Remote sensing -- South Africa , Atmosphere, Upper , Tomography -- Scientific applications -- South Africa , Global Positioning System
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5497 , http://hdl.handle.net/10962/d1005283 , Ionosphere -- Remote sensing -- South Africa , Atmosphere, Upper , Tomography -- Scientific applications -- South Africa , Global Positioning System
  • Description: Global Positioning System (GPS) satellites and receivers are used to derive total electron content (TEC) from the time delay and phase advance of the radiowaves as they travels through the ionosphere. TEC is defined as the integralof the electron density along the satellite-receiver signal path. Electron densityprofiles can be determined from these TEC values using ionospheric tomographic inversion techniques such as Multi-Instrument Data Analysis System (MIDAS).This thesis reports on a study aimed at evaluating the suitability of ionospheric tomography as a tool to derive one-dimensional electron density profiles, using the MIDAS inversion algorithm over Grahamstown, South Africa (33.30◦S, 26.50◦E). The evaluation was done by using ionosonde data from the Louisvale (28.50◦S, 21.20◦E) and Madimbo (22.40◦S, 30.90◦E) stations to create empirical orthonormal functions (EOFs). These EOFs were used by MIDAS in the inversion process to describe the vertical variation of the electron density. Profiles derived from the MIDAS algorithm were compared with profiles obtained from the international Reference Ionosphere (IRI) 2001 model and with ionosonde profiles from the Grahamstown ionosonde station. The optimised MIDAS profiles show a good agreement with the Grahamstown ionosonde profiles. The South African Bottomside Ionospheric Model (SABIM) was used to set the limits within which MIDAS was producing accurate peak electron density (NmF2) values and to define accuracy in this project, with the understanding that the national model (SABIM) is currently the best model for the Grahamstown region. Analysis show that MIDAS produces accurate results during the winter season, which had the lowest root mean square (rms) error of 0.37×1011[e/m3] and an approximately 86% chance of producing NmF2 closer to the actual NmF2 value than the national model SABIM. MIDAS was found to also produce accurate NmF2 values at 12h00 UT, where an approximately 88% chance of producing an accurate NmF2 value, which may deviate from the measured value by 0.72×1011[e/m3], was determined. In conclusion, ionospheric tomographic inversion techniques show promise in the reconstruction of electron density profiles over South Africa, and are worth pursuing further in the future.
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  • Date Issued: 2008

Measuring the RFI environment of the South African SKA site

- Manners, Paul John

  • Authors: Manners, Paul John
  • Date: 2007
  • Subjects: Radio telescopes , Radio telescopes -- South Africa , Radio astronomy , Radio astronomy -- South Africa , Square Kilometer Array (Spacecraft) , Radio -- Interference -- Measurement
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5474 , http://hdl.handle.net/10962/d1005259 , Radio telescopes , Radio telescopes -- South Africa , Radio astronomy , Radio astronomy -- South Africa , Square Kilometer Array (Spacecraft) , Radio -- Interference -- Measurement
  • Description: The Square Kilometre Array (SKA) Project is an international effort to build the world’s largest radio telescope. It will be 100 times more sensitive than any other radio telescope currently in existence and will consist of thousands of dishes placed at baselines up to 3000 km. In addition to its increased sensitivity it will operate over a very wide frequency range (current specification is 100 MHz - 22 GHz) and will use frequency bands not primarily allocated to radio astronomy. Because of this the telescope needs to be located at a site with low levels of radio frequency interference (RFI). This implies a site that is remote and away from human activity. In bidding to host the SKA, South Africa was required to conduct an RFI survey at its proposed site for a period of 12 months. Apart from this core site, where more than half the SKA dishes may potentially be deployed, the measurement of remote sites in Southern Africa was also required. To conduct measurements at these sites, three mobile measurement systems were designed and built by the South African SKA Project. The design considerations, implementation and RFI measurements recorded during this campaign will be the focus for this dissertation.
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  • Date Issued: 2007

Particle precipitation effects on the South African ionosphere

- Sibanda, Patrick

  • Authors: Sibanda, Patrick
  • Date: 2007
  • Subjects: Ionosphere -- South Africa , Precipitation (Chemistry) -- South Africa
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5481 , http://hdl.handle.net/10962/d1005267 , Ionosphere -- South Africa , Precipitation (Chemistry) -- South Africa
  • Description: Particle precipitation involves the injection of energetic particles into the ionosphere which could increase the ionisation and conductivity of the upper atmosphere. The goal of this study was to examine the ionospheric response and changes due to particle precipitation in the region over South Africa, using a combination of groundbased and satellite instruments. Particle precipitation events were identified from satellite particle flux measurements of the Defence Meteorological Satellite Program (DMSP). Comprehensive studies were done on the events of 5 April, 2000 and 7 October, 2000. Analysis of the data from the satellite instruments indicates that no particle precipitation was observed over the South African region during these events and that it is unlikely to occur during other such events. To validate the data, methods and tools used in this study, precipitation in the South Atlantic anomaly (SAA) region is used. Satellite ion density measurements revealed that strong density enhancements occurred over the SAA region at satellite altitudes during the precipitation events, but this did not occur in the South African region. The measurements also revealed how the ionisation enhancements in the SAA region correlated with geomagnetic and solar activities. Particle precipitation and convective electric fields are two major magnetospheric energy sources to the upper atmosphere in the auroral and the SAA regions. These increase dramatically during geomagnetic storms and can disturb thermospheric circulation in the atmosphere and alter the rates of production and recombination of the ionised species. Ionosonde observations at Grahamstown, South Africa (33.30S, 26.50E), provided the data to build a picture of the response of the ionosphere over the South African region to particle precipitation during the precipitation events. This analysis showed that, within the confines of the available data, no direct connections between particle precipitation events and disturbances in the ionosphere over this region were revealed.
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  • Date Issued: 2007

The C-Band : continuum mapping and measuring the radio environment

- King, Oliver Gray

  • Authors: King, Oliver Gray
  • Date: 2007 , 2013-07-01
  • Identifier: vital:5449 , http://hdl.handle.net/10962/d1004744
  • Description: Voice-enabled applications, applications that interact with a user via an audio channel, are used extensively today. Their use is growing as speech related technologies improve, as speech is one of the most natural methods of interaction. They can provide customer support as IVRs, can be used as an assistive technology, or can become an aural interface to the Internet. Given that the telephone is used extensively throughout the globe, the number of potential users of voice-enabled applications is very high. VoiceXML is a popular, open, high-level, standard means of creating voice-enabled applications which was designed to bring the benefits of web based development to services. While VoiceXML is an ideal language for creating these applications, VoiceXML gateways, the hardware and software responsible for interpreting VoiceXML applications and interfacing with the PSTN, are still expensive and so there is a need for a low-cost gateway. Asterisk, and open-source, TDM/VoIP telephony platform, can be used as a low-cost PSTN interface. This thesis investigates adding a VoiceXML service to Asterisk, creating a low-cost VoiceXML prototype gateway which is able to render voice-enabled applications. Following the Component-Based Software Engineering (CBSE) paradigm, the VoiceXML gateway is divided into a set of components which are sourced from the open-source community, and integrated to create the gateway. The browser requires a VoiceXML interpreter (OpenVXI), a Text-To-Speech engine (Festival) and a speech recognition engine (Sphinx 4). The integration of the components results in a low-cost, open-source VoiceXML gateway. System tests show that the integration of the components was successful, and that the system can handle concurrent calls. A fully compliant version of the gateway can be used in the real world to render voice-enabled applications at a low cost.
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  • Date Issued: 2007

A global ionospheric F2 region peak electron density model using neural networks and extended geophysically relevant inputs

- Oyeyemi, Elijah Oyedola

  • Authors: Oyeyemi, Elijah Oyedola
  • Date: 2006
  • Subjects: Neural networks (Computer science) Ionospheric electron density Ionosphere Ionosphere -- Mathematical models
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5470 , http://hdl.handle.net/10962/d1005255
  • Description: This thesis presents my research on the development of a neural network (NN) based global empirical model of the ionospheric F2 region peak electron density using extended geophysically relevant inputs. The main principle behind this approach has been to utilize parameters other than simple geographic co-ordinates, on which the F2 peak electron density is known to depend, and to exploit the technique of NNs, thereby establishing and modeling the non-linear dynamic processes (both in space and time)associated with the F2 region electron density on a global scale. Four different models have been developed in this work. These are the foF2 NN model, M(3000)F2 NN model, short-term forecasting foF2 NN, and a near-real time foF2 NN model. Data used in the training of the NNs were obtained from the worldwide ionosonde stations spanning the period 1964 to 1986 based on availability, which included all periods of calm and disturbed magnetic activity. Common input parameters used in the training of all 4 models are day number (day of the year, DN), Universal Time (UT), a 2 month running mean of the sunspot number (R2), a 2 day running mean of the 3-hour planetary magnetic index ap (A16), solar zenith angle (CHI), geographic latitude (q), magnetic dip angle (I), angle of magnetic declination (D), angle of meridian relative to subsolar point (M). For the short-term and near-real time foF2 models, additional input parameters related to recent past observations of foF2 itself were included in the training of the NNs. The results of the foF2 NN model and M(3000)F2 NN model presented in this work, which compare favourably with the IRI (International Reference Ionosphere) model successfully demonstrate the potential of NNs for spatial and temporal modeling of the ionospheric parameters foF2 and M(3000)F2 globally. The results obtained from the short-term foF2 NN model and nearreal time foF2 NN model reveal that, in addition to the temporal and spatial input variables, short-term forecasting of foF2 is much improved by including past observations of foF2 itself. Results obtained from the near-real time foF2 NN model also reveal that there exists a correlation between measured foF2 values at different locations across the globe. Again, comparisons of the foF2 NN model and M(3000)F2 NN model predictions with that of the IRI model predictions and observed values at some selected high latitude stations, suggest that the NN technique can successfully be employed to model the complex irregularities associated with the high latitude regions. Based on the results obtained in this research and the comparison made with the IRI model (URSI and CCIR coefficients), these results justify consideration of the NN technique for the prediction of global ionospheric parameters. I believe that, after consideration by the IRI community, these models will prove to be valuable to both the high frequency (HF) communication and worldwide ionospheric communities.
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  • Date Issued: 2006

A VLBI polarisation study of 43 GHZ SiO masers towards VY CMA

- Richter, Laura

  • Authors: Richter, Laura
  • Date: 2006
  • Subjects: Very long baseline interferometry , Polarization (Light) , Masers
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5498 , http://hdl.handle.net/10962/d1005284
  • Description: This thesis reports the calibration, imaging and analysis of one epoch of VLBI observations of the v (italics) = J (italics) = 1-0 transition of SiO towards VY CMa. Full polarisation information was recorded, allowing high resolution synthesis maps of each of the four Stokes parameters to be produced. A total of 81 maser components were extracted from the total intensity map, each approximately 1 mas in size. The emission spans approximately 100 x 80 mas in right ascension and declination and is concentrated to the east. The maser component positions were fitted to a ring of radius ~ 3.2R₊ (italics), or 7.2 x 1O¹⁴ cm for a stellar distance of 1.5 kpc. If the stellar position is assumed to be the centre of this ring then almost all of the maser components fall within the inner dust shell radius, which is at ~ 5R (italics)ϰ All of the maser components fall between 1.5R (italics)ϰ and 6R (italics)ϰ. A velocity gradient with position angle was observed in the sparsely filled western region of the maser ring. If interpreted as evidence of shell rotation, this gradient implies a rotational velocity of v (italics) rot (subscirpt) sin i (italics) = 18 km.s⁻¹. The fractional circular and linear polarisations of the maser spots were derived from the Stokes parameter maps. The mean fractional circular polarisation of the masers components was ~ 2 percent and the median fractional linear polarisation was ~ 6 percent, with many spots displaying over ~ 30 percent linear polarisation. The mean circular polarisation implies a magnetic field of ~ 4 G in the SiO maser region if the polarisation is due to Zeeman splitting. Two maser components display a rotation of linear polarisation position angle with velocity, possibly implying a connection between the magnetic field and the velocity field variations in the region of these components.
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  • Date Issued: 2006

Ionospheric total electron content variability and its influence in radio astronomy

- Botai, Ondego Joel

  • Authors: Botai, Ondego Joel
  • Date: 2006
  • Subjects: Electrons , Global Positioning System , Global Positioning System -- Data processing , Ionosphere , Ionospheric radio wave propagation
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5473 , http://hdl.handle.net/10962/d1005258 , Electrons , Global Positioning System , Global Positioning System -- Data processing , Ionosphere , Ionospheric radio wave propagation
  • Description: Ionospheric phase delays of radio signals from Global Positioning System (GPS) satellites have been used to compute ionospheric Total Electron Content (TEC). An extended Chapman profle model is used to estimate the electron density profles and TEC. The Chapman profle that can be used to predict TEC over the mid-latitudes only applies during day time. To model night time TEC variability, a polynomial function is fitted to the night time peak electron density profles derived from the online International Reference Ionosphere (IRI) 2001. The observed and predicted TEC and its variability have been used to study ionospheric in°uence on Radio Astronomy in South Africa region. Di®erential phase delays of the radio signals from Radio Astronomy sources have been simulated using TEC. Using the simulated phase delays, the azimuth and declination o®sets of the radio sources have been estimated. Results indicate that, pointing errors of the order of miliarcseconds (mas) are likely if the ionospheric phase delays are not corrected for. These delays are not uniform and vary over a broad spectrum of timescales. This implies that fast frequency (referencing) switching, closure phases and fringe ¯tting schemes for ionospheric correction in astrometry are not the best option as they do not capture the real state of the ionosphere especially if the switching time is greater than the ionospheric TEC variability. However, advantage can be taken of the GPS satellite data available at intervals of a second from the GPS receiver network in South Africa to derive parameters which could be used to correct for the ionospheric delays. Furthermore GPS data can also be used to monitor the occurrence of scintillations, (which might corrupt radio signals) especially for the proposed, Square Kilometer Array (SKA) stations closer to the equatorial belt during magnetic storms and sub-storms. A 10 minute snapshot of GPS data recorded with the Hermanus [34:420 S, 19:220 E ] dual frequency receiver on 2003-04-11 did not show the occurrence of scintillations. This time scale is however too short and cannot be representative. Longer time scales; hours, days, seasons are needed to monitor the occurrence of scintillations.
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  • Date Issued: 2006

An investigation into improved ionospheric F1 layer predictions over Grahamstown, South Africa

- Jacobs, Linda

  • Authors: Jacobs, Linda
  • Date: 2005
  • Subjects: Ionosphere , Ionospheric electron density -- South Africa -- Grahamstown , Neural networks (Computer Science)
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5511 , http://hdl.handle.net/10962/d1008094 , Ionosphere , Ionospheric electron density -- South Africa -- Grahamstown , Neural networks (Computer Science)
  • Description: This thesis describes an analysis of the F1 layer data obtained from the Grahamstown (33.32°S, 26.500 E), South Africa ionospheric station and the use of this data in improving a Neural Network (NN) based model of the F1 layer of the ionosphere. An application for real-time ray tracing through the South African ionosphere was identified, and for this application real-time evaluation of the electron density profile is essential. Raw real-time virtual height data are provided by a Lowell Digisonde (DPS), which employs the automatic scaling software, ARTIST whose output includes the virtual-toreal height data conversion. Experience has shown that there are times when the ray tracing performance is degraded because of difficulties surrounding the real-time characterization of the F1 region by ARTIST. Therefore available DPS data from the archives of the Grahamstown station were re-scaled manually in order to establish the extent of the problem and the times and conditions under which most inaccuracies occur. The re-scaled data were used to update the F1 contribution of an existing NN based ionospheric model, the LAM model, which predicts the values of the parameters required to produce an electron density profile. This thesis describes the development of three separate NNs required to predict the ionospheric characteristics and coefficients that are required to describe the F1 layer profile. Inputs to the NNs include day number, hour and measures of solar and magnetic activity. Outputs include the value of the critical frequency of the F1 layer, foF1, the real height of reflection at the peak, hmFl, as well as information on the state of the F1 layer. All data from the Grahamstown station from 1973 to 2003 was used to train these NNs. Tests show that the predictive ability of the LAM model has been improved by incorporating the re-scaled data.
  • Full Text:
  • Date Issued: 2005

Modeling and measurement of torqued procession in radio pulsars

- Tiplady, Adrian John

  • Authors: Tiplady, Adrian John
  • Date: 2005
  • Subjects: Pulsars , Radio telescopes , Radio astronomy , Precession , Hartebeeshoek Radio Astronomy Observatory (HartRAO)
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5475 , http://hdl.handle.net/10962/d1005260
  • Description: The long term isolated pulsar monitoring program, which commenced in 1984 using the 26 m radio telescope at the Hartebeeshoek Radio Astronomy Observatory (HartRAO), has produced high resolution timing residual data over long timespans. This has enabled the analysis of observed spin down behaviour for 27 braking pulsars, most of which have dataspans longer than 14 years. The phenomenology of observed timing residuals of certain pulsars can be explained by pseudo periodic effects such as precession. Analytic and numerical models are developed to study the kinematic and dynamic behaviour of isolated but torqued precessing pulsars. The predicted timing residual behaviour of the models is characterised, and confronted with timing data from selected pulsars. Cyclic variations in the observed timing residuals of PSR B1642-03, PSR B1323-58 and PSR B1557-50 are fitted with a torqued precession model. The phenomenology of the observed timing behaviour of these pulsars can be explained by the precession models, but precise model fitting was not possible. This is not surprising given that the complexity of the pulsar systems is not completely described by the model. The extension of the pulsar monitoring program at HartRAO is used as motivation for the design and development of a new low cost, multi-purpose digital pulsar receiver. The instrument is implemented using a hybrid filterbank architecture, consisting of an analogue frontend and digital backend, to perform incoherent dedispersion. The design of a polyphase filtering system, which will consolidate multiple processing units into a single filtering solution, is discussed for future implementation.
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  • Date Issued: 2005

A neural network based ionospheric model for the bottomside electron density profile over Grahamstown, South Africa

- McKinnell, L A

  • Authors: McKinnell, L A
  • Date: 2003
  • Subjects: Neural networks (Computer science) Ionospheric electron density -- South Africa -- Grahamstown
  • Language: English
  • Type: Thesis , Doctoral , PhD
  • Identifier: vital:5477 , http://hdl.handle.net/10962/d1005262
  • Description: This thesis describes the development and application of a neural network based ionospheric model for the bottomside electron density profile over Grahamstown, South Africa. All available ionospheric data from the archives of the Grahamstown (33.32ºS, 26.50ºE) ionospheric station were used for training neural networks (NNs) to predict the parameters required to produce the final profile. Inputs to the model, called the LAM model, are day number, hour, and measures of solar and magnetic activity. The output is a mathematical description of the bottomside electron density profile for that particular input set. The two main ionospheric layers, the E and F layers, are predicted separately and then combined at the final stage. For each layer, NNs have been trained to predict the individual ionospheric characteristics and coefficients that were required to describe the layer profile. NNs were also applied to the task of determining the hours between which an E layer is measurable by a groundbased ionosonde and the probability of the existence of an F1 layer. The F1 probability NN is innovative in that it provides information on the existence of the F1 layer as well as the probability of that layer being in a L-condition state - the state where an F1 layer is present on an ionogram but it is not possible to record any F1 parameters. In the event of an L-condition state being predicted as probable, an L algorithm has been designed to alter the shape of the profile to reflect this state. A smoothing algorithm has been implemented to remove discontinuities at the F1-F2 boundary and ensure that the profile represents realistic ionospheric behaviour in the F1 region. Tests show that the LAM model is more successful at predicting Grahamstown electron density profiles for a particular set of inputs than the International Reference Ionosphere (IRI). It is anticipated that the LAM model will be used as a tool in the pin-pointing of hostile HF transmitters, known as single-site location.
  • Full Text:
  • Date Issued: 2003

Progress towards the development and implementation of an unambiguous copper wire fingerprinting system

- Poole, Martin

  • Authors: Poole, Martin
  • Date: 2003
  • Subjects: Electroplating , Copper , Telecommunication , Theft -- Prevention
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5484 , http://hdl.handle.net/10962/d1005270 , Electroplating , Copper , Telecommunication , Theft -- Prevention
  • Description: The Telecommunications industry in Southern Africa is faced with the problem of theft of the signal carrying copper wire, both from the ground and from telephone poles. In many cases, if the offenders are caught, the prosecuting party has no way of proving that the wire is the property of any one Telecommunication company, as any inked markings on the insulating sheaths have been burned off along with the insulation and protective coatings themselves. Through this work we * describe the problem, * specify the necessary and preferred technical properties of a viable solution, * report the preliminary investigations into the devising of an unambiguous "fingerprinting" of the 0.5 mm wires, including some of those solutions that, upon investigation, appear non-viable, * describe the development and implementation of an electrochemical marker with detection mechanism which has shown in proof-of-principle to work, * outline the road-map of necessary future work.
  • Full Text:
  • Date Issued: 2003

Distributed control applications using local area networks: a LAN based power control system at Rhodes University

- Sullivan, Anthony John

  • Authors: Sullivan, Anthony John
  • Date: 2002
  • Subjects: Embedded computer systems , Local area networks (Computer networks) , Linux
  • Language: English
  • Type: Thesis , Masters , MSc
  • Identifier: vital:5461 , http://hdl.handle.net/10962/d1005246 , Embedded computer systems , Local area networks (Computer networks) , Linux
  • Description: This thesis describes the design and development of both the hardware and software of an embedded, distributed control system using a LAN infrastructure for communication between nodes. The primary application of this system is for power monitoring and control at Rhodes University. Both the hardware and software have been developed to provide a modular and scalable system capable of growing and adapting to meet the changing demands placed on it. The software includes a custom written Internet Protocol stack for use in the embedded environment, with a small code footprint and low processing overheads. There is also Linux-based control software, which includes a web-based device management interface and graphical output. Problems specific to the application are discussed as well as their solutions, with particular attention to the constraints of an embedded system.
  • Full Text:
  • Date Issued: 2002
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