A 150 MHz all sky survey with the Precision Array to Probe the Epoch of Reionization
- Authors: Chege, James Kariuki
- Date: 2020
- Subjects: Epoch of reionization -- Research , Astronomy -- Observations , Radio interferometers
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/117733 , vital:34556
- Description: The Precision Array to Probe the Epoch of Reionization (PAPER) was built to measure the redshifted 21 cm line of hydrogen from cosmic reionization. Such low frequency observations promise to be the best means of understanding the cosmic dawn; when the first galaxies in the universe formed, and also the Epoch of Reionization; when the intergalactic medium changed from neutral to ionized. The major challenges to these observations is the presence of astrophysical foregrounds that are much brighter than the cosmological signal. Here, I present an all-sky survey at 150 MHz obtained from the analysis of 300 hours of PAPER observations. Particular focus is given to the calibration and imaging techniques that need to deal with the wide field of view of a non-tracking instrument. The survey covers ~ 7000 square degrees of the southern sky. From a sky area of 4400 square degrees out of the total survey area, I extract a catalogue of sources brighter than 4 Jy whose accuracy was tested against the published GLEAM catalogue, leading to a fractional difference rms better than 20%. The catalogue provides an all-sky accurate model of the extragalactic foreground to be used for the calibration of future Epoch of Reionization observations and to be subtracted from the PAPER observations themselves in order to mitigate the foreground contamination.
- Full Text:
- Date Issued: 2020
A Bayesian approach to tilted-ring modelling of galaxies
- Authors: Maina, Eric Kamau
- Date: 2020
- Subjects: Bayesian statistical decision theory , Galaxies , Radio astronomy , TiRiFiC (Tilted Ring Fitting Code) , Neutral hydrogen , Spectroscopic data cubes , Galaxy parametrisation
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/145783 , vital:38466
- Description: The orbits of neutral hydrogen (H I) gas found in most disk galaxies are circular and also exhibit long-lived warps at large radii where the restoring gravitational forces of the inner disk become weak (Spekkens and Giovanelli 2006). These warps make the tilted-ring model an ideal choice for galaxy parametrisation. Analysis software utilizing the tilted-ring-model can be grouped into two and three-dimensional based software. Józsa et al. (2007b) demonstrated that three dimensional based software is better suited for galaxy parametrisation because it is affected by the effect of beam smearing only by increasing the uncertainty of parameters but not with the notorious systematic effects observed for two-dimensional fitting techniques. TiRiFiC, The Tilted Ring Fitting Code (Józsa et al. 2007b), is a software to construct parameterised models of high-resolution data cubes of rotating galaxies. It uses the tilted-ring model, and with that, a combination of some parameters such as surface brightness, position angle, rotation velocity and inclination, to describe galaxies. TiRiFiC works by directly fitting tilted-ring models to spectroscopic data cubes and hence is not affected by beam smearing or line-of-site-effects, e.g. strong warps. Because of that, the method is unavoidable as an analytic method in future Hi surveys. In the current implementation, though, there are several drawbacks. The implemented optimisers search for local solutions in parameter space only, do not quantify correlations between parameters and cannot find errors of single parameters. In theory, these drawbacks can be overcome by using Bayesian statistics, implemented in Multinest (Feroz et al. 2008), as it allows for sampling a posterior distribution irrespective of its multimodal nature resulting in parameter samples that correspond to the maximum in the posterior distribution. These parameter samples can be used as well to quantify correlations and find errors of single parameters. Since this method employs Bayesian statistics, it also allows the user to leverage any prior information they may have on parameter values.
- Full Text:
- Date Issued: 2020
A feasibility study into the possibility of ionospheric propagation of low VHF (30-35 MHZ) signals between South Africa and Central Africa
- Authors: Coetzee, Petrus Johannes
- Date: 2009
- Subjects: Communications, Military -- South Africa , Communications, Military -- Africa, Central , Digital communications -- South Africa , Digital communications -- Africa, Central , Signals and signaling -- South Africa , Signals and signaling -- Africa, Central , Artificial satellites in telecommunication -- South Africa , Artificial satellites in telecommunication -- Africa, Central , Shortwave radio
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5465 , http://hdl.handle.net/10962/d1005250 , Communications, Military -- South Africa , Communications, Military -- Africa, Central , Digital communications -- South Africa , Digital communications -- Africa, Central , Signals and signaling -- South Africa , Signals and signaling -- Africa, Central , Artificial satellites in telecommunication -- South Africa , Artificial satellites in telecommunication -- Africa, Central , Shortwave radio
- Description: The role of the South African National Defence Force (SANDF) has changed considerably in the last decade. The emphasis has moved from protecting the country's borders to peacekeeping duties in Central Africa and even further North. Communications between the peacekeeping missions and the military bases back in South Africa is vital to ensure the success of these missions. Currently use is made of satellite as well as High Frequency (HF) communications. There are drawbacks associated with these technologies (high cost and low data rates/interference respectively). Successful long distance ionospheric propagation in the low Very High Frequency (VHF) range will complement the existing infrastructure and enhance the success rate of these missions. This thesis presents a feasibility study to determine under what ionospheric conditions such low VHF communications will be possible. The International Reference Ionosphere (IRI) was used to generate ionospheric data for the reflection point(s) of the signal. The peak height of the ionospheric F2 layer (hmF2) was used to calculate the required antenna elevation angle. Once the elevation angle is known it is possible to calculate the required F2 layer critical frequency (foF2). The required foF2 value was calculated by assuming a Maximum Useable Frequency (MUF) of 20% higher than the planned operational frequency. It was determined that single hop propagation is possible during the daytime if the smoothed sunspot number (SSN) exceeds 15. The most challenging requirement for successful single hop propagation is the need of an antenna height of 23 m. For rapid deployment and semi-mobile operations within a jungle environment it may prove to be a formidable obstacle.
- Full Text:
- Date Issued: 2009
A feasibility study into total electron content prediction using neural networks
- 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.
- Full Text:
- Date Issued: 2008
A light-emitting-diode pulsing system for measurement of time-resolved luminescence
- Authors: Uriri, Solomon Akpore
- Date: 2015
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:20976 , http://hdl.handle.net/10962/5788
- Description: A new light-emitting-diode based pulsing system for measurement of time-resolved luminescence has been developed. The light-emitting-diodes are pulsed at various pulse-widths by a 555-timer operated as a monostable multivibrator. The light-emitting-diodes are arranged in a dural holder connected in parallel in sets of four, each containing four diodes in series. The output pulse from the 555-timer is fed into an 2N7000 MOSFET to produce a pulse-current of 500 mA to drive the set of 16 light-emitting-diodes. This size of current is sufficient to drive the diodes with each driven at a pulse-current of 90 mA with a possible maximum of 110 mA per diode. A multichannel scaler is used to trigger the pulsing system and to record data at selectable dwell times. The system is capable of generating pulse-widths in the range of microseconds upwards.
- Full Text:
- Date Issued: 2015
A new empirical model for the peak ionospheric electron density using neural networks
- Authors: McKinnell, L A
- Date: 1997
- Subjects: Ionospheric electron density Neural networks (Computer science)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5478 , http://hdl.handle.net/10962/d1005264
- Description: This thesis describes the search for a temporal model for predicting the peak ionospheric electron density-(foF2). Existing models, such as the International Reference Ionosphere (IRI) and 8KYCOM, were used to predict the 12 noon foF2 value over Grahamstown (26°E, 33°8). An attempt was then made to find a model that would improve upon these results. The traditional method of linear regression was used as a first step towards a new model. It was found that this would involve a multi variable regression that is reliant on guessing the optimum variables to be used in the final equation. An extremely complicated modelling equation involving many terms would result. Neural networks (NNs) are introduced as a new technique for predicting foF2. They are also applied, for the first time, to the problem of determining the best predictors of foF2. This quantity depends upon day number, level of solar activity and level of magnetic activity. The optimum averaging lengths of the solar activity index and the magnetic activity index were determined by appling NNs, using the criterion that the best indices are those that give the lowest rms error between the measured and predicted foF2. The optimum index for solar activity was found to be a 2-month running mean value of the daily sunspot number and for magnetic activity a 2-day averaged A index was found to be optimum. In addition, it was found that the response of foF2 to magnetic activity changes is highly non-linear and seasonally dependent. Using these indices as inputs, the NN trained successfully to predict foF2 with an rms error of 0.946 MHz on the daily testing values. Comparison with the IRI showed an improvement of 40% on the rms error. It is also shown that the NN will predict the noon value of foF2 to the same level of accuracy for unseen data of the same type.
- Full Text:
- Date Issued: 1997
A pilot wide-field VLBI survey of the GOODS-North field
- Authors: Akoto-Danso, Alexander
- Date: 2019
- Subjects: Radio astronomy , Very long baseline interferometry , Radio interometers , Imaging systems in astronomy , Hubble Space Telescope (Spacecraft) -- Observations
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/72296 , vital:30027
- Description: Very Long Baseline Interferometry (VLBI) has significant advantages in disentangling active galactic nuclei (AGN) from star formation, particularly at intermediate to high-redshift due to its high angular resolution and insensitivity to dust. Surveys using VLBI arrays are only just becoming practical over wide areas with numerous developments and innovations (such as multi-phase centre techniques) in observation and data analysis techniques. However, fully automated pipelines for VLBI data analysis are based on old software packages and are unable to incorporate new calibration and imaging algorithms. In this work, the researcher developed a pipeline for VLBI data analysis which integrates a recent wide-field imaging algorithm, RFI excision, and a purpose-built source finding algorithm specifically developed for the 64kx64k wide-field VLBI images. The researcher used this novel pipeline to process 6% (~ 9 arcmin2 of the total 160 arcmin2) of the data from the CANDELS GOODS- North extragalactic field at 1.6 GHz. The milli-arcsec scale images have an average rms of a ~ 10 uJy/beam. Forty four (44) candidate sources were detected, most of which are at sub-mJy flux densities, having brightness temperatures and luminosities of >5x105 K and >6x1021 W Hz-1 respectively. This work demonstrates that automated post-processing pipelines for wide-field, uniform sensitivity VLBI surveys are feasible and indeed made more efficient with new software, wide-field imaging algorithms and more purpose-built source- finders. This broadens the discovery space for future wide-field surveys with upcoming arrays such as the African VLBI Network (AVN), MeerKAT and the Square Kilometre Array (SKA).
- Full Text:
- Date Issued: 2019
A real time Fast Fourier Transform analyser
- Authors: Fisher, John Stanley
- Date: 1980
- Subjects: Fourier transformations Ionosondes Algorithms Computer simulation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5439 , http://hdl.handle.net/10962/d1001992
- Description: From the requirements of the Ionosonde digitisation project, undertaken by Rhodes University Antarctic Research Group, it was decided to use the Fast Fourier Transform to compute the spectrum analysis. Several FFT algorithms are reviewed and properties discussed, and the Ccoley Tukey algorithm chosen for utilization. The hardware implementation of this algorithm, and the microprogram control of the whole system are discussed in detail, and such design aspects that required computer simulation are also treated in detail. The final testing of the analyser is shown, and includes a test using data from an ionosonde sounding. The conclusions contain details of extensions to the analysers present operation, required by plans to place the whole Chirpsounder under microprocessor control
- Full Text:
- Date Issued: 1980
A real time HF beacon monitoring station for South Africa
- Authors: Mudzingwa, Courage
- Date: 2009
- Subjects: Radio , Shortwave radio , Radio and television towers
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5486 , http://hdl.handle.net/10962/d1005272 , Radio , Shortwave radio , Radio and television towers
- Description: High frequency, HF (3 - 30 MHz), radio communications are greatly affected by ionospheric conditions. Both civilian and military users need reliable, real time propagation information to show at any time the feasibility of communicating to any part of the world on a particular frequency band. For this thesis, an automated receiving/monitoring station for the Northern California DX Foundation (NCDXF)/ International Amateur Radio Union (IARU) International Beacon Project was setup at the Hermanus Magnetic Observatory, HMO (34.42oS, 19.22oE) to monitor international beacons on 20 m, 17 m, 15 m, 12 m and 10 m bands. The beacons form a world wide multiband network. The task of monitoring the beacons was broken down into two steps. Initially the single band station, at 14.10 MHz, was installed and later it was upgraded to a multiband station capable of automatically monitoring all the five HF bands. The single band station setup involved the construction and installation of the half-wave dipole antenna, construction and installation of an HF choke balun; and the choice of Faros 1.3 as the appropriate monitoring software. The multiband monitoring station set-up involved the installation of an MFJ-1778 G5RV multiband antenna, construction and installation of a Communication Interface - V (CI-V) level converter and configuring the Faros 1.3 software to monitor the beacons on all five HF bands. Then a web page was created on the HMO space weather website (http://spaceweather.hmo.ac.za). Here, the real-time signal to noise ratio (SNR) and short path (SP)/long path (LP) plots are uploaded every 3 minutes, showing real time HF propagation conditions on the five HF bands. Historical propagation data are archived for later analysis. A preliminary data analysis was done to confirm the peration of the monitoring station. The archived data were analysed and compared to ICEPAC (Ionospheric Communications Enhanced Profile Analysis and Circuit) predictions. Results show that the real-time signal plots as well as the archive of historical signal plots, convey information on ropagation conditions to users in terms that are easy to interpret and understand.
- Full Text:
- Date Issued: 2009
A study of atmospheric oscillations in the meteor region above Grahamstown
- Authors: Malinga, Sandile B
- Date: 1995
- Subjects: Mesosphere , Thermosphere , Atmosphere, Upper -- South Africa -- Grahamstown , Meteorology -- Methodology
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5490 , http://hdl.handle.net/10962/d1005276 , Mesosphere , Thermosphere , Atmosphere, Upper -- South Africa -- Grahamstown , Meteorology -- Methodology
- Description: The dynamics of the atmospheric meteor region have been studied using the data obtained with - - the Grahamstown (33°16'S, 26<>:30'E) meteor radar between the years 1987 and 1993 inclusive. Harmonic analysis and the maximum entropy method (MEM) , were used for the spectral characterization ofthe wind above Grahamstown. The prevailing wind, tidal (periods 12- and 24- h) and other (periods 8- and 6-h) oscillations were extracted from the data using the guidelines agreed upon by the A TMAP community. Above Grahamstown the zonal and meridional prevailing winds were found to be predominantly eastward and equatorward respectively. Tidal amplitudes are comparable to the magnitude of the prevailing wind vector, with the diurnal tide being stronger than the semidiurnal tide. The phase differences between ,the zonal and meridional components of the semidiurnal and diurnal tides are - 2 hand - 5 h respectively, which is in reasonable agreement with the corresponding expected values of3 hand 6 h. The tidal wind vectors are on average elliptically polarized with anticlockwise rotation. Longitudinal and day-to-day tidal variations were studied. From the longitudinal study, the semidiurnal tide was found to be dominated by migrating modes, while the diurnal tidal behaviour suggests the presence of nonmigrating modes with zonal wavenumber s = 4. Tides were found to be variable from day to day with little apparent correlation between the zonal and meridional components of the respective tides.
- Full Text:
- Date Issued: 1995
A study of the fluorescence excitation spectrum of crystalline anthracene
- Authors: Driver, Adrian Stanford
- Date: 1960
- Subjects: Fluorimetry , Anthracene crystals -- Spectra
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5518 , http://hdl.handle.net/10962/d1011463 , Fluorimetry , Anthracene crystals -- Spectra
- Description: The work described in this thesis was performed at the Physics Department, Rhodes University during 1958 and 1959 under the supervision of Professor J.A. Gledhill. Use was made of a vacuum ultra-violet spectrograph which had been constructed in the Physics Department (1.1) and modifications to be described were made to this instrument. The instrument was used for studying the effects of oxygen on the fluorescence excitation spectrum of Anthracene.
- Full Text:
- Date Issued: 1960
A study of why some physic concepts in the South African Physical Science curriculum are poorly understood in order to develop a targeted action-research intervention for Newton’s second law
- Authors: Cobbing, Kathleen Margaret
- Date: 2020
- Subjects: Physics -- Study and teaching (Secondary) -- South Africa , Physics -- Examinations, questions, etc. -- South Africa , Motion -- Study and teaching (Secondary) -- South Africa
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/146903 , vital:38575
- Description: Globally, many students show a poor understanding of concepts in high school physics and lack the necessary problem-solving skills that the course demands. The application of Newton’s second law was found to be particularly problematic through document analysis of South African examination feedback reports, as well as from an analysis of the physics examinations at a pair of well-resourced South African independent schools that follow the Independent Examination Board curriculum. Through an action-research approach, a resource for use by students was designed and modified to improve students’ understanding of this concept, while modelling problemsolving methods. The resource consisted of brief revision notes, worked examples and scaffolded exercises. The design of the resource was influenced by the theory of cognitive apprenticeship, cognitive load theory and conceptual change theory. One of the aims of the resource was to encourage students to translate between the different representations of a problem situation: symbolic, abstract, model and concrete. The impact of this resource was evaluated at a pair of schools using a mixed methods approach. This incorporated pre- and post-tests for a quantitative assessment, qualitative student evaluations and the analysis of examination scripts. There was an improvement from pre- to post-test for all four iterations of the intervention and these improvements were shown to be significant. The use of the resource led to an increase in the quality and quantity of diagrams drawn by students in subsequent assessments.
- Full Text:
- Date Issued: 2020
A VLBI polarisation study of 43 GHZ SiO masers towards VY CMA
- 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.
- Full Text:
- Date Issued: 2006
A VLBI study of OH masers in a proto-planetary nebula OH 0.9+1.3
- 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.
- Full Text:
- Date Issued: 2008
Accelerated implementations of the RIME for DDE calibration and source modelling
- Authors: Van Staden, Joshua
- Date: 2021
- Subjects: Radio astronomy , Radio inferometers , Radio inferometers -- Calibration , Radio astronomy -- Data processing , Radio inferometers -- Data processing , Radio inferometers -- Calibration -- Data processing
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/172422 , vital:42199
- Description: Second- and third-generation calibration methods filter out subtle effects in interferometer data, and therefore yield significantly higher dynamic ranges. The basis of these calibration techniques relies on building a model of the sky and corrupting it with models of the effects acting on the sources. The sensitivities of modern instruments call for more elaborate models to capture the level of detail that is required to achieve accurate calibration. This thesis implements two types of models to be used in for second- and third-generation calibration. The first model implemented is shapelets, which can be used to model radio source morphologies directly in uv space. The second model implemented is Zernike polynomials, which can be used to represent the primary beam of the antenna. We implement these models in the CODEX-AFRICANUS package and provide a set of unit tests for each model. Additionally, we compare our implementations against other methods of representing these objects and instrumental effects, namely NIFTY-GRIDDER against shapelets and a FITS-interpolation method against the Zernike polynomials. We find that to achieve sufficient accuracy, our implementation of the shapelet model has a higher runtime to that of the NIFTY-GRIDDER. However, the NIFTY-GRIDDER cannot simulate a component-based sky model while the shapelet model can. Additionally, the shapelet model is fully parametric, which allows for integration into a parameterised solver. We find that, while having a smaller memory footprint, our Zernike model has a greater computational complexity than that of the FITS-interpolated method. However, we find that the Zernike implementation has floating-point accuracy in its modelling, while the FITS-interpolated model loses some accuracy through the discretisation of the beam.
- Full Text:
- Date Issued: 2021
An investigation into improved ionospheric F1 layer predictions over Grahamstown, South Africa
- 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
An investigation of the structure of the Southern Hemisphere radio-meteor streams
- Authors: Roux, David Gerhardus
- Date: 1988
- Subjects: Radio meteorology -- Southern Hemisphere , Solar system
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5448 , http://hdl.handle.net/10962/d1004587 , Radio meteorology -- Southern Hemisphere , Solar system
- Description: Our current knowledge of the Solar System, with a particular emphasis on the systems of interplanetary objects, is reviewed, and the theory of meteors and the reflection of radio waves from meteoric ionization is then discussed. A description of the meteor radar is given and a method of calibrating the antenna beam is developed. The main project comprises two parts: (a) A general survey of the radar echorate for 20 major and minor meteor streams and the sporadic meteor background, conducted from Grahamstown over the period 1986 April to 1988 January, is described. Definite shower activity was observed for all of the major and some of the minor showers. (b) Based on a scheme proposed by previous workers (Morton & Jones), a method of recovering meteor radiant distributions from the distribution of echo directions is developed. We devise a technique of compensating for possible distortions of the resulting radiant maps, which may arise due to the arisotropic antenna beam. This involves a system of echo-weighting. Radiant maps which showed considerably less distortion than those of the above workers were obtained without the weighting procedure. It is concluded that, although the method in its present form introduces spurious features into the maps, the principle is sound and should eventually be refined to produce the desired compensation
- Full Text:
- Date Issued: 1988
Assignment of spin and parity to states in the nucleus ¹⁹⁶T1
- Authors: Uwitonze, Pierre Celestin
- Date: 2015
- Subjects: Nuclear spin , Particles (Nuclear physics) -- Chirality
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5558 , http://hdl.handle.net/10962/d1017903
- Description: This work presents a study of high-spin states in the nucleus ¹⁹⁶Tl via γ-spectroscopy. ¹⁹⁶Tl was produced via the ¹⁹⁷Au(⁴He,5n) ¹⁹⁶Tl reaction at a beam energy of 63 MeV. The γ-γ coincidence measurements were performed using the AFRODITE γ-spectrometer array at iThemba LABS. The previous level scheme of ¹⁹⁶Tl has been extended up to an excitation of 4071 keV including 24 new γ-ray transitions. The spin and parity assignment to levels was made from the directional correlation of oriented nuclei (DCO) and linear polarization anisotropy ratios. An analysis of the B(M1)/B(E2) ratios was found to be consistent with the configuration of πh₉/₂♁vi₁₃/₂ for the ground state band. Although no chiral band was found in ¹⁹⁶TI and ¹⁹⁸TI.
- Full Text:
- Date Issued: 2015
Automation of source-artefact classification
- Authors: Sebokolodi, Makhuduga Lerato Lydia
- Date: 2017
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/4920 , vital:20743
- Description: The high sensitivities of modern radio telescopes will enable the detection of very faint astrophysical sources in the distant Universe. However, these high sensitivities also imply that calibration artefacts, which were below the noise for less sensitive instruments, will emerge above the noise and may limit the dynamic range capabilities of these instruments. Detecting faint emission will require detection thresholds close to the noise and this may cause some of the artefacts to be incorrectly detected as real emission. The current approach is to manually remove the artefacts, or set high detection thresholds in order to avoid them. The former will not be possible given the large quantities of data that these instruments will produce, and the latter results in very shallow and incomplete catalogues. This work uses the negative detection method developed by Serra et al. (2012) to distinguish artefacts from astrophysical emission in radio images. We also present a technique that automates the identification of sources subject to severe direction-dependent (DD) effects and thus allows them to be flagged for DD calibration. The negative detection approach is shown to provide high reliability and high completeness catalogues for simulated data, as well as a JVLA observation of the 3C147 field (Mitra et al., 2015). We also show that our technique correctly identifies sources that require DD calibration for datasets from the KAT-7, LOFAR, JVLA and GMRT instruments.
- Full Text:
- Date Issued: 2017
Beta decay of 100/400 Zr produced in neutron-induced fission of natural uranium
- Authors: Kamoto, Thokozani
- Date: 2016
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/3024 , vital:20353
- Description: Fission fragments, produced by neutron bombardment of natural uranium at the Physics Department, Jyväskylä, Finland, are studied in this work. The data had been sorted into 25 Y — y coincidence matrices which were then analysed. In this work we aimed to identify the fission products using Y-Y coincidence analysis and then study the beta-decay of some of the fission products. Sixteen fission products ranging from A = 94 to A = 136 were identified. Out of these fission products beta decay of the A = 100 (100/40 Zr – 100/41 Nb – 100/42 Mo) chain was studied in greater detail. We have also studied the variation of the relative intensities as a function of time of the 159-, 528-, 600-, 768-, 928- and 1502-keV Y-rav lines in 100/42 Mo and the profiles of the relative intensities have been modelled with the variation of the activity of 100/41 Nb against time. Configuration assignments of 100 Zr and 100/42 Mo are discussed.
- Full Text:
- Date Issued: 2016