Machine learning methods for calibrating radio interferometric data
- Authors: Zitha, Simphiwe Nhlanhla
- Date: 2019
- Subjects: Calibration , Radio astronomy -- Data processing , Radio astronomy -- South Africa , Karoo Array Telescope (South Africa) , Radio telescopes -- South Africa , Common Astronomy Software Application (Computer software)
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/97096 , vital:31398
- Description: The applications of machine learning have created an opportunity to deal with complex problems currently encountered in radio astronomy data processing. Calibration is one of the most important data processing steps required to produce high dynamic range images. This process involves the determination of calibration parameters, both instrumental and astronomical, to correct the collected data. Typically, astronomers use a package such as Common Astronomy Software Applications (CASA) to compute the gain solutions based on regular observations of a known calibrator source. In this work we present applications of machine learning to first generation calibration (1GC), using the KAT-7 telescope environmental and pointing sensor data recorded during observations. Applying machine learning to 1GC, as opposed to calculating the gain solutions in CASA, has shown evidence of reducing computation, as well as accurately predict the 1GC gain solutions representing the behaviour of the antenna during an observation. These methods are computationally less expensive, however they have not fully learned to generalise in predicting accurate 1GC solutions by looking at environmental and pointing sensors. We call this multi-output regression model ZCal, which is based on random forest, decision trees, extremely randomized trees and K-nearest neighbor algorithms. The prediction error obtained during the testing of our model on testing data is ≈ 0.01 < rmse < 0.09 for gain amplitude per antenna, and 0.2 rad < rmse <0.5 rad for gain phase. This shows that the instrumental parameters used to train our model more strongly correlate with gain amplitude effects than phase.
- Full Text:
- Date Issued: 2019
- Authors: Zitha, Simphiwe Nhlanhla
- Date: 2019
- Subjects: Calibration , Radio astronomy -- Data processing , Radio astronomy -- South Africa , Karoo Array Telescope (South Africa) , Radio telescopes -- South Africa , Common Astronomy Software Application (Computer software)
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/97096 , vital:31398
- Description: The applications of machine learning have created an opportunity to deal with complex problems currently encountered in radio astronomy data processing. Calibration is one of the most important data processing steps required to produce high dynamic range images. This process involves the determination of calibration parameters, both instrumental and astronomical, to correct the collected data. Typically, astronomers use a package such as Common Astronomy Software Applications (CASA) to compute the gain solutions based on regular observations of a known calibrator source. In this work we present applications of machine learning to first generation calibration (1GC), using the KAT-7 telescope environmental and pointing sensor data recorded during observations. Applying machine learning to 1GC, as opposed to calculating the gain solutions in CASA, has shown evidence of reducing computation, as well as accurately predict the 1GC gain solutions representing the behaviour of the antenna during an observation. These methods are computationally less expensive, however they have not fully learned to generalise in predicting accurate 1GC solutions by looking at environmental and pointing sensors. We call this multi-output regression model ZCal, which is based on random forest, decision trees, extremely randomized trees and K-nearest neighbor algorithms. The prediction error obtained during the testing of our model on testing data is ≈ 0.01 < rmse < 0.09 for gain amplitude per antenna, and 0.2 rad < rmse <0.5 rad for gain phase. This shows that the instrumental parameters used to train our model more strongly correlate with gain amplitude effects than phase.
- Full Text:
- Date Issued: 2019
Designing and implementing a new pulsar timer for the Hartebeesthoek Radio Astronomy Observatory
- 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.
- Full Text:
- Date Issued: 2008
- 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.
- Full Text:
- Date Issued: 2008
Radio observation of the Gum Nebula Region
- Authors: Woermann, Beate
- Date: 1997
- Subjects: Radio sources (Astronomy) Nebulae -- Observations Radio astronomy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5485 , http://hdl.handle.net/10962/d1005271
- Description: This thesis describes the results of an investigation of the physical properties of the Gum Nebula. For this investigation a radio continuum map of the region was made at 2326 MHz and resolution ⅓° with the HartRAO antenna. This map was used to generate spectral index images and an infrared to radio flux density ratio (IRR) image. The latter image shows that the IRR of the nebula is in the range 20 to 250, identifying it as an old SNR. Several spectral index images of this region were generated using two different methods, one based on the isolation of the nebula from its background radiation, the other based on TT-plots (Turtle et al., 1962). The two methods yield similar results, which show that the nebula has a thermal shell with a non-thermal region in its interior. Below the galactic plane the thermal region dominates and above the plane the nonthermal region. These results suggest a model of an old SNR with an H II region shell. Spectral line observations of hydrogen recombination lines and hydroxyl (OH) were made with the HartRAO and the Mopra telescopes. The detection of hydrogen recombination lines at four positions in the thermal regions of the nebula give electron temperatures and emission measures in the ranges 4000 to 6000 K and 220 to 460 pc.cm⁻⁶ respectively. The turbulent velocities are of the order of 20 km/s. A search for shocked OH lines at 1667 MHz and 1720 MHz in the Gum Nebula gave results that were negative, but numerous unshocked 1667 MHz OH lines were detected. The latter were used in a test for an expansion of the nebula. The most plausible fit to the data gives an expansion centre at l = 260.5°, b = -2.5° and at a distance of 0.7 kpc from us. The front face angular radius and expansion velocity are 10.5° and 16 km/s respectively. The back face angular radius and expansion velocity are 8.50 and 7 km/s respectively.
- Full Text:
- Date Issued: 1997
- Authors: Woermann, Beate
- Date: 1997
- Subjects: Radio sources (Astronomy) Nebulae -- Observations Radio astronomy
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5485 , http://hdl.handle.net/10962/d1005271
- Description: This thesis describes the results of an investigation of the physical properties of the Gum Nebula. For this investigation a radio continuum map of the region was made at 2326 MHz and resolution ⅓° with the HartRAO antenna. This map was used to generate spectral index images and an infrared to radio flux density ratio (IRR) image. The latter image shows that the IRR of the nebula is in the range 20 to 250, identifying it as an old SNR. Several spectral index images of this region were generated using two different methods, one based on the isolation of the nebula from its background radiation, the other based on TT-plots (Turtle et al., 1962). The two methods yield similar results, which show that the nebula has a thermal shell with a non-thermal region in its interior. Below the galactic plane the thermal region dominates and above the plane the nonthermal region. These results suggest a model of an old SNR with an H II region shell. Spectral line observations of hydrogen recombination lines and hydroxyl (OH) were made with the HartRAO and the Mopra telescopes. The detection of hydrogen recombination lines at four positions in the thermal regions of the nebula give electron temperatures and emission measures in the ranges 4000 to 6000 K and 220 to 460 pc.cm⁻⁶ respectively. The turbulent velocities are of the order of 20 km/s. A search for shocked OH lines at 1667 MHz and 1720 MHz in the Gum Nebula gave results that were negative, but numerous unshocked 1667 MHz OH lines were detected. The latter were used in a test for an expansion of the nebula. The most plausible fit to the data gives an expansion centre at l = 260.5°, b = -2.5° and at a distance of 0.7 kpc from us. The front face angular radius and expansion velocity are 10.5° and 16 km/s respectively. The back face angular radius and expansion velocity are 8.50 and 7 km/s respectively.
- Full Text:
- Date Issued: 1997
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
- 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
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
- 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
Single station TEC modelling during storm conditions
- Authors: Uwamahoro, Jean Claude
- Date: 2016
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/3812 , vital:20545
- Description: It has been shown in ionospheric research that modelling total electron content (TEC) during storm conditions is a big challenge. In this study, mathematical equations were developed to estimate TEC over Sutherland (32.38oS, 20.81oE), during storm conditions, using the Empirical Orthogonal Function (EOF) analysis, combined with regression analysis. TEC was derived from GPS observations and a geomagnetic storm was defined for Dst ≤ -50 nT. The inputs for the model were chosen based on the factors that influence TEC variation, such as diurnal, seasonal, solar and geomagnetic activity variation, and these were represented by hour of the day, day number of the year, F10.7 and A index respectively. The EOF model was developed using GPS TEC data from 1999 to 2013 and tested on different storms. For the model validation (interpolation), three storms were chosen in 2000 (solar maximum period) and three others in 2006 (solar minimum period), while for extrapolation six storms including three in 2014 and three in 2015 were chosen. Before building the model, TEC values for the selected 2000 and 2006 storms were removed from the dataset used to construct the model in order to make the model validation independent on data. A comparison of the observed and modelled TEC showed that the EOF model works well for storms with non-significant ionospheric TEC response and storms that occurred during periods of low solar activity. High correlation coefficients between the observed and modelled TEC were obtained showing that the model covers most of the information contained in the observed TEC. Furthermore, it has been shown that the EOF model developed for a specific station may be used to estimate TEC over other locations within a latitudinal and longitudinal coverage of 8.7o and 10.6o respectively. This is an important result as it reduces the data dimensionality problem for computational purposes. It may therefore not be necessary for regional storm-time TEC modelling to compute TEC data for all the closest GPS receiver stations since most of the needed information can be extracted from measurements at one location.
- Full Text:
- Date Issued: 2016
- Authors: Uwamahoro, Jean Claude
- Date: 2016
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/3812 , vital:20545
- Description: It has been shown in ionospheric research that modelling total electron content (TEC) during storm conditions is a big challenge. In this study, mathematical equations were developed to estimate TEC over Sutherland (32.38oS, 20.81oE), during storm conditions, using the Empirical Orthogonal Function (EOF) analysis, combined with regression analysis. TEC was derived from GPS observations and a geomagnetic storm was defined for Dst ≤ -50 nT. The inputs for the model were chosen based on the factors that influence TEC variation, such as diurnal, seasonal, solar and geomagnetic activity variation, and these were represented by hour of the day, day number of the year, F10.7 and A index respectively. The EOF model was developed using GPS TEC data from 1999 to 2013 and tested on different storms. For the model validation (interpolation), three storms were chosen in 2000 (solar maximum period) and three others in 2006 (solar minimum period), while for extrapolation six storms including three in 2014 and three in 2015 were chosen. Before building the model, TEC values for the selected 2000 and 2006 storms were removed from the dataset used to construct the model in order to make the model validation independent on data. A comparison of the observed and modelled TEC showed that the EOF model works well for storms with non-significant ionospheric TEC response and storms that occurred during periods of low solar activity. High correlation coefficients between the observed and modelled TEC were obtained showing that the model covers most of the information contained in the observed TEC. Furthermore, it has been shown that the EOF model developed for a specific station may be used to estimate TEC over other locations within a latitudinal and longitudinal coverage of 8.7o and 10.6o respectively. This is an important result as it reduces the data dimensionality problem for computational purposes. It may therefore not be necessary for regional storm-time TEC modelling to compute TEC data for all the closest GPS receiver stations since most of the needed information can be extracted from measurements at one location.
- Full Text:
- Date Issued: 2016
Modelling storm-time TEC changes using linear and non-linear techniques
- Authors: Uwamahoro, Jean Claude
- Date: 2019
- Subjects: Magnetic storms , Astronomy -- Computer programs , Imaging systems in astronomy , Ionospheric storms , Electrons -- Measurement , Magnetosphere -- Observations
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/92908 , vital:30762
- Description: Statistical models based on empirical orthogonal functions (EOF) analysis and non-linear regression analysis (NLRA) were developed for the purpose of estimating the ionospheric total electron content (TEC) during geomagnetic storms. The well-known least squares method (LSM) and Metropolis-Hastings algorithm (MHA) were used as optimization techniques to determine the unknown coefficients of the developed analytical expressions. Artificial Neural Networks (ANNs), the International Reference Ionosphere (IRI) model, and the Multi-Instrument Data Analysis System (MIDAS) tomographic inversion algorithm were also applied to storm-time TEC modelling/reconstruction for various latitudes of the African sector and surrounding areas. This work presents some of the first statistical modeling of the mid-latitude and low-latitude ionosphere during geomagnetic storms that includes solar, geomagnetic and neutral wind drivers.Development and validation of the empirical models were based on storm-time TEC data derived from the global positioning system (GPS) measurements over ground receivers within Africa and surrounding areas. The storm criterion applied was Dst 6 −50 nT and/or Kp > 4. The performance evaluation of MIDAS compared with ANNs to reconstruct storm-time TEC over the African low- and mid-latitude regions showed that MIDAS and ANNs provide comparable results. Their respective mean absolute error (MAE) values were 4.81 and 4.18 TECU. The ANN model was, however, found to perform 24.37 % better than MIDAS at estimating storm-time TEC for low latitudes, while MIDAS is 13.44 % more accurate than ANN for the mid-latitudes. When their performances are compared with the IRI model, both MIDAS and ANN model were found to provide more accurate storm-time TEC reconstructions for the African low- and mid-latitude regions. A comparative study of the performances of EOF, NLRA, ANN, and IRI models to estimate TEC during geomagnetic storm conditions over various latitudes showed that the ANN model is about 10 %, 26 %, and 58 % more accurate than EOF, NLRA, and IRI models, respectively, while EOF was found to perform 15 %, and 44 % better than NLRA and IRI, respectively. It was further found that the NLRA model is 25 % more accurate than the IRI model. We have also investigated for the first time, the role of meridional neutral winds (from the Horizontal Wind Model) to storm-time TEC modelling in the low latitude, northern and southern hemisphere mid-latitude regions of the African sector, based on ANN models. Statistics have shown that the inclusion of the meridional wind velocity in TEC modelling during geomagnetic storms leads to percentage improvements of about 5 % for the low latitude, 10 % and 5 % for the northern and southern hemisphere mid-latitude regions, respectively. High-latitude storm-induced winds and the inter-hemispheric blows of the meridional winds from summer to winter hemisphere have been suggested to be associated with these improvements.
- Full Text:
- Date Issued: 2019
- Authors: Uwamahoro, Jean Claude
- Date: 2019
- Subjects: Magnetic storms , Astronomy -- Computer programs , Imaging systems in astronomy , Ionospheric storms , Electrons -- Measurement , Magnetosphere -- Observations
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/92908 , vital:30762
- Description: Statistical models based on empirical orthogonal functions (EOF) analysis and non-linear regression analysis (NLRA) were developed for the purpose of estimating the ionospheric total electron content (TEC) during geomagnetic storms. The well-known least squares method (LSM) and Metropolis-Hastings algorithm (MHA) were used as optimization techniques to determine the unknown coefficients of the developed analytical expressions. Artificial Neural Networks (ANNs), the International Reference Ionosphere (IRI) model, and the Multi-Instrument Data Analysis System (MIDAS) tomographic inversion algorithm were also applied to storm-time TEC modelling/reconstruction for various latitudes of the African sector and surrounding areas. This work presents some of the first statistical modeling of the mid-latitude and low-latitude ionosphere during geomagnetic storms that includes solar, geomagnetic and neutral wind drivers.Development and validation of the empirical models were based on storm-time TEC data derived from the global positioning system (GPS) measurements over ground receivers within Africa and surrounding areas. The storm criterion applied was Dst 6 −50 nT and/or Kp > 4. The performance evaluation of MIDAS compared with ANNs to reconstruct storm-time TEC over the African low- and mid-latitude regions showed that MIDAS and ANNs provide comparable results. Their respective mean absolute error (MAE) values were 4.81 and 4.18 TECU. The ANN model was, however, found to perform 24.37 % better than MIDAS at estimating storm-time TEC for low latitudes, while MIDAS is 13.44 % more accurate than ANN for the mid-latitudes. When their performances are compared with the IRI model, both MIDAS and ANN model were found to provide more accurate storm-time TEC reconstructions for the African low- and mid-latitude regions. A comparative study of the performances of EOF, NLRA, ANN, and IRI models to estimate TEC during geomagnetic storm conditions over various latitudes showed that the ANN model is about 10 %, 26 %, and 58 % more accurate than EOF, NLRA, and IRI models, respectively, while EOF was found to perform 15 %, and 44 % better than NLRA and IRI, respectively. It was further found that the NLRA model is 25 % more accurate than the IRI model. We have also investigated for the first time, the role of meridional neutral winds (from the Horizontal Wind Model) to storm-time TEC modelling in the low latitude, northern and southern hemisphere mid-latitude regions of the African sector, based on ANN models. Statistics have shown that the inclusion of the meridional wind velocity in TEC modelling during geomagnetic storms leads to percentage improvements of about 5 % for the low latitude, 10 % and 5 % for the northern and southern hemisphere mid-latitude regions, respectively. High-latitude storm-induced winds and the inter-hemispheric blows of the meridional winds from summer to winter hemisphere have been suggested to be associated with these improvements.
- Full Text:
- Date Issued: 2019
Forecasting solar cycle 24 using neural networks
- Authors: Uwamahoro, Jean
- Date: 2009
- Subjects: Solar cycle , Neural networks (Computer science) , Ionosphere , Ionospheric electron density , Ionospheric forecasting , Solar thermal energy
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5468 , http://hdl.handle.net/10962/d1005253 , Solar cycle , Neural networks (Computer science) , Ionosphere , Ionospheric electron density , Ionospheric forecasting , Solar thermal energy
- Description: The ability to predict the future behavior of solar activity has become of extreme importance due to its effect on the near-Earth environment. Predictions of both the amplitude and timing of the next solar cycle will assist in estimating the various consequences of Space Weather. Several prediction techniques have been applied and have achieved varying degrees of success in the domain of solar activity prediction. These techniques include, for example, neural networks and geomagnetic precursor methods. In this thesis, various neural network based models were developed and the model considered to be optimum was used to estimate the shape and timing of solar cycle 24. Given the recent success of the geomagnetic precusrsor methods, geomagnetic activity as measured by the aa index is considered among the main inputs to the neural network model. The neural network model developed is also provided with the time input parameters defining the year and the month of a particular solar cycle, in order to characterise the temporal behaviour of sunspot number as observed during the last 10 solar cycles. The structure of input-output patterns to the neural network is constructed in such a way that the network learns the relationship between the aa index values of a particular cycle, and the sunspot number values of the following cycle. Assuming January 2008 as the minimum preceding solar cycle 24, the shape and amplitude of solar cycle 24 is estimated in terms of monthly mean and smoothed monthly sunspot number. This new prediction model estimates an average solar cycle 24, with the maximum occurring around June 2012 [± 11 months], with a smoothed monthly maximum sunspot number of 121 ± 9.
- Full Text:
- Date Issued: 2009
- Authors: Uwamahoro, Jean
- Date: 2009
- Subjects: Solar cycle , Neural networks (Computer science) , Ionosphere , Ionospheric electron density , Ionospheric forecasting , Solar thermal energy
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5468 , http://hdl.handle.net/10962/d1005253 , Solar cycle , Neural networks (Computer science) , Ionosphere , Ionospheric electron density , Ionospheric forecasting , Solar thermal energy
- Description: The ability to predict the future behavior of solar activity has become of extreme importance due to its effect on the near-Earth environment. Predictions of both the amplitude and timing of the next solar cycle will assist in estimating the various consequences of Space Weather. Several prediction techniques have been applied and have achieved varying degrees of success in the domain of solar activity prediction. These techniques include, for example, neural networks and geomagnetic precursor methods. In this thesis, various neural network based models were developed and the model considered to be optimum was used to estimate the shape and timing of solar cycle 24. Given the recent success of the geomagnetic precusrsor methods, geomagnetic activity as measured by the aa index is considered among the main inputs to the neural network model. The neural network model developed is also provided with the time input parameters defining the year and the month of a particular solar cycle, in order to characterise the temporal behaviour of sunspot number as observed during the last 10 solar cycles. The structure of input-output patterns to the neural network is constructed in such a way that the network learns the relationship between the aa index values of a particular cycle, and the sunspot number values of the following cycle. Assuming January 2008 as the minimum preceding solar cycle 24, the shape and amplitude of solar cycle 24 is estimated in terms of monthly mean and smoothed monthly sunspot number. This new prediction model estimates an average solar cycle 24, with the maximum occurring around June 2012 [± 11 months], with a smoothed monthly maximum sunspot number of 121 ± 9.
- Full Text:
- Date Issued: 2009
An analysis of sources and predictability of geomagnetic storms
- Authors: Uwamahoro, Jean
- Date: 2011
- Subjects: Ionospheric storms Solar flares Interplanetary magnetic fields Magnetospheric substorms Coronal mass ejections Space environment Neural networks (Computer science)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5451 , http://hdl.handle.net/10962/d1005236
- Description: Solar transient eruptions are the main cause of interplanetary-magnetospheric disturbances leading to the phenomena known as geomagnetic storms. Eruptive solar events such as coronal mass ejections (CMEs) are currently considered the main cause of geomagnetic storms (GMS). GMS are strong perturbations of the Earth’s magnetic field that can affect space-borne and ground-based technological systems. The solar-terrestrial impact on modern technological systems is commonly known as Space Weather. Part of the research study described in this thesis was to investigate and establish a relationship between GMS (periods with Dst ≤ −50 nT) and their associated solar and interplanetary (IP) properties during solar cycle (SC) 23. Solar and IP geoeffective properties associated with or without CMEs were investigated and used to qualitatively characterise both intense and moderate storms. The results of this analysis specifically provide an estimate of the main sources of GMS during an average 11-year solar activity period. This study indicates that during SC 23, the majority of intense GMS (83%) were associated with CMEs, while the non-associated CME storms were dominant among moderate storms. GMS phenomena are the result of a complex and non-linear chaotic system involving the Sun, the IP medium, the magnetosphere and ionosphere, which make the prediction of these phenomena challenging. This thesis also explored the predictability of both the occurrence and strength of GMS. Due to their nonlinear driving mechanisms, the prediction of GMS was attempted by the use of neural network (NN) techniques, known for their non-linear modelling capabilities. To predict the occurrence of storms, a combination of solar and IP parameters were used as inputs in the NN model that proved to predict the occurrence of GMS with a probability of 87%. Using the solar wind (SW) and IP magnetic field (IMF) parameters, a separate NN-based model was developed to predict the storm-time strength as measured by the global Dst and ap geomagnetic indices, as well as by the locally measured K-index. The performance of the models was tested on data sets which were not part of the NN training process. The results obtained indicate that NN models provide a reliable alternative method for empirically predicting the occurrence and strength of GMS on the basis of solar and IP parameters. The demonstrated ability to predict the geoeffectiveness of solar and IP transient events is a key step in the goal towards improving space weather modelling and prediction.
- Full Text:
- Date Issued: 2011
- Authors: Uwamahoro, Jean
- Date: 2011
- Subjects: Ionospheric storms Solar flares Interplanetary magnetic fields Magnetospheric substorms Coronal mass ejections Space environment Neural networks (Computer science)
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5451 , http://hdl.handle.net/10962/d1005236
- Description: Solar transient eruptions are the main cause of interplanetary-magnetospheric disturbances leading to the phenomena known as geomagnetic storms. Eruptive solar events such as coronal mass ejections (CMEs) are currently considered the main cause of geomagnetic storms (GMS). GMS are strong perturbations of the Earth’s magnetic field that can affect space-borne and ground-based technological systems. The solar-terrestrial impact on modern technological systems is commonly known as Space Weather. Part of the research study described in this thesis was to investigate and establish a relationship between GMS (periods with Dst ≤ −50 nT) and their associated solar and interplanetary (IP) properties during solar cycle (SC) 23. Solar and IP geoeffective properties associated with or without CMEs were investigated and used to qualitatively characterise both intense and moderate storms. The results of this analysis specifically provide an estimate of the main sources of GMS during an average 11-year solar activity period. This study indicates that during SC 23, the majority of intense GMS (83%) were associated with CMEs, while the non-associated CME storms were dominant among moderate storms. GMS phenomena are the result of a complex and non-linear chaotic system involving the Sun, the IP medium, the magnetosphere and ionosphere, which make the prediction of these phenomena challenging. This thesis also explored the predictability of both the occurrence and strength of GMS. Due to their nonlinear driving mechanisms, the prediction of GMS was attempted by the use of neural network (NN) techniques, known for their non-linear modelling capabilities. To predict the occurrence of storms, a combination of solar and IP parameters were used as inputs in the NN model that proved to predict the occurrence of GMS with a probability of 87%. Using the solar wind (SW) and IP magnetic field (IMF) parameters, a separate NN-based model was developed to predict the storm-time strength as measured by the global Dst and ap geomagnetic indices, as well as by the locally measured K-index. The performance of the models was tested on data sets which were not part of the NN training process. The results obtained indicate that NN models provide a reliable alternative method for empirically predicting the occurrence and strength of GMS on the basis of solar and IP parameters. The demonstrated ability to predict the geoeffectiveness of solar and IP transient events is a key step in the goal towards improving space weather modelling and prediction.
- Full Text:
- Date Issued: 2011
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
- 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
Properties of traveling ionospheric disturbances (TIDs) over the Western Cape, South Africa
- Authors: Tyalimpi, Vumile Mike
- Date: 2015
- Subjects: Doppler radar , Geographic information systems , Traveling ionospheric disturbances -- south Africa , Ionospheric disturbances -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5557 , http://hdl.handle.net/10962/d1017901
- Description: Travelling Ionospheric Disturbances (TIDs) are said to be produced by atmospheric gravitational waves propagating through the neutral ionosphere. These are smaller in amplitude and period when compared to most ionospheric disturbances and hence more difficult to measure. Very little is known about the properties of the travelling ionospheric disturbances (TIDs) over the Southern Hemisphere regions since studies have been conducted mostly over the Northern Hemisphere regions. This study presents a framework, using a High Frequency (HF) Doppler radar to investigate the physical properties and the possible driving mechanisms of TIDs. This research focuses on studying the characteristics of the TIDs, such as period, velocity and temporal variations, using HF Doppler measurements taken in South Africa. By making use of a Wavelet Analysis technique, the TIDs’ characteristics were determined. A statistical summary on speed and direction of propagation of the observed TIDs was performed. The winter medium scale travelling ionospheric disturbances (MSTIDs) observed are generally faster than the summer MSTIDs. For all seasons, the MSTIDs had a preferred south-southwest direction of propagation. Most of the large scale travelling ionospheric disturbances (LSTIDs) were observed during the night and of these, the spring LSTIDs were fastest when compared to autumn and summer LSTIDs. The general direction of travel of the observed LSTIDs is south-southeast. Total Electron Content (TEC), derived from Global Positioning System (GPS) measurements, were used to validate some of the TID results obtained from the HF Doppler data. The Horizontal Wind Model (HWM07), magnetic K index, and solar terminators were used to determine the possible sources of the observed TIDs. Only 41% of the observed TIDs were successfully linked to their possible sources of excitation. The information gathered from this study will be valuable in future radio communications and will serve as means to improve the existing ionospheric models over the South African region.
- Full Text:
- Date Issued: 2015
- Authors: Tyalimpi, Vumile Mike
- Date: 2015
- Subjects: Doppler radar , Geographic information systems , Traveling ionospheric disturbances -- south Africa , Ionospheric disturbances -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5557 , http://hdl.handle.net/10962/d1017901
- Description: Travelling Ionospheric Disturbances (TIDs) are said to be produced by atmospheric gravitational waves propagating through the neutral ionosphere. These are smaller in amplitude and period when compared to most ionospheric disturbances and hence more difficult to measure. Very little is known about the properties of the travelling ionospheric disturbances (TIDs) over the Southern Hemisphere regions since studies have been conducted mostly over the Northern Hemisphere regions. This study presents a framework, using a High Frequency (HF) Doppler radar to investigate the physical properties and the possible driving mechanisms of TIDs. This research focuses on studying the characteristics of the TIDs, such as period, velocity and temporal variations, using HF Doppler measurements taken in South Africa. By making use of a Wavelet Analysis technique, the TIDs’ characteristics were determined. A statistical summary on speed and direction of propagation of the observed TIDs was performed. The winter medium scale travelling ionospheric disturbances (MSTIDs) observed are generally faster than the summer MSTIDs. For all seasons, the MSTIDs had a preferred south-southwest direction of propagation. Most of the large scale travelling ionospheric disturbances (LSTIDs) were observed during the night and of these, the spring LSTIDs were fastest when compared to autumn and summer LSTIDs. The general direction of travel of the observed LSTIDs is south-southeast. Total Electron Content (TEC), derived from Global Positioning System (GPS) measurements, were used to validate some of the TID results obtained from the HF Doppler data. The Horizontal Wind Model (HWM07), magnetic K index, and solar terminators were used to determine the possible sources of the observed TIDs. Only 41% of the observed TIDs were successfully linked to their possible sources of excitation. The information gathered from this study will be valuable in future radio communications and will serve as means to improve the existing ionospheric models over the South African region.
- Full Text:
- Date Issued: 2015
TiRiFiG, a graphical 3D kinematic modelling tool
- Authors: Twum, Samuel Nyarko
- Date: 2019
- Subjects: Tilted Ring Fitting GUI , Astronomy -- Observations , Galaxies -- Observations , Galaxies -- Measurement , Galaxies -- Measurement -- Data processing , Kinematics
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/76409 , vital:30558
- Description: Galaxy kinematics is of crucial importance to understanding the structure, formation and evolution of galaxies. The studies of mass distributions giving rise to the missing mass problem, first raised by Zwicky (1933), give us an insight into dark matter distributions which are tightly linked to cosmology. Neutral hydrogen (H i) has been widely used as a tracer in the kinematic studies of galaxies. The Square Kilometre Array (SKA) and its precursors will produce large Hi datasets which will require kinematic modelling tools to extract kinematic parameters such as rotation curves. TiRiFiC (Józsa et al., 2007) is an example of such a tool for 3D kinematic modelling of resolved spectroscopic observations of rotating disks in terms of the tilted-ring model with varying complexities. TiRiFiC can be used to model a large number (20+) of parameters which are set in a configuration file (.def) for its execution. However, manually editing these parameters in a text editor is uncomfortable. In this work, we present TiRiFiG, Tilted Ring Fitting GUI, which is the graphical user interface that provides an easy way for parameter inputs to be modified in an interactive manner.
- Full Text:
- Date Issued: 2019
- Authors: Twum, Samuel Nyarko
- Date: 2019
- Subjects: Tilted Ring Fitting GUI , Astronomy -- Observations , Galaxies -- Observations , Galaxies -- Measurement , Galaxies -- Measurement -- Data processing , Kinematics
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/76409 , vital:30558
- Description: Galaxy kinematics is of crucial importance to understanding the structure, formation and evolution of galaxies. The studies of mass distributions giving rise to the missing mass problem, first raised by Zwicky (1933), give us an insight into dark matter distributions which are tightly linked to cosmology. Neutral hydrogen (H i) has been widely used as a tracer in the kinematic studies of galaxies. The Square Kilometre Array (SKA) and its precursors will produce large Hi datasets which will require kinematic modelling tools to extract kinematic parameters such as rotation curves. TiRiFiC (Józsa et al., 2007) is an example of such a tool for 3D kinematic modelling of resolved spectroscopic observations of rotating disks in terms of the tilted-ring model with varying complexities. TiRiFiC can be used to model a large number (20+) of parameters which are set in a configuration file (.def) for its execution. However, manually editing these parameters in a text editor is uncomfortable. In this work, we present TiRiFiG, Tilted Ring Fitting GUI, which is the graphical user interface that provides an easy way for parameter inputs to be modified in an interactive manner.
- Full Text:
- Date Issued: 2019
The development of an ionospheric storm-time index for the South African region
- Authors: Tshisaphungo, Mpho
- Date: 2021-04
- Subjects: Ionospheric storms -- South Africa , Global Positioning System , Neural networks (Computer science) , Regression analysis , Ionosondes , Auroral electrojet , Geomagnetic indexes , Magnetic storms -- South Africa
- Language: English
- Type: thesis , text , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/178409 , vital:42937 , 10.21504/10962/178409
- Description: This thesis presents the development of a regional ionospheric storm-time model which forms the foundation of an index to provide a quick view of the ionospheric storm effects over South African mid-latitude region. The model is based on the foF2 measurements from four South African ionosonde stations. The data coverage for the model development over Grahamstown (33.3◦S, 26.5◦E), Hermanus (34.42◦S, 19.22◦E), Louisvale (28.50◦S, 21.20◦E), and Madimbo (22.39◦S, 30.88◦E) is 1996-2016, 2009-2016, 2000-2016, and 2000-2016 respectively. Data from the Global Positioning System (GPS) and radio occultation (RO) technique were used during validation. As the measure of either positive or negative storm effect, the variation of the critical frequency of the F2 layer (foF2) from the monthly median values (denoted as _foF2) is modeled. The modeling of _foF2 is based on only storm time data with the criteria of Dst 6 -50 nT and Kp > 4. The modeling methods used in the study were artificial neural network (ANN), linear regression (LR) and polynomial functions. The approach taken was to first test the modeling techniques on a single station before expanding the study to cover the regional aspect. The single station modeling was developed based on ionosonde data over Grahamstown. The inputs for the model which related to seasonal variation, diurnal variation, geomagnetic activity and solar activity were considered. For the geomagnetic activity, three indices namely; the symmetric disturbance in the horizontal component of the Earth’s magnetic field (SYM − H), the Auroral Electrojet (AE) index and local geomagnetic index A, were included as inputs. The performance of a single station model revealed that, of the three geomagnetic indices, SYM − H index has the largest contribution of 41% and 54% based on ANN and LR techniques respectively. The average correlation coefficients (R) for both ANN and LR models was 0.8, when validated during the selected storms falling within the period of model development. When validated using storms that fall outside the period of model development, the model gave R values of 0.6 and 0.5 for ANN and LR respectively. In addition, the GPS total electron content (TEC) derived measurements were used to estimate foF2 data. This is because there are more GPS receivers than ionosonde locations and the utilisation of this data increases the spatial coverage of the regional model. The estimation of foF2 from GPS TEC was done at GPS-ionosonde co-locations using polynomial functions. The average R values of 0.69 and 0.65 were obtained between actual and derived _foF2 over the co-locations and other GPS stations respectively. Validation of GPS TEC derived foF2 with RO data over regions out of ionospheric pierce points coverage with respect to ionosonde locations gave R greater than 0.9 for the selected storm period of 4-8 August 2011. The regional storm-time model was then developed based on the ANN technique using the four South African ionosonde stations. The maximum and minimum R values of 0.6 and 0.5 were obtained over ionosonde and GPS locations respectively. This model forms the basis towards the regional ionospheric storm-time index. , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2021
- Full Text:
- Date Issued: 2021-04
- Authors: Tshisaphungo, Mpho
- Date: 2021-04
- Subjects: Ionospheric storms -- South Africa , Global Positioning System , Neural networks (Computer science) , Regression analysis , Ionosondes , Auroral electrojet , Geomagnetic indexes , Magnetic storms -- South Africa
- Language: English
- Type: thesis , text , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/178409 , vital:42937 , 10.21504/10962/178409
- Description: This thesis presents the development of a regional ionospheric storm-time model which forms the foundation of an index to provide a quick view of the ionospheric storm effects over South African mid-latitude region. The model is based on the foF2 measurements from four South African ionosonde stations. The data coverage for the model development over Grahamstown (33.3◦S, 26.5◦E), Hermanus (34.42◦S, 19.22◦E), Louisvale (28.50◦S, 21.20◦E), and Madimbo (22.39◦S, 30.88◦E) is 1996-2016, 2009-2016, 2000-2016, and 2000-2016 respectively. Data from the Global Positioning System (GPS) and radio occultation (RO) technique were used during validation. As the measure of either positive or negative storm effect, the variation of the critical frequency of the F2 layer (foF2) from the monthly median values (denoted as _foF2) is modeled. The modeling of _foF2 is based on only storm time data with the criteria of Dst 6 -50 nT and Kp > 4. The modeling methods used in the study were artificial neural network (ANN), linear regression (LR) and polynomial functions. The approach taken was to first test the modeling techniques on a single station before expanding the study to cover the regional aspect. The single station modeling was developed based on ionosonde data over Grahamstown. The inputs for the model which related to seasonal variation, diurnal variation, geomagnetic activity and solar activity were considered. For the geomagnetic activity, three indices namely; the symmetric disturbance in the horizontal component of the Earth’s magnetic field (SYM − H), the Auroral Electrojet (AE) index and local geomagnetic index A, were included as inputs. The performance of a single station model revealed that, of the three geomagnetic indices, SYM − H index has the largest contribution of 41% and 54% based on ANN and LR techniques respectively. The average correlation coefficients (R) for both ANN and LR models was 0.8, when validated during the selected storms falling within the period of model development. When validated using storms that fall outside the period of model development, the model gave R values of 0.6 and 0.5 for ANN and LR respectively. In addition, the GPS total electron content (TEC) derived measurements were used to estimate foF2 data. This is because there are more GPS receivers than ionosonde locations and the utilisation of this data increases the spatial coverage of the regional model. The estimation of foF2 from GPS TEC was done at GPS-ionosonde co-locations using polynomial functions. The average R values of 0.69 and 0.65 were obtained between actual and derived _foF2 over the co-locations and other GPS stations respectively. Validation of GPS TEC derived foF2 with RO data over regions out of ionospheric pierce points coverage with respect to ionosonde locations gave R greater than 0.9 for the selected storm period of 4-8 August 2011. The regional storm-time model was then developed based on the ANN technique using the four South African ionosonde stations. The maximum and minimum R values of 0.6 and 0.5 were obtained over ionosonde and GPS locations respectively. This model forms the basis towards the regional ionospheric storm-time index. , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2021
- Full Text:
- Date Issued: 2021-04
M3: Mining Mini-Halos with MeerKAT
- Authors: Trehaeven, Keegan Somerset
- Date: 2023-10-13
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424754 , vital:72181
- Description: This work aims to showcase the MeerKAT telescope’s capabilities and related calibration and imaging software in studying the emission of radio mini-halos. These diffuse radio synchrotron sources surround a Brightest Cluster Galaxy (BCG) in relatively relaxed clusters out to a few 100 kpc in size. They are difficult to image because of their relatively low surface brightness and small angular size. Hence, they could not be studied in great detail by previous generations of radio telescopes and much about their nature, particularly the exact production mechanism, is not yet fully understood. Thus, for the first time, MeerKAT observed a sample of five galaxy clusters to investigate the central radio mini-halo in each. Studying these sources requires the deepest images generated from the data and the effective subtraction of any projected sources obscuring or contaminating the underlying diffuse emission. Therefore, I describe the data reduction used to create third-generation calibrated, primary beam corrected, point source subtracted Stokes I L-band continuum images of these clusters. For first- and second-generation calibration, I use the CARACal pipeline, which implements software optimised explicitly for MeerKAT data. For third-generation calibration, I use the faceted approach of killMS and DDFacet, and then I perform visibility-plane point source subtraction to disentangle the compact and diffuse emissions. I then measured the size, flux density, in-band spectral properties, and radio power of the central mini-halos. I present the first new mini-halo detection by MeerKAT (MACS J2140.2-2339, Trehaeven et al. accepted), the first spectral index maps of these mini-halos, which show very interesting distributions, and a ∼100 kpc II southern extension to the ACO 3444 mini-halo previously unseen in archival VLA data. Thereafter, I present a multi-wavelength case study for two complementary mini-halos from our sample and show via a radio-to-X-ray spatial correlation test that they might be caused by different particle (re)-acceleration mechanisms. Through these initial science results, I have shown that future observations of radio mini-halos with MeerKAT are an exciting prospect that can lead to a better understanding of the fundamental physics behind these sources. , Thesis (MSc) -- Faculty of Science, Physics and Electronics, 2023
- Full Text:
- Date Issued: 2023-10-13
- Authors: Trehaeven, Keegan Somerset
- Date: 2023-10-13
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/424754 , vital:72181
- Description: This work aims to showcase the MeerKAT telescope’s capabilities and related calibration and imaging software in studying the emission of radio mini-halos. These diffuse radio synchrotron sources surround a Brightest Cluster Galaxy (BCG) in relatively relaxed clusters out to a few 100 kpc in size. They are difficult to image because of their relatively low surface brightness and small angular size. Hence, they could not be studied in great detail by previous generations of radio telescopes and much about their nature, particularly the exact production mechanism, is not yet fully understood. Thus, for the first time, MeerKAT observed a sample of five galaxy clusters to investigate the central radio mini-halo in each. Studying these sources requires the deepest images generated from the data and the effective subtraction of any projected sources obscuring or contaminating the underlying diffuse emission. Therefore, I describe the data reduction used to create third-generation calibrated, primary beam corrected, point source subtracted Stokes I L-band continuum images of these clusters. For first- and second-generation calibration, I use the CARACal pipeline, which implements software optimised explicitly for MeerKAT data. For third-generation calibration, I use the faceted approach of killMS and DDFacet, and then I perform visibility-plane point source subtraction to disentangle the compact and diffuse emissions. I then measured the size, flux density, in-band spectral properties, and radio power of the central mini-halos. I present the first new mini-halo detection by MeerKAT (MACS J2140.2-2339, Trehaeven et al. accepted), the first spectral index maps of these mini-halos, which show very interesting distributions, and a ∼100 kpc II southern extension to the ACO 3444 mini-halo previously unseen in archival VLA data. Thereafter, I present a multi-wavelength case study for two complementary mini-halos from our sample and show via a radio-to-X-ray spatial correlation test that they might be caused by different particle (re)-acceleration mechanisms. Through these initial science results, I have shown that future observations of radio mini-halos with MeerKAT are an exciting prospect that can lead to a better understanding of the fundamental physics behind these sources. , Thesis (MSc) -- Faculty of Science, Physics and Electronics, 2023
- Full Text:
- Date Issued: 2023-10-13
Modeling and measurement of torqued procession in radio pulsars
- 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.
- Full Text:
- Date Issued: 2005
- 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.
- Full Text:
- Date Issued: 2005
A statistical study of travelling ionospheric disturbances over the African-European and American sectors
- Authors: Thaganyana, Golekamang Piet
- Date: 2023-03-31
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/422541 , vital:71956 , DOI 10.21504/10962/422543
- Description: This research presents a long-term statistical study of travelling ionospheric disturbances (TIDs) of low- and high-latitude origin over the American and African-European sectors between 2010 and 2018. The TIDs of low latitude origin (hereafter known as poleward TIDs) were studied in both quiet and disturbed conditions, whereas the equatorward TIDs were only studied during quiet conditions. The Kp > 4 and Dst_ -50 nT was used as a criterion for geomagnetic disturbed conditions, while the four geomagnetically quiet days were selected each month based on Kp < 3. Observations of TIDs are made using Global Navigational Satellite Systems (GNSS) total electron content derived data. During quiet conditions, seven and two transhemispheric TIDs were identified over the African-European and American sectors, respectively. The observed TIDs originated from the wintertime hemisphere and propagated into the summertime hemisphere. The horizontal velocity, periods, and horizontal wavelengths of TIDs are in range of cH = 120-274 m/s, 48-80 min and _H = 379-1104 km, respectively. These quiet-time equatorward TIDs have been associated with tertiary gravity waves (GWs) from the dissipation of secondary GWs which are in turn generated from the dissipation of mountain waves (MWs) as a result of excited orographic forcing. The poleward TIDs during geomagnetically quiet conditions over the African and American sectors occur mainly during local daytime. Poleward TIDs were observed mostly in the African-European sector than the American sector. Their horizontal propagation velocities and periods range between 129-280 m/s and 39-70 min over African-European and American sectors. Although the mechanisms responsible for launching quiet-time poleward TIDs have not been established in this study, lower atmospheric processes such as convection systems, sudden stratospheric warming and cold weather fronts may have a role in their generation. During geomagnetic storms in the African sector, almost all poleward TIDs (with the exception of two cases) during the main phase were large-scale with horizontal velocities and periods ranging from 250-503 m/s and 30 min to 2 hours. During recovery phase, poleward TIDs fall under the category of medium scale. In the American sector, the majority of poleward TIDs occurred during the storm's main phase, as opposed to the African-European sector, which experienced a significant number of poleward TIDs during the recovery phase. The periods and horizontal velocities of TIDs range from 45 min-1.5 h and 180-296 m/s during main phase. During the recovery phase, the horizontal velocity and period range from 177-271 m/s and 40-1.5 h, respectively. Overall, it has been shown that statistically, changes in equatorial electrodynamics related to enhanced eastward electric _eld and hence increased equatorial electrojet (vertical E_B drift) correlates highly with the reported poleward TIDs. , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2023
- Full Text:
- Date Issued: 2023-03-31
- Authors: Thaganyana, Golekamang Piet
- Date: 2023-03-31
- Subjects: Uncatalogued
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/422541 , vital:71956 , DOI 10.21504/10962/422543
- Description: This research presents a long-term statistical study of travelling ionospheric disturbances (TIDs) of low- and high-latitude origin over the American and African-European sectors between 2010 and 2018. The TIDs of low latitude origin (hereafter known as poleward TIDs) were studied in both quiet and disturbed conditions, whereas the equatorward TIDs were only studied during quiet conditions. The Kp > 4 and Dst_ -50 nT was used as a criterion for geomagnetic disturbed conditions, while the four geomagnetically quiet days were selected each month based on Kp < 3. Observations of TIDs are made using Global Navigational Satellite Systems (GNSS) total electron content derived data. During quiet conditions, seven and two transhemispheric TIDs were identified over the African-European and American sectors, respectively. The observed TIDs originated from the wintertime hemisphere and propagated into the summertime hemisphere. The horizontal velocity, periods, and horizontal wavelengths of TIDs are in range of cH = 120-274 m/s, 48-80 min and _H = 379-1104 km, respectively. These quiet-time equatorward TIDs have been associated with tertiary gravity waves (GWs) from the dissipation of secondary GWs which are in turn generated from the dissipation of mountain waves (MWs) as a result of excited orographic forcing. The poleward TIDs during geomagnetically quiet conditions over the African and American sectors occur mainly during local daytime. Poleward TIDs were observed mostly in the African-European sector than the American sector. Their horizontal propagation velocities and periods range between 129-280 m/s and 39-70 min over African-European and American sectors. Although the mechanisms responsible for launching quiet-time poleward TIDs have not been established in this study, lower atmospheric processes such as convection systems, sudden stratospheric warming and cold weather fronts may have a role in their generation. During geomagnetic storms in the African sector, almost all poleward TIDs (with the exception of two cases) during the main phase were large-scale with horizontal velocities and periods ranging from 250-503 m/s and 30 min to 2 hours. During recovery phase, poleward TIDs fall under the category of medium scale. In the American sector, the majority of poleward TIDs occurred during the storm's main phase, as opposed to the African-European sector, which experienced a significant number of poleward TIDs during the recovery phase. The periods and horizontal velocities of TIDs range from 45 min-1.5 h and 180-296 m/s during main phase. During the recovery phase, the horizontal velocity and period range from 177-271 m/s and 40-1.5 h, respectively. Overall, it has been shown that statistically, changes in equatorial electrodynamics related to enhanced eastward electric _eld and hence increased equatorial electrojet (vertical E_B drift) correlates highly with the reported poleward TIDs. , Thesis (PhD) -- Faculty of Science, Physics and Electronics, 2023
- Full Text:
- Date Issued: 2023-03-31
Distributed control applications using local area networks: a LAN based power control system at Rhodes University
- 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
- 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
Development of an ionospheric map for Africa
- Authors: Ssessanga, Nicholas
- Date: 2014
- Subjects: Ionosondes Ionosphere Ionosphere -- Observations Ionosphere -- Research -- Africa Ionospheric electron density -- Africa Ionospheric critical frequencies
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5519 , http://hdl.handle.net/10962/d1011498
- Description: This thesis presents research pertaining to the development of an African Ionospheric Map (AIM). An ionospheric map is a computer program that is able to display spatial and temporal representations of ionospheric parameters such as, electron density and critical plasma frequencies, for every geographical location on the map. The purpose of this development was to make the most optimum use of all available data sources, namely ionosondes, satellites and models, and to implement error minimisation techniques in order to obtain the best result at any given location on the African continent. The focus was placed on the accurate estimation of three upper atmosphere parameters which are important for radio communications: critical frequency of the F2 layer (foF2), Total Electron Content (TEC) and the maximum usable frequency over a distance of 3000 km (M3000F2). The results show that AIM provided a more accurate estimation of the three parameters than the internationally recognised and recommended ionosphere model (IRI-2012) when used on its own. Therefore, the AIM is a more accurate solution than single independent data sources for applications requiring ionospheric mapping over the African continent.
- Full Text:
- Date Issued: 2014
- Authors: Ssessanga, Nicholas
- Date: 2014
- Subjects: Ionosondes Ionosphere Ionosphere -- Observations Ionosphere -- Research -- Africa Ionospheric electron density -- Africa Ionospheric critical frequencies
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5519 , http://hdl.handle.net/10962/d1011498
- Description: This thesis presents research pertaining to the development of an African Ionospheric Map (AIM). An ionospheric map is a computer program that is able to display spatial and temporal representations of ionospheric parameters such as, electron density and critical plasma frequencies, for every geographical location on the map. The purpose of this development was to make the most optimum use of all available data sources, namely ionosondes, satellites and models, and to implement error minimisation techniques in order to obtain the best result at any given location on the African continent. The focus was placed on the accurate estimation of three upper atmosphere parameters which are important for radio communications: critical frequency of the F2 layer (foF2), Total Electron Content (TEC) and the maximum usable frequency over a distance of 3000 km (M3000F2). The results show that AIM provided a more accurate estimation of the three parameters than the internationally recognised and recommended ionosphere model (IRI-2012) when used on its own. Therefore, the AIM is a more accurate solution than single independent data sources for applications requiring ionospheric mapping over the African continent.
- Full Text:
- Date Issued: 2014
Investigation into the extended capabilities of the new DPS-4D ionosonde
- Authors: Ssessanga, Nicholas
- Date: 2011
- Subjects: Ionosondes , Ionosphere , Ionosphere -- Observations -- South Africa -- Hermanus (Cape of Good Hope) , Ionosphere -- Research -- South Africa -- Hermanus (Cape of Good Hope)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5472 , http://hdl.handle.net/10962/d1005257 , Ionosondes , Ionosphere , Ionosphere -- Observations -- South Africa -- Hermanus (Cape of Good Hope) , Ionosphere -- Research -- South Africa -- Hermanus (Cape of Good Hope)
- Description: The DPS-4D is the latest version of digital ionosonde developed by the UMLCAR (University of Massachusetts in Lowell Center for Atmospheric Research) in 2008. This new ionosonde has advances in both the hardware and software which allows for the promised advanced capabilities. The aim of this thesis was to present results from an experiment undertaken using the Hermanus DPS-4D (34.4°S 19.2°E, South Africa), the first of this version to be installed globally, to answer a science question outside of the normally expected capabilities of an ionosonde. The science question posed focused on the ability of the DPS-4D to provide information on day-time Pc3 pulsations evident in the ionosphere. Day-time Pc3 ULF waves propagating down through the ionosphere cause oscillations in the Doppler shift of High Frequency (HF) radio transmissions that are correlated with the magnetic pulsations recorded on the ground. Evidence is presented which shows that no correlation exists between the ground magnetic pulsation data and DPS-4D ionospheric data. The conclusion was reached that although the DPS-4D is more advanced in its eld of technology than its predecessors it may not be used to observe Pc3 pulsations.
- Full Text:
- Date Issued: 2011
- Authors: Ssessanga, Nicholas
- Date: 2011
- Subjects: Ionosondes , Ionosphere , Ionosphere -- Observations -- South Africa -- Hermanus (Cape of Good Hope) , Ionosphere -- Research -- South Africa -- Hermanus (Cape of Good Hope)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5472 , http://hdl.handle.net/10962/d1005257 , Ionosondes , Ionosphere , Ionosphere -- Observations -- South Africa -- Hermanus (Cape of Good Hope) , Ionosphere -- Research -- South Africa -- Hermanus (Cape of Good Hope)
- Description: The DPS-4D is the latest version of digital ionosonde developed by the UMLCAR (University of Massachusetts in Lowell Center for Atmospheric Research) in 2008. This new ionosonde has advances in both the hardware and software which allows for the promised advanced capabilities. The aim of this thesis was to present results from an experiment undertaken using the Hermanus DPS-4D (34.4°S 19.2°E, South Africa), the first of this version to be installed globally, to answer a science question outside of the normally expected capabilities of an ionosonde. The science question posed focused on the ability of the DPS-4D to provide information on day-time Pc3 pulsations evident in the ionosphere. Day-time Pc3 ULF waves propagating down through the ionosphere cause oscillations in the Doppler shift of High Frequency (HF) radio transmissions that are correlated with the magnetic pulsations recorded on the ground. Evidence is presented which shows that no correlation exists between the ground magnetic pulsation data and DPS-4D ionospheric data. The conclusion was reached that although the DPS-4D is more advanced in its eld of technology than its predecessors it may not be used to observe Pc3 pulsations.
- Full Text:
- Date Issued: 2011
Addressing flux suppression, radio frequency interference, and selection of optimal solution intervals during radio interferometric calibration
- Authors: Sob, Ulrich Armel Mbou
- Date: 2020
- Subjects: CubiCal (Software) , Radio -- Interference , Imaging systems in astronomy , Algorithms , Astronomical instruments -- Calibration , Astronomy -- Data processing
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/147714 , vital:38663
- Description: The forthcoming Square Kilometre Array is expected to provide answers to some of the most intriguing questions about our Universe. However, as it is already noticeable from MeerKAT and other precursors, the amounts of data produced by these new instruments are significantly challenging to calibrate and image. Calibration of radio interferometric data is usually biased by incomplete sky models and radio frequency interference (RFI) resulting in calibration artefacts that limit the dynamic range and image fidelity of the resulting images. One of the most noticeable of these artefacts is the formation of spurious sources which causes suppression of real emissions. Fortunately, it has been shown that calibration algorithms employing heavy-tailed likelihood functions are less susceptible to this due to their robustness against outliers. Leveraging on recent developments in the field of complex optimisation, we implement a robust calibration algorithm using a Student’s t likelihood function and Wirtinger derivatives. The new algorithm, dubbed the robust solver, is incorporated as a subroutine into the newly released calibration software package CubiCal. We perform statistical analysis on the distribution of visibilities and provide an insight into the functioning of the robust solver and describe different scenarios where it will improve calibration. We use simulations to show that the robust solver effectively reduces the amount of flux suppressed from unmodelled sources both in direction independent and direction dependent calibration. Furthermore, the robust solver is shown to successfully mitigate the effects of low-level RFI when applied to a simulated and a real VLA dataset. Finally, we demonstrate that there are close links between the amount of flux suppressed from sources, the effects of the RFI and the employed solution interval during radio interferometric calibration. Hence, we investigate the effects of solution intervals and the different factors to consider in order to select adequate solution intervals. Furthermore, we propose a practical brute force method for selecting optimal solution intervals. The proposed method is successfully applied to a VLA dataset.
- Full Text:
- Date Issued: 2020
- Authors: Sob, Ulrich Armel Mbou
- Date: 2020
- Subjects: CubiCal (Software) , Radio -- Interference , Imaging systems in astronomy , Algorithms , Astronomical instruments -- Calibration , Astronomy -- Data processing
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/147714 , vital:38663
- Description: The forthcoming Square Kilometre Array is expected to provide answers to some of the most intriguing questions about our Universe. However, as it is already noticeable from MeerKAT and other precursors, the amounts of data produced by these new instruments are significantly challenging to calibrate and image. Calibration of radio interferometric data is usually biased by incomplete sky models and radio frequency interference (RFI) resulting in calibration artefacts that limit the dynamic range and image fidelity of the resulting images. One of the most noticeable of these artefacts is the formation of spurious sources which causes suppression of real emissions. Fortunately, it has been shown that calibration algorithms employing heavy-tailed likelihood functions are less susceptible to this due to their robustness against outliers. Leveraging on recent developments in the field of complex optimisation, we implement a robust calibration algorithm using a Student’s t likelihood function and Wirtinger derivatives. The new algorithm, dubbed the robust solver, is incorporated as a subroutine into the newly released calibration software package CubiCal. We perform statistical analysis on the distribution of visibilities and provide an insight into the functioning of the robust solver and describe different scenarios where it will improve calibration. We use simulations to show that the robust solver effectively reduces the amount of flux suppressed from unmodelled sources both in direction independent and direction dependent calibration. Furthermore, the robust solver is shown to successfully mitigate the effects of low-level RFI when applied to a simulated and a real VLA dataset. Finally, we demonstrate that there are close links between the amount of flux suppressed from sources, the effects of the RFI and the employed solution interval during radio interferometric calibration. Hence, we investigate the effects of solution intervals and the different factors to consider in order to select adequate solution intervals. Furthermore, we propose a practical brute force method for selecting optimal solution intervals. The proposed method is successfully applied to a VLA dataset.
- Full Text:
- Date Issued: 2020