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
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
Observations of glitches in PSR 0833-45 and 1641-45
- Authors: Flanagan, Claire Susan
- Date: 1996
- Subjects: Pulsars Neutron stars
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5480 , http://hdl.handle.net/10962/d1005266
- Description: An eleven-year series of radio timing observations of 0833- 45 (Vela) and PSR 1641- 45 is presented. During this time, five large spin-ups ("glitches") were observed in 0833- 45 and one in 1641-45. The stellar response to these events is investigated, and the three relat ively long complete inter-glitch intervals in 0833-45 are modeled. The results are of relevance to studies of the interiors of neutron stars. The initial aim of the project - to obtain good observational coverage of large glitches in the Vela pulsar - was successfully achieved, and high quality observations of the periods between glitches were obtained as a by-product. The results of the analysis presented here provide support for the existence of both linear and non-linear coupling in the Vela pulsar, and put a limit on the former in PSR 1641- 45. The recently observed existence of a rapidly recovering component of part of a glitch in Vela was verified in the subsequent glitch, although there is now evidence to contradict the suggestion that this component involves a particular region of the star that is implicated in every glitch. Observations of a recent glitch in the same pulsar have resolved a small component of the spin-up; such a component has not been reported for any other large glitch.
- Full Text:
- Date Issued: 1996
Physics and applications of scintillation detectors
- Authors: Brooks, Francis Dey
- Date: 1996
- Subjects: Nuclear physics Scintillation counters Nuclear counters Detectors
- Language: English
- Type: Thesis , Doctoral , DSc
- Identifier: vital:5482 , http://hdl.handle.net/10962/d1005268
- Description: The papers submitted in this volume present contributions and reviews on the physics of the scintillation process together with contributions to the development of scintillation detection techniques and the use of these techniques in nuclear physics research and in the applications of nuclear methods to other fields.
- Full Text:
- Date Issued: 1996
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
The effects of particle precipitation on the ionosphere in the South Atlantic Anomaly Region
- Authors: Haggard, Raymond
- Date: 1994
- Subjects: Precipitation (Chemistry) Ionosphere Ionospheric electron density
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5463 , http://hdl.handle.net/10962/d1005248
- Description: The first ground based observations of aeronomic phenomena in the South Atlantic Anomaly Region are presented. These data show that enhancements in foF2 and foE can be directly attributed to precipitated electron energy fluxes in the Anomaly Region. The regular occurrence of particle induced sporadic-E ionization is also presented together with the first measurable 391.4 nm airglow radiation of about 16 R. The first comprehensive survey of energy fluxes carried by energetic particles using satellites is also presented for both daytime and nighttime as well as the seasonal fluctuations. We found that the nocturnally precipitated electron energy fluxes varied between 1 x 10⁻⁴ and 38 x 10⁻⁴ erg cm²s⁻¹, depending upon magnetic activity and season, whereas the daytime precipitated electron energy fluxes tended to vary between 1 x 10⁻³ and 8 x 10⁻³ erg cm⁻²s⁻¹, with a tendency to decrease during magnetically active periods. Electron density and temperature contours as well as NO⁺ and 0⁺ ions contours for nighttime are also presented. The main conclusion of the study is that precipitating electrons provide a significant and sometimes dominant source of ionization in the ionosphere over the South Atlantic Anomaly Region.
- Full Text:
- Date Issued: 1994
The search for an ionospheric model suitable for real-time applications in HF radio communications
- Authors: Mercer, Christopher Crossley
- Date: 1994
- Subjects: Ionospheric radio wave propagation
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5488 , http://hdl.handle.net/10962/d1005274 , Ionospheric radio wave propagation
- Description: Statement of work: In essence the research work was to focus on the development of an ionospheric model suitable for real time HF frequency prediction and direction finding applications. The modelling of the ionosphere had to be generic in nature, sufficient to ensure that the CSIR could simultaneously secure commercial competitiveness in each of the three niche market areas aforementioned, while requiring only minimal changes to software architecture in the case of each application. A little research quickly showed that the development of an ionospheric model capable of driving a HFDFSSL system in "real time" would result in one having to make only slight re-structuring of the software to facilitate application of the same model in the areas of real time frequency prediction and spectrum management. The decision made at the outset of the project to slant the research toward the development of a model best suited for HF direction finding applications is reflected in the avenues followed during the course of the modelling process
- Full Text:
- Date Issued: 1994
Aspects of the symplectic and metric geometry of classical and quantum physics
- Authors: Russell, Neil Eric
- Date: 1993
- Subjects: Symplectic manifolds Geometry, Differential Geometric quantization Quantum theory Clifford algebras
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5452 , http://hdl.handle.net/10962/d1005237
- Description: I investigate some algebras and calculi naturally associated with the symplectic and metric Clifford algebras. In particular, I reformulate the well known Lepage decomposition for the symplectic exterior algebra in geometrical form and present some new results relating to the simple subspaces of the decomposition. I then present an analogous decomposition for the symmetric exterior algebra with a metric. Finally, I extend this symmetric exterior algebra into a new calculus for the symmetric differential forms on a pseudo-Riemannian manifold. The importance of this calculus lies in its potential for the description of bosonic systems in Quantum Theory.
- Full Text:
- Date Issued: 1993
Data reduction techniques for Very Long Baseline Interferometric spectropolarimetry
- Authors: Kemball, Athol James
- Date: 1993
- Subjects: Very long baseline interferometry Radio interferometers Data reduction -- Research
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5457 , http://hdl.handle.net/10962/d1005242
- Description: This thesis reports the results of an investigation into techniques for the calibration and imaging of spectral line polarization observations in Very Long Baseline Interferometry (VLBI). A review is given of the instrumental and propagation effects which need to be removed in the course of calibrating such obervations, with particular reference to their polarization dependence. The removal of amplitude and phase errors and the determination of the instrumental feed response is described. The polarization imaging of such data is discussed with particular reference to the case of poorly sampled cross-polarization data. The software implementation of the algorithms within the Astronomical Image Processing System (AlPS) is discussed and the specific case of spectral line polarization reduction for data observed using the MK3 VLBI system is considered in detail. VLBI observations at two separate epochs of the 1612 MHz OH masers towards the source IRC+ 10420 are reduced as part of this work. Spectral line polarization maps of the source structure are presented, including a discussion of source morphology and variability. The source is sigmficantly circularly polarized at VLBI resolution, but does not display appreciable linear polarization. A proper motion study of the circumstellar envelope is presented, which supports an ellipsoidal kinematic model with anisotropic radial outflow. Kinematic modelling of the measured proper motions suggests a distance to the source of ~ 3 kpc. The cirumstellar magnetic field strength in the masing regions is determined as 1-3 mG, assuming Zeeman splitting as the polarization mechanism.
- Full Text:
- Date Issued: 1993
The utilization of tilting-filter photometry in airglow and auroral research
- Authors: Dore, Ian Stuart
- Date: 1992
- Subjects: Photometry -- Research , Airglow -- Research , Auroras -- Research , Auroral photography -- Research
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5494 , http://hdl.handle.net/10962/d1005280 , Photometry -- Research , Airglow -- Research , Auroras -- Research , Auroral photography -- Research
- Description: This thesis describes the application of tilting-filter photometry to the study of the airglow and aurora. Previous South African photometric research is reviewed. Optical instrumentation and techniques used in airglow and auroral research are reviewed. The transmission characteristics of narrowband interference filters are discussed. The analogue meridian-scanning tilting-filter photometry system used at Sanae, Antarctica is described. Shortcomings of this system have been identified, and modifications have been made to improve its spatial and temporal resolution. Details are given of the computer-controlled digital photometry system which replaced the analogue system. Equations are derived for the conversion of raw photometric data (analogue chart deflections or digital photon counts) to absolute emission intensities. The accuracy of the intensities obtained depends on the absolute calibration of the photometer, the transmission characteristics of the filter used to isolate the spectral feature of interest, and the effects of atmospheric extinction and scattering. The influence of these factors on observed emission intensities is discussed. Various models used to determine atmospheric correction factors are reviewed. It is shown that atmospheric correction factors can have a significant effect on both emission intensities and intensity ratios. The procedure used to determine the transmission characteristics of interference filters is described, as is the procedure used to cross-calibrate secondary light sources. The transmission characteristics of the filters and the brightnesses of the light sources were both found to have changed appreciably with age. The observation of a magnetospheric substorm at Sanae (L ≃ 4) is used to illustrate the use of a meridian-scanning tilting-filter photometer system in auroral research. The ratio I(557.7)/ I(391.4) observed at Sanae was found to be lower than expected, as were the OJ airglow emission intensities. A prototype digital photometer system was used aboard a ship, to observe the airglow in the region of the South Atlantic Anomaly. Significant N₂⁺ lNG emissions at 391.4 nm were measured, confirming the presence of discernable particle precipitation in the region. The 0I557.7 and 630.0 nm intensities measured from the ship were found to be lower than expected. This, combined with low airglow and auroral intensities measured at Sanae, is a cause for concern. It is recommended that further checks be made regarding the brightness of the calibration sources.
- Full Text:
- Date Issued: 1992
Computer control of an HF chirp radar
- Authors: Griggs, Desmond Bryan
- Date: 1991
- Subjects: Radar , Radar meteorology , Computerized instruments , Ionosondes
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5455 , http://hdl.handle.net/10962/d1005240 , Radar , Radar meteorology , Computerized instruments , Ionosondes
- Description: This thesis describes the interfacing of an IBM compatible microcomputer to a BR Communications chirp sounder. The need for this is twofold: Firstly for control of the sounder including automatic scheduling of operations, and secondly for data capture. A signal processing card inside the computer performs a Fast Fourier Transform on the sampled data from two phase matched receivers. The transformed data is then transferred to the host computer for further processing, display and storage on hard disk or magnetic tape, all in real time. Critical timing functions are provided by another card in the microcomputer, the timing controller. Built by the author, the design and operation of this sub-system is discussed in detail. Additional circuitry is required to perform antenna and filter switching, and a possible design thereof is also presented by the author. The completed system, comprising the chirp sounder, the PC environment, and the signal switching circuitry, has a dual purpose. It can operate as either a meteor radar, using a fixed frequency (currently 27,99 MHz), or as an advanced chirp ionosonde allowing frequency sweeps from 1,6 to 30 MHz. In the latter case fixed frequency doppler soundings are also possible. Examples of data recorded in the various modes are given.
- Full Text:
- Date Issued: 1991
Radio studies of ionized hydrogen in the southern Milky Way
- Authors: Gaylard, Michael John
- Date: 1990
- Subjects: Interstellar hydrogen Milky Way Astrophysics
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5440 , http://hdl.handle.net/10962/d1001993
- Description: This thesis describes the results of a survey of the HI42ɑ recombination line emission at 2.3 GHz from HII regions in the Southern Milky Way, carried out with the 26 m diameter Hartebeesthoek radio telescope. The Galactic Longitude range covered was 290° to 40°. Single recombination lines were detected from 375 positions. Multiple lines were observed towards 90 positions in the inner Galaxy. No line emission could be detected in 28 positions. Continuum antenna temperatures were estimated from drift scans or radio maps observed for the purpose. LTE electron temperatures and turbulent velocities of the HII regions were calculated where possible. The properties of the sample were compared to those observed in HI09ɑ surveys. The lines observed from over 50 positions were first detections, of which half were associated with optically-identified HII regions. In about 150 cases the lines were only the second to be detected from those HII regions. The processes of the radio emission, detection, and analysis were simulated numerically. The detectability of the emission and the magnitude of non-LTE effects and pressure-broadening in multi-component HII regions was predicted and compared to observations. The radio luminosity function of the HII regions was determined over a range of three orders of magnitude in intrinsic brightness for the first time, using techniques which corrected for different types of incompleteness in the samples. The luminosity function was compared to those in five selected spiral galaxies, and shown to lie between those of M33 and M81. An alternate form of the luminosity function was developed for use with a numerical model of the spiral arm structure of the Milky Way. The physical parameters defining the major spiral arms were established by comparing synthesized diagrams of radial velocity versus Galactic Longitude with those actually observed. The faint, extended HII regions S9 and RCW129 in Scorpius, the Barnard Loop in Orion, and S296 in Canis Major were analyzed, using all available data. All the recombination lines from these HII regions were first detections
- Full Text:
- Date Issued: 1990
The observation of extended sources with the Hartebeesthoek radio telescope
- Authors: Mountfort, Peter Ian
- Date: 1990
- Subjects: Radio telescopes
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5479 , http://hdl.handle.net/10962/d1005265
- Description: The Hartebeesthoek Radio Telescope is well suited to mapping large areas of sky at 2.3 GHz because of the stability and sensitivity of the noise-adding radiometer (Nicolson, 1970) and cryogenic amplifier used at this frequency, the relatively large 20' beam of the 26 m dish antenna, and its high-speed drive capability. Telescope control programs were written for the Observatory's online computer for automated mapping. Effort centred on removing the curved baseline or 'background' from each Declination (Dec) scan, due to atmospheric and ground radiation contributions varying as the antenna is scanned. Initially these backgrounds were measured over a wide range of Hour Angle (HA) for the Dec range of a map, and an interpolated curve subtracted from each on-source scan for its HA. A common base level was established by comparison with drift scans (observed with the antenna stationary). These different observations (on- and off-source Dec scans and drift scans) were combined into one in the Skymap system by performing Dec scans at a fixed starting HA for a period long enough to permit 'cold sky' and the source to drift through. A background formed by fitting a smooth curve through the lowest sample at each Dec provides a consistent relative base level for all the scans in an observation. A high scanning speed is used so that observations may fruitfully be repeated three times and interleaved to build a reliable, fully sampled map. As each observation has its own background removed, it may be made at any HA. For comparison, maps of Upper Scorpio produced by the earlier method (Baart et al., 1980) and the Magellanic Cloud region produced by Skymap (Mountfort et al., 1987) are shown. Skymap provides a simple and flexible mapping method which relies on the stability of the noise-adding radiometer and high-speed repeated scans to produce good maps of large or small extent with little computation. Correction for drift is more difficult than with systems which use intersecting scans, such as the 'nodding' scans used by Haslam et al. (1981) or the Azimuth scans of Reich (1982).
- Full Text:
- Date Issued: 1990
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
The establishment of a Lidar facility at Rhodes University
- Authors: Grant, Richard Peter James Seton
- Date: 1988
- Subjects: Optical radar , LIDAR , Receiver , Transmitter , Photon counting electronics , Aerosol scattering , Temperature profiles , Stratosphere
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5445 , http://hdl.handle.net/10962/d1001998
- Description: LIDAR is the optical equivalent of RADAR. A LIDAR facility has been established at Rhodes University using a flashlamp-pumped dye laser as the transmitter and a photomultiplier tube at the focus of a searchlight mirror as the receiver. The setting up of the receiver and transmitter as well as the design and construction of the photon counting electronics is described. The LIDAR has been used to measure aerosol scattering ratios and temperature profiles in the stratosphere and these results are presented with the algorithms and software used to reduce the data. Finally some recommendations are made for future work
- Full Text:
- Date Issued: 1988
The selection and evaluation of grey-level thresholds applied to digital images
- Authors: Brink, Anton David
- Date: 1988
- Subjects: Image processing -- Digital techniques
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5443 , http://hdl.handle.net/10962/d1001996
- Description: Many applications of image processing require the initial segmentation of the image by means of grey-level thresholding. In this thesis, the problems of automatic threshold selection and evaluation are addressed in order to find a universally applicable thresholding method. Three previously proposed threshold selection techniques are investigated, and two new methods are introduced. The results of applying these methods to several different images are evaluated using two threshold evaluation techniques, one subjective and one quantitative. It is found that no threshold selection technique is universally acceptable, as different methods work best with different images and applications
- Full Text:
- Date Issued: 1988
The development and evaluation of a Nd:YAG laser incorporating an unstable resonator
- Authors: De Kock, Trevor Neil
- Date: 1986
- Subjects: Nd-YAG lasers , Lasers -- Resonators , Laser beams
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5512 , http://hdl.handle.net/10962/d1008566 , Nd-YAG lasers , Lasers -- Resonators , Laser beams
- Description: Introduction: For approximately the last eight years the Laser Section of the National Physical Research Laboratory (NPRL) has been interested in inter alia, pulsed solid-state lasers and in particular, Nd:YAG. Investigations of various resonator types were undertaken with a view to the improvement of the laser parameters such as output energy, pulse width, beam quality and sensitivity to mirror misalignment. In 1980 a Nd: YAG laser employing a rotating prism Q-switch was constructed (Preussler (1980)). It involves rotating one of the two cavity reflectors so that they are parallel for only a brief instant in time. Typically the prism must rotate at a speed of 20 000 r.p.m. to ensure a single pulse output. Such lasers suffer from the tendency to emit multiple pulses, they are very noisy and they require frequent maintenance because of the short lifetime of the bearings. A resonator employing conventional curved mirrors and an electro-optical Q-switch was constructed in 1980 (Robertson & Preussler (1982)). In 1981 an electro-optically Q-swi tched laser making use of a crossed Porro-prism resonator was investigated due to its relative insensitivity to misalignment of the reflectors compared with the conventional mirror resonator (Nortier (1981)). Improvements in terms of output power, beam divergence and beam quality can be achieved by making use of a so-called unstable resonator. Such a laser has been investigated and is reported on in this study. Chapter 2 provides some background into laser theory and operation while chapter 3 deals with the theory of the unstable resonator. Chapter 4 provides details of the experimental equipment and techniques used in the work and chapter 5 discusses the evaluation of the project and results obtained.
- Full Text:
- Date Issued: 1986
Computer control of a barry research chirpsounder
- Authors: Evans, Geoffrey Philip
- Date: 1985
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5507 , http://hdl.handle.net/10962/d1007495
- Description: This thesis describes the design and development of a computer-based controller together with additional hardware that greatly extends the capabilities of a Barry Research VOS-1 Chirpsounder. The measurement of the virtual height of the ionosphere as a function of frequency using pulse- and frequency-modulated carrier wave (FMCW techniques is described and the concept of the so called "digital" ionosonde is introduced. The modifications required for the standard Chirpsounder to perform as a versatile digital chirp ionosonde are discussed. Simplified block diagrams are used to describe the Controller hardware which is fully described in two comprehensive service manuals which have been included as appendices. Important aspects of the Controller software and data storage formats are described in detail. The emphasis is then placed on system capabilities. An operators' software manual which describes system initialization and operation in terms of system commands is included as an appendix. Results of tests at both Grahamstown, South Africa , and at the SANAE base in the Antarctic are presented.
- Full Text:
- Date Issued: 1985
Advanced ionospheric chirpsounding
- Authors: Poole, Allon William Victor
- Date: 1984
- Subjects: Ionospheric sounds Ionosphere -- Research
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5446 , http://hdl.handle.net/10962/d1001999
- Description: This dissertation reports research into the theory and practical application of linear frequency modulated ionospheric sounding, as an alternative to the more usual technique of pulse modulation. A comparison of this technique with that of conventional pulse sounders is given, based on the concepts of matched filters and ambiguity functions for both modulations. A theory is developed to relate the group range and phase velocity of the ionospheric target to the phase and frequency of the difference signal at the receiver output. A method is then described whereby the group range and phase velocity of the reflection point as well as the amplitude, arrival angle and polarisation mode of the reflected energy can be measured. A description of the implementation of the technique is given together with some initial results. Finally, some suggestions for improvements are given
- Full Text:
- Date Issued: 1984
Calibration and interpretation of A 2.3 GHz continuum survey
- Authors: Greybe, Andrew
- Date: 1984
- Subjects: Radio astronomy , Astronomical observatories , Galaxies
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5502 , http://hdl.handle.net/10962/d1007210 , Radio astronomy , Astronomical observatories , Galaxies
- Description: This thesis continues the Rhodes 2.3 GHz Survey of the Southern Sky. It consists of two parts : a data processing part and an astronomical analysis part. In the data processing part the data for the regions 4HR to 15HR, -80° to -61° and 12HR to 23HR . -27° to -7° are presented in contour map format. A beam pattern of the Hartebeesthoek telescope at 13 cm is constructed from drift scans of the radio source TAU A. This is used to investigate the data filtering techniques applied to the Rhodes Survey. It is proposed that a set of widely spaced scans which have been referred to the South Celestial Pole can provide a single calibrated baselevel for the Rhodes Survey. The observing technique and the necessary reduction programs to create a coarse grid of antenna temperatures of the Southern Sky using these observation are developed. Preliminary results for this technique are presented as a map of the region 18HR to 6HR, 90° to 30° with a 5°x5° resolution. On the astronomical side two studies are undertaken. The region 13HR to 23HR, -61° to -7° is searched for large extended areas of emission. 7 features occurring at intermediate galactic latitudes are found. They are interpreted as follows: one of them is the classical HII region surrounding the star Zeta Ophiuchi (l",b")=(6.7°,22.4°), and the rest are combinations of thermal and nonthermal emission from galactic features. The galactic equator profile for 24°> L > -58° is studied. It is dominated by a plateau of emission for L < -26°. This is interpreted as a combination of thermal and nonthermal radiation emitted by a ring of gas symmetric about the galactic centre with a radius of 4 - 6 kpc.
- Full Text:
- Date Issued: 1984