Browsing by Author "Ndlovu, Sphumelele Colin."

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Development of an integrated model and system to enable optimal efficiency of the HartRAO LLR signal path.
(2017) Ndlovu, Sphumelele Colin.; Combrinck, Ludwig.; Akombelwa, Mulemwa.; Chetty, Naven.
The Lunar Laser Ranger (LLR) system under development at the Hartebeesthoek Radio Astronomy Observatory (Hartford) in South Africa is being built to accurately measure the Earth-Moon distance (at 1 cm level) through the use of short laser pulses, a single photon detection system, an accurate timing system and other sophisticated components. This LLR system is unique in Africa and indeed in the entire Southern Hemisphere. The system utilizes a 1 m diameter optical telescope, which was donated to the project by the Observatoire de la Côte d’Azur of France. In this work, the author discusses the development of an integrated model that will be utilized to obtain optimal efficiency of the HartRAO-LLR system. The model is used to estimate the expected number of returned photons by considering a number of parameters which affects the laser beam pulses as they traverse the atmosphere from the LLR telescope to the Moon and back to the telescope. Factors such as the apparent Earth-Moon range, atmospheric extinction, laser beam characteristics, optical path efficiencies and others, affect the estimated (predicted by software) and actual (measured) number of returned photons for the HartRAO-LLR station. The estimated average signal return rate (which is dependent on a number of factors) of the HartRAO-LLR ranges between 0 to 12 photons per minute, which is in agreement with the available data from five globally distributed LLR stations. It also correlated with the estimated returns that were obtained using least squares parameter estimations. They were in agreement by an average difference of 0.00272. Our estimated signal returns are strongly affected by two-way atmospheric extinction (atmospheric and cirrus cloud transmissions), variations in the laser beam incident angle on the retroreflectors located on the Moon as well as the varying Earth-Moon range. A new parameter, named lunar reflectivity ranging between 0 and 1, was introduced in the link budget equation to consider the effects of Moon Phases on the returned photons. Modelling the returned number of photons and comparing these to the actual number received leads to an understanding of the effects of numerous variables on the total laser path efficiency. Total system efficiency can be improved as well, as particular atmospheric conditions will not allow LLR to be successful on certain days. For these days, the system can be utilized for other purposes such as maintenance or satellite laser ranging.
Experimental measurement of the fluctuations of a laser beam due to thermal turbulence.
(2013) Ndlovu, Sphumelele Colin.; Chetty, Naven.
In this work, we considered and developed a new method to detect and quantify the fluctuations of a laser beam due to thermal turbulence. The new method consisted of a single laser beam propagating in air and passing through a point diffraction interferometer (PDI). Stable interferograms were thus formed by diffraction of light at the PDI pinhole. Such interferograms underwent phase shifts due to the application of simulated thermal turbulence on the propagating laser beam. These phase shifts were then used to obtain atmospheric turbulence parameters such as the atmospheric turbulence strength, temperature near the propagating beam and the scintillation index. Chapter 1 of this thesis is an introduction and discussion of the theory on the propagation of laser beams in air. Gaussian beam propagation, turbulence detectors, Rytov’s theory and Kolmogorov’s theory of turbulence is also discussed in detail. artefact descriptive experimental procedure is then provided. This chapter focuses on the behavior of a laser beam propagating under the conditions of weak turbulence and relates the Rytov weak fluctuations to the Kolmogorov spectrum since the Rytov variance can be exactly equal to the scintillation index under the conditions of weak turbulence. Two unpublished scientific papers were submitted for publication to the Canadian Journal of Physics and Europian Journal of Remote Sensing. Chapter 2 consist of paper 1 which is based on the development of the experiment and it describes the apparatus in detail as well as it explains the experimental procedure. The preliminary results presented in paper 1 showed that a PDI can produce stable interferograms that can be used to extract the atmospheric turbulence parameters and thus, the PDI method can be used for atmospheric detection and ranging. In chapter 3, we discussed and analysed the experimental results, where the phase shifts were used to estimate the temperature that caused the purturbations on the interferograms. In chapter 4, we concluded about the use of a PDI as a remote sensing technique.