Development of an integrated model and system to enable optimal efficiency of the HartRAO LLR signal path.
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Date
2017
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Abstract
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.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.