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General Introduction
Space geodetic techniques such as the Global Navigation Satellite Systems (GNSS), Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI), Lunar Laser Ranging (LLR) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), provide data for scientific research ranging from fundamental physics, geophysics, geodynamics to implementation and maintenance of the International Terrestrial Reference Frame (ITRF) and International Celestial Reference Frame (ICRF). The accuracies of these techniques have increased from metres (during the 1960’s) to sub-centimetre level (1990’s-present). With the increasing accuracies of these techniques, geophysical processes are better understood today, including interplanetary science such as the Earth-Moon system. These improvements have also benefited the public, for example, implementation and maintenance of reference frames such as ITRF.
Geodetic network capacity has been limited, in the Southern Hemisphere, but particularly in Africa, due to high costs involved in commissioning the geodetic instruments and lack of human capacity. However, the network geometry has been improving particularly for GNSS ground-based stations, while the VLBI and LLR stations are still limited. Many efforts have been made to extend the current networks of all the techniques, in particular the Hartebeesthoek Radio Astronomy Observatory (HartRAO) by installing GNSS equipment in Africa, Antarctica, Marion Island and Gough Island. Currently, HartRAO is working on improving the network of all the space geodetic techniques in the Southern Hemisphere.
This includes adding an LLR station and a new geodetic VLBI antenna (historically termed VLBI2010, but is now called VGOS, where VGOS is the acronym for VLBI Global Observing System (GGOS) as well as a Russian Federation SLR system at the observatory.
Above all other space-based techniques, the LLR technique has the weaker geometrical network. At the moment, there are only three LLR active stations i.e. the Apache Point Observatory Lunar Laser-ranging Operation (APOLLO 3.5 m telescope), New Mexico, USA; McDonald Laser Ranging Station (MLRS 0.75 m telescope) currently not supporting LLR due to lack of financial support, Texas, USA and Observatoire de la Côte d’ Azur (OCA 1.54 m Cassegrain telescope), France. There are a few upcoming LLR stations that will increase the network volume i.e. Matera Laser Ranging Observatory (already providing some data), Italy; Geodetic Observatory Wettzell, Germany; Mount Stromlo Satellite Laser Ranging Observatory, Australia and HartRAO, South Africa (source: http://ilrs.gsfc.nasa.gov/).
Chapter 1 General Introduction
1.1. Introduction
1.2. Problem outline
1.3. Research objectives
1.4. Significance of the research
1.5. Challenges in lunar laser ranging technologies
1.6. Thesis outline
1.7. Concluding remarks
Chapter 2 A review of the lunar laser ranging technique
2.1. Introduction
2.2. Historical developments of lunar laser ranging
2.3. Time measurement systems and timescales as applied to LLR
2.4. Basic principles of lunar laser ranging
2.5. External factors contributing to observed time delay
2.5.1. Tidal correction on station position
2.5.2. Tropospheric delay correction
2.5.3. Degradation of lunar retro-reflectors
2.5.4. Timing systems
2.6. Towards understanding O – C parameters and their sources
2.7. Examples of lunar laser ranging systems
2.7.1. Apache Point Observatory Lunar Laser-ranging Operation (APOLLO)
2.7.2. Observatoire de la Côte d’ Azur (OCA)
2.8. Lunar laser ranging contribution to science and society
2.9. Proposed geodetic site at Matjiesfontein
2.9.1. Introductio
2.9.2. Progress made on the ex-OCA 1 m telescope
2.10. Concluding remarks
Chapter 3 Site selection for the lunar laser ranger timing antenna
3.1. Introduction
3.2. Material and methodolog
3.2.1. Study area and data
3.2.2. Methodology
3.3. Results and discussion
3.4. Concluding remarks
Chapter 4 Integration of the lunar laser ranging timing subsystem
4.1. Introduction
4.2. A brief history on time and frequency standards
4.3. Lunar Laser Ranging at HartRAO
4.4. Environmental factors affecting time and frequency transfer
4.4.1. Ionosphere
4.4.2. Troposphere
4.4.3. On site ambient air temperature
4.4.4. Thermal monitoring system for the LLR control room
4.5. Timing subsystem for the lunar laser ranger
4.6. Frequency stability of 4380A rubidium clock
4.7. Photon detection subsystem and signal calibration
4.8. Concluding remarks
Chapter 5 The performance of hydrogen maser clocks at HartRAO
5.1. Introduction
5.2. Hydrogen maser clocks at HartRAO
5.3. Basic principles of the VLBI technique
5.4. Data and analysis method
5.4.1. Long-term data
5.4.2. Short-term data
5.5. Results and discussion
5.6. Concluding remarks
