Development of an Integrated Timing System for the HartRAO Lunar Laser Ranging Station

Abstract

The Hartebeesthoek Radio Astronomy Observatory is currently developing a Lunar Laser Ranger (LLR) station based on a 1 m classical Cassegrain telescope that was donated to the project by the Observatoire de la Côte d’ Azur (OCA) of France. The LLR project at HartRAO currently consists of different subsystems that will be integrated to form a complete functioning system. This integrated system will produce sub-centimetre accuracy ranging data, allowing the distance from the Earth to the Moon to be determined with high accuracy and be used for a number of scientific investigations. Such subsystems include the laser systems (one suitable for Satellite Laser Ranging (SLR), the other for LLR) timing and photon detection systems, data analysis software, steering and control hardware, and integrated software modules such as the pointing model for the telescope. This research project focused on the development of the timing subsystem for this new LLR station. In particular, it involved the determination and evaluation of the oscillator error budget, development of MATLAB scripts for analysis of the rubidium oscillator drift and ageing characteristics as well as GPS analysis of possible multi-path effects. The technique of the LLR involves ranging to the Moon by transmitting short laser pulses (about 0.03 m) from the ranging station to the retro-reflectors that are located on the surface of the Moon. Different models exist that are used to correct for these errors. However, the stability of the local clock forms the basis of ensuring that accurate Time of Flight (ToF) is measured accurately and precisely. This requires picosecond (10-12) level or better accuracy to minimise systematic errors in the ToF measurement. Hence, the aim of this study was to develop an integrated timing system for this new station that will meet this stringent requirement. A 4380A rubidium timing reference system was acquired for the LLR station at HartRAO. This reference timing system has a timing accuracy of less than 10 ns Root Mean Square (RMS), a frequency accuracy of better than 1×10^-13 over a 1 day period. An Allan deviation (locked to the Global Positioning System – (GPS)) of 6×10^-13 at 1 second and a phase noise of -110 dBc/Hz at 1 Hz. It has temperature stability of ~1×10^-12/℃. This timing system is locked to GPS time and updated accordingly to ensure that it is aligned with Universal Coordinated Time (UTC). The installation site for the GPS antenna was preliminary investigated for the extent of multipath from the surrounding features. The GPS antenna requires a stable platform and the location of the antenna must be chosen to minimise the effects of multi-path, which can affect the measured position and as a result affect the consistency of GPS time that is used to update the rubidium clock. Results from this experiment indicate that a cut-off angle of 10º-20º in GPS observations can minimise reflections from the ground and nearby objects. The rubidium 4380A proved to be a stable timing system with predictable clock behaviour over short time intervals. However, to achieve 1 mm ranging precision, a high photon return rate ought to be achieved. The estimated return rate of 5 photons per minute implies that the LLR must range for longer than 30 minutes in order to collect an adequate number of photons to statistically achieve sub-centimetre ranging precision during calculation of the normal points. The new LLR station once completed will be the only station in Africa to be capable of ranging to the retro-reflectors located on the surface of the Moon. It will also improve the current ILRS network by providing data that are important in improving the accuracy of the Moon’s orbit parameters and reduce network dependent biases. This new LLR station will contribute to both local and global communities to meet the scientific objectives of the currently growing space science endeavours by many countries as well as supporting socio-economic developments.