Abstract

The Hartebeesthoek Radio Astronomy Observatory (HartRAO) in South Africa is developing an observational system capable of ranging picosecond laser pulses to the retro-reflectors on the Moon, a technique called Lunar Laser Ranger (LLR). The new LLR station in the Southern Hemisphere to be installed at Matjiesfontein in the Karoo, will contribute towards a better understanding of the dynamics of the Earth-Moon system. One of the most fundamental applications of the LLR system is to determine the Earth-Moon orbit parameters at high accuracy levels. As the observable of the LLR technique is the time-of-flight of the laser photons to and from the Moon, a picosecond level (1 picosecond = 10-12 second) accuracy timing system is one of the key design considerations required for the new LLR system. Here were present preliminary results of an integrated design of the timing sub-system for the HartRAO LLR station. All sub-systems that directly affect the timing system performance are described, including operational and environmental conditions. Based on the specifications of the equipment under consideration, preliminary results indicate that the timing sub-system will achieve a sub-picosecond Allan deviation at 1 s, phase noise of -113 dBc at 1 Hz, and a drift rate of 5 × 10-11 per month (free run). Furthermore, the cumulative RMS noise from the proposed system is estimated to be 21 mm per pulse. This noise level will require the station to range to the Moon for a longer period (at a rate of 5 photons per minute) in order to collect adequate photons to statistically achieve millimetre precision for normal point calculation. The current design specifications present a promising future for millimetre ranging at HartRAO.

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