Abstract
Hydrogen maser frequency standards are commonly utilised in various space geodetic techniques such as Very Long Baseline Interferometry (VLBI) as local reference clocks. The Hartebeesthoek Radio Astronomy Observatory in South Africa is currently operating two maser frequency standards i.e., an EFOS28 and an iMaser72 for the 15 m and 26 m VLBI radio telescopes respectively, an older EFOS6 is a standby spare. This study utilised the least-squares method to derive clock parameters, which indicates the performance levels of the masers by making use of the offset measurements obtained between hydrogen maser clock 1 PPS and GNSS 1 PPS for a period of 35 days. The masers were also compared using a frequency comparator (VCH-314) for a time period of 100 s. The results indicate that the performances of both Masers are relatively similar to each other, with short-term and long-term results indicating good agreement. The iMaser72 has a better standard error of 0.0039 μs compared to the standard error of 0.0059 μs for the EFOS28 maser clock. In general, both masers performed at an expected level required for radio astronomy and geodetic VLBI applications. The method used in this study proved to be useful in managing local hydrogen maser clocks to ensure accurate VLBI observations are obtained.
Highlights
Most geodetic techniques rely on the accuracy and stability of their reference clocks; the clock characteristics have a direct influence on precision and accuracy of the observations
Other clocks are mainly used in Satellite Laser Ranging (SLR) or Lunar Laser Ranging (LLR) applications
In the early days of Very Long Baseline Interferometry (VLBI) experiments, the telescopes observing the same source in a network were configured as a Connected Element Interferometer (CEI), where the observing telescopes were connected to a single oscillator by a fibre cable (Edwards et al, 1992)
Summary
Most geodetic techniques rely on the accuracy and stability of their reference clocks; the clock characteristics have a direct influence on precision and accuracy of the observations. Ely et al, (2014) and Seubert and Ely (2015) have reported on a small mercury-ion atomic clock with Allan deviation of less than 10-14 per day for use in deep space applications through the Deep Space Atomic Clock (DSAC) programme of NASA. The technique contributes towards the International Terrestrial Reference Frame (ITRF) by providing positions of the VLBI antennas and the reference frame scale component (Schuh and Böhm 2012) These data products are important for a number of applications such as establishment and maintenance of reference frames and determination of orbit parameters for spacecraft and satellites. The telescopes function within the framework of the International VLBI Service for Astrometry and Geodesy (IVS) and participate in various international programmes They use hydrogen maser clocks as their local reference timing systems. Its manufacturer specifications are 1.26×10-13 at 1 s Allan deviation and it has a thermal stability of 3.5×10-15 / C
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