Geodetic VLBI Results after 26m Telescope Bearing Replacement 2011-05-28

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Geodetic VLBI Results after 26m Telescope Bearing Replacement 2011-05-28

A major motivation in repairing and returning to service the 26m radio telescope at Hartebeesthoek after the main bearing failure in 2008 October was its role in providing regular precise absolute position determination. It did this by operating with other radio telescopes in a global network carrying out geodetic Very Long Baseline Interferometry (VLBI), using distant quasars as fixed reference points in space. These experiments began in 1986 as part of the NASA Crustal Dynamics Project for measuring plate tectonic motion, and provided a 22 year time series of data.

The HartRAO telescope was the only operational radio telescope in Africa with geodetic VLBI capability. Its abolutely referenced position provided an anchor for the co-located NASA MOBLAS-6 Satellite Laser Ranging System (SLR), Global Navigation Satellite System (GNSS) receivers and DORIS satellite orbit determination system. This made HartRAO a key fiducial station for the International Terrestrial Reference Frame (ITRF), and it had also become the reference point for the survey system of South Africa.

Therefore a critical question was how accurately the repaired telescope had been returned to its original position, in order to continue the position time series without a discontinuity. Major mechanical repairs on some other radio telescopes are known to have shifted their positions and introduced offsets in their data time series.

The first post-repair VLBI was for geodetic purposes, carried out on 2010 August 11. Further geodetic VLBI's have been carried out at typically weekly intervals since then. The results of these are now coming in from the six data processing centres of the International VLBI Service for Astrometry and Geodesy (IVS).

Graphs of the baseline lengths between the HartRAO 26m telescope and representative radio telescopes on other continents are shown below, courtesy of the IVS Analysis Coordinator. The baseline lengths change steadily with time owing to the steady movement of the Earth's tectonic plates, and sometimes abruptly when the position of a telescope is shifted by a nearby earthquake at a boundary between plates. An example of the latter is shown below, a consequence of the magnitude 8.8 earthquake in Chile on 2010 February 27.

The graphs reproduced below and data analyses from all the data centres and show that there has been no need to introduce a discontinuity (or "episodic motion" in geodetic VLBI wording) for the HartRAO 26m telescope position after the repair.

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Graph of baseline length between the HartRAO 26m radio telescope and Hobart 26m radio telescope in Tasmania. The data gap between 2008 October and 2010 August was caused by the bearing failure of the HartRAO 26m telescope.

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Graph of baseline length between the HartRAO 26m radio telescope and the Seshan 25m radio telescope in China.

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Graph of baseline length between the HartRAO 26m telescope and the 6m TIGO telescope near Concepcion in Chile. The 2010 earthquake in Chile epicentre was about 115 km distant and changed the baseline from HartRAO to TIGO by 1.864 +/- 0.009 m. Total displacement at TIGO was 3.19m.

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Graph of baseline length between HartRAO 26m radio telescope and the Tsukuba 32m radio telescope in Japan.

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Graph of baseline length between the HartRAO 26m radio telescope and the Westford 18m radio telescope in Massachusetts, United States of America.

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Graph of the baseline length between the HartRAO 26m radio telescope and the Wetzell 20m radio telescope in Germany.