HartRAO Home > news > CONT08 2008/08/12-26
The 26m Hartebeesthoek telescope is normally used for 24 hours once a week for these measurements.
CONT08 is part of a series of special campaigns to obtain the highest possible quality data for two weeks. These are used to measure a variety of geodetic parameters. One example is the vertical tidal motion of the Earth's surface caused by the Sun and Moon, for comparison with the predictions of numerical models of the effect. Another aim is to measure the Total Electron Content (TEC) of the ionosphere - the upper part of the Earth's atmosphere where the Sun's radiation ionises the gas ("knocks" electrons off the neutral molecules).
More detailed information about the objectives can be found on the website of the International VLBI Service (IVS).
HartRAO was previously involved in the successful CONT02 and CONT05 campaigns.
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The locations of eleven radio telescopes participating in cont08 are shown above. The long baselines in the north-south and east-west directions are essential to maximise the data quality. Only two participating telescopes are located in the southern hemisphere to provide the long north-south baselines - the 6m diameter TIGO near Concepcion in Chile, and the 26m Hartebeesthoek telescope in South Africa.
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HartRAO VLBI Programme Leader Jonathan Quick and VLBI Coordinator Marisa Nickola discuss the progress of CONT08 on August 13. The Mark5A VLBI terminal is in the rack behind them.
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HartRAO electronics technician Lerato Masongwa checks the phase calibration. Lerato and ran the first night session of CONT08 at HartRAO.
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HartRAO Space Geodesy PhD student Roelf Botha enters weather data into the VLBI log after a long night running CONT08.
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Photographed on the morning of 13 August 2008, the Harteebesthoek 26m telescope was tracking quasar 0743+259 for CONT08. In the background, the XDM Karoo Array Telescope prototype was carrying out a receiver stability test while pointing at the South Celestial Pole.
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The NASA MOBLAS-6 satellite laser ranger (SLR) operated by HartRAO was in action as well. Here it is seen tracking the satellite Lageos-1 across the sky, with the 26m telescope (still on 0743+259) in the background. Lageos-1 is a satellite in low Earth orbit whose function is to map the Earth's gravity field. The SLR fires laser pulses at the satellite. These are reflected back towards the SLR by an array of corner reflectors on the satellite, and are received by a large telescope on the SLR. The SLR measures the (constantly changing) travel time of the light to the satellite and back. Thus the distance to the satellite can be deduced, and its orbit determined to within centimetres as its orbital path changes slightly owing to the variations in the Earth's gravity field caused by the continents, oceans and structures below the Earth's surface.
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Inside the SLR control trailer, operator Christina Langa was putting in fine corrections to the tracking of the satellite in order to maximise the return signal strength.
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The third geodetic research instrument located at HartRAO is a Global Positioning System (GPS) basestation, whose antenna is seen in the image above, taken on the morning of 14 August. Its co-location with the 26m telescope and SLR enables their position measurements to be tied together with high precision. However, dual frequency GPS data can be processed to derive additional information.
One parameter that can be derived, from the differential delay between the two GPS frequencies, is the Total Electron Content (TEC) of the ionosphere. Near real-time maps of the global TEC derived from GPS are available from JPL. This map uses data from the GPS basestation at HartRAO, and the HartRAO operated station at the South African Astronomical Observatory near Sutherland in the Karoo.
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A second parameter is the Total Zenith Delay (TZD). From this the total Precipitable Water Vapour (PWV) above the site can be estimated. The SUOMINET website provides charts of these results from the HartRAO GPS receiver. The data for the first few days of CONT08 are shown above.
As CONT08 progressed, technical problems inevitably occurred. The 13cm and 3.5cm wavelength receivers used for these geodetic VLBI experiments are cooled to -257 degrees Celcius using helium gas. The compressors for the helium are located in a room below the antenna. On the evening of Sunday 17 August the helium line to the refrigerators cooling the two receivers starting leaking badly in the telescope structure. Without proper helium flow the receivers started warming up. Rapid action was taken by technician Jacques Grobler who was running the VLBI. He and technician Piet Louw changed the hose and Jonathan Quick started the vacuum pump on the antenna from home via the internet. The situation was retrieved with just a 30 minute gap in observing.
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Jacques Grobler is seen above on August 20 at the top of the north ladder of the 26m telescope, where he was checking for leaks in the helium lines. These are identifiable in the picture by their woven steel outer layer. The lowest helium hose visible was the one that was replaced on Sunday 17 August.
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Helium leaks can also occur in the lines in the cable tray running up the fixed structure. Jacques is just visible near the centre of the picture, behind the top of the cable tray.