HartRAO Home > news > Hart 15m telescope commissioning - UR1203 - 2012/12/18
Geodetic VLBI is the technique of using radio-loud quasars far out in the Universe as essentially fixed beacons in space thereby allowing networks of radio telescopes to measure their relative positions very accurately. Rapid repeated observations of the sources allow the short term changes in the Earth's axis' rotational rate and polar orientation to be measured very precisely. Long term observations are used to measure effects such as the current motion of the Earth's tectonic plates ("continental drift" on geological timescales).
The converted 15m telescope at HartRAO is well suited for use in the geodetic VLBI experiments that are aimed at providing near real-time measurement of the Earth's rotation rate and polar wobble. The 15m telescope is able to slew from radio source to radio source much faster than the larger 26m telescope at HartRAO. It is also able to observe down to the horizon in all directions, whereas the 26m telescope has a large arc below the South Celestial Pole which it cannot observe owing to its equatorial mount.
As part of the commissioning process for the 15m telescope it has participated in several of these experiments. Most recently it participated in the Ultra-Rapid experiment UR1023 on 2012 December 17-18. Development of the 15m telescope control system had reached the stage that it could be remotely controlled and configured for the VLBI through the internet. Hence it was possible for VLBI Programme Leader Jonathan Quick to set up and run this experiment on a public holiday in South Africa, without physically coming to the observatory.
Other radio telescopes participating in this experiment were the 32m telescope at Tsukuba in Japan, the 20m telescope at Onsala Space Observatory in Sweden and the 12m AuScope telescope at Hobart in Tasmania, Australia. Between each observation, as the telescopes slew to their next target, the recorded data from the observation just completed are streamed through the internet to the correlator at Tsukuba and processed in real-time to obtain interference fringes of the rceived radio emission between each pair of telescopes. From these measurements the Earth rotation rate, known technically as UT1-UTC (or dUT1), and the offset of the direction of the Earth's pole in X and Y coordinates, can be compared to predictions.
Does this have practical relevance? Yes, Global Navigation Satellite Systems (GNSS) such a the US Global Positioning System (GPS) need these polar orientation parameters and Earth rotation rate data to keep positions determined relative to the satellites in orbit linked to true positions on the Earth's surface.
Left click on image for large version. Credit: Michael Gaylard /
HartRAO
HartRAO 26m telescope (left) and 15m telescope (right) at 06h30 UT on Dec 18 during UR1023. The
26m telescope is seen carrying out an independent research programme. The
15m telescope is pointing low in the North-East to observe quasar NRAO512
simultaneously with the Tsukuba telescope in Japan .
The Tsukuba correlator team provide a webpage at http://www.spacegeodesy.go.jp/vlbi/dUT1/ (updated once a minute) of the live status of these ultra-rapid geodetic VLBI experiments. The Tsukuba team also provide a list of past experiments, at http://www.spacegeodesy.go.jp/vlbi/dUT1/sess.html. These experiments initially began live estimation of the variations in the Earth's rotation rate (dUT1) between Onsala and Tsukuba, but expanded to include the two Southern telescopes, thereby also permitting live estimation of the orientation of the Earth's axis of rotation (Xp,Yp). The HartRAO 26m and 15m telescope have been regular participants in these experiments since 2012 August 13.
Left click on image for plots of all four signal bands at X-band. Credit: Tsukuba
Correlator team
The peak in the centre of the noise in the plot above is the interference fringe
obtained from the radio emission of quasar NRAO512 in a simultaneous observation by the Hart 15m telescope and Tsukuba
32m telescope at 06h18 UT on Dec 18. This is one of ten signal bands
recorded between 8.2 and 8.5 GHz. Click on the image for plots of a selection
of four of these X-band channels.
Left click on image for plots of the radio source positions as seen by all four telescopes. Credit: Tsukuba
Correlator team
The 35 hour long UR1023 experiment was planned to include 453 observations by the
HartRAO 15m telescope. A few scans were not observed owing to a technical
problem that was corrected through the internet. The plot above
shows the positions of the radio sources in the sky during each of
the observations by the HartRAO 15m telescope.
Left click on image for full size plot. Credit: Tsukuba
Correlator team
The difference between time provided by Earth rotation (UT1) and time
provided by a uniformly running atomic clock (UTC) is shown above for part
of the UR1023 experiment. The vertical scale is in microseconds.
Left click on image for full size plot. Credit: Tsukuba
Correlator team
The difference between the measured pole position and the predicted pole
position in the X direction is shown above for part
of the UR1023 experiment. The vertical scale is in seconds of arc - there
are 3600 arc seconds per degree of angle.
Left click on image for full size plot. Credit: Tsukuba
Correlator team
The difference between the measured pole position and the predicted pole
position in the Y direction is shown above for part
of the UR1023 experiment. The vertical scale is in seconds of arc.