Sibling Radio Telescopes for Geodesy

The upcoming VLBI2010 Geodetic Observing System (VGOS) has the potential to improve the performance of geodetic Very Long Baseline Interferometry (VLBI) by about an order of magnitude. The concept includes co-located radio telescopes, either as two identical twin-telescope dishes or as a sibling telescope, a combination of a large legacy telescope and a new small VGOS antenna. Common observations at one site require new observing modes to be developed, but it also offers new analysis options for improved results. In addition, both telescopes need to be accurately connected to each other. The so-called local tie is essential to connect the present VLBI reference frame to the new VGOS frame, as well as to connect VLBI to the other space geodetic techniques making up the International Terrestrial Reference Frame (ITRF). In project Sibling Telescopes, observations with the Australian AuScope VLBI array, a prototype of the future VGOS system, will be used to optimise observing with sibling telescopes. This comprises new scheduling strategies, improved analysis, and the generation of actual results for the Ho (26m) - Hb (12m) antenna pair in Hobart. Improved planning of the observations will be developed using thorough simulations, answering the question whether it is better to observe the same source or different sources with the sibling telescope. Applying the developed scheduling tool, observations will be planned for the AUSTRAL observing program, using telescopes in Australia, New Zealand, and South Africa. In the analysis of the collected data common parameters will be combined. These are the identical troposphere above the sibling telescope, the parameters of the atomic clock at the station providing the frequency standard to both telescopes, and large-scale antenna movements due to plate tectonics and geodynamical processes. Special attention will be given to the determination of the local tie between the two radio antennas and the comparison between the solution from the VLBI observations and the tie vector determined by local surveying. Precise intra- technique ties will help to improve the inter-technique ties to the other co-located geodetic space techniques, the backbone of the ITRF as a multi-technique solution. The project Sibling Telescopes will be conducted at the University of Tasmania, operating the Australian AuScope VLBI array. The frequent AUSTRAL sessions offer the perfect test bed for the proposed research. During the return phase at the Vienna University of Technology, the findings will be extended to global networks. The developed tools and strategies will allow the optimal usage of the collected data as well as of the upcoming VGOS system, a global multi-million Euro research infrastructure project, targeted to fulfill future scientific and societal demands on Geosciences.

Measuring the Earth using radio telescopes had its breakthrough some 30 years ago, with the first proven measurement of continental plate tectonics. Today, the very long baseline interferometry technique (VLBI) is an important addition to the Earth measuring systems, besides for example the well-known satellite systems GPS or Galileo. The global VLBI system currently undergoes a total renewal, with new telescopes as well as new observing and recording systems expected to increase current accuracies by up to one order of magnitude. In this project, new observing strategies have been developed and applied to extensive measurements with the Australian VLBI telescopes. The results led to first practical proof of certain aspects of this new VGOS system. The host institution, the University of Tasmania, is operating the Australian geodetic VLBI network consisting of three new radio telescopes. While fully compatible with the legacy system, these also already meet some of the VGOS standards. The new antenna capabilities such as faster slew speeds and higher recording rates were first tested and then fully utilised in roughly 150 AUSTRAL observing sessions of 24h duration. The results in terms of the precision of continental baseline lengths could be improved by up to a factor of two. Main aspect of this project was the use of so-called sibling telescopes, where a legacy telescope of high sensitivity but slow slew speed is co-located with one of the new kind having lower sensitivity but high slew speeds. The development of a new observing mode the star mode allowed the sibling telescope in Hobart to perform a unique and novel observing series, as a contribution to the new International Celestial Reference Frame (ICRF3). The very practically designed project plan further allowed improvements in efficient observing and organisation of VLBI experiments. The dynamic observing efforts are currently under development and may change the way how future global VLBI experiments are organised and conducted. In addition, some of the observations are still being analysed, with more results expected in the areas of antenna systematics as well as stability and variations of local aspects of the Hobart sibling telescope.