Twin Telescopes at Onsala and Wettzell and their contribution to the VGOS System

During the last years the International VLBI Service for Geodesy and Astrometry (IVS) spent efforts to improve the accuracy of the geodetic Very Long Baseline Interferometry (VLBI) system to 1 mm for station positions and 0.1 mm/yr for station velocities. To achieve these ambitious goals the VLBI2010 Global Observing System (VGOS) concept was developed, which includes broadband observations with fast-slewing antennas and suggests twin telescopes to reduce the source switching interval and increase the number of observations. Wettzell in Germany has already installed a twin telescope and further twin telescopes will be built in the coming years at Onsala (Sweden) and Ny-Alesund (Spitsbergen, Norway). In this study, the Vienna VLBI Software (VieVS) is used to schedule and simulate a global VLBI network using the sites that participated in the CONT11 campaign. We compare schedules using the legacy telescopes at Onsala and Wettzell with schedules where either one or both legacy telescopes are replaced by VGOS twin telescopes. The scheduling was done with the source based strategy with four sources at a time, and multidirectional mode for the twin telescopes. The evaluation concerns the numbers of observations and scans, as well as baseline length repeatability and atmospheric parameters. A higher number of observations and scans can be seen at all sites for the schedules including twin telescopes. Especially at the sites with twin telescopes the number of scans and observations nearly doubles. Also the Caroline Schonberger, Paul Gnilsen, Johannes Bohm Technische Universitat Wien, Karlsplatz 13, A-1040 Wien, Austria Rudiger Haas Chalmers University of Technology, Onsala Space Observatory, SE-439 92 Onsala, Sweden zenith wet delay (ZWD) estimation improves at sites with twin telescopes. However, so far no significant improvement was found for baseline length repeatabilities and further investigations are ongoing.