Optimum design of geodetic VLBI networks and observing strategies

Geodetic VLBI (Very Long Baseline Interferometry) is the principle technique for determining small variations of the spin rate of the Earth, parameters for precession and nutation, and the Celestial Reference Frame realized by the coordinates of extragalactic radio sources. With ever increasing demands on the accuracy, reliability and temporal resolution of these parameters the International VLBI Service for Geodesy and Astrometry (IVS) has to contemplate the optimum design of its networks and observing strategies. Therefore, thorough and systematic simulation studies strongly encouraged by the IVS will be carried out within the project. The investigations will comprise various tasks and options, e.g. observing with multiple antennas at one geodetic VLBI station using the same clock (allowing more than one observation at a time or shorter integration times), the application of one `global` antenna type for all sites, the selection of optimum locations for new radio telescopes to improve the global coverage with VLBI stations, or the question of the cutoff elevation angle. The project will deliver exact numerical results for the accuracies and correlations between the parameters achieved with certain network designs and observing strategies. Since many accuracy assessments presently used to project VLBI onto 2010 are based on pure assumptions, this project is very important for the IVS to find the optimum setup of future geodetic VLBI systems. The simulation studies are based on three programs: the scheduling software SKED, the VLBI software package OCCAM, and a new software VV-SIM (Vienna VLBI-Simulations) which will be freely available. VV-SIM will take the schedules from SKED to e.g. create sky-plots with regard to certain time intervals, and it will process SINEX (Software Indenpendent Exchange) files from OCCAM to interpret and illustrate the results (e.g. formal errors of the parameters or the correlations between them). Besides providing feedback to the IVS regularily, all the experience gathered in the project will be used to schedule a continuous global VLBI campaign planned by the IVS for 2008 (CONT08).

Geodetic Very Long Baseline Interferometry (VLBI) is the unique technique to determine the earth angular velocity, the parameters of nutation and precession and the Celestial Reference Frame (CRF) realized by the positions of extragalactic radio sources. Due to increasing demands on accuracy, reliability, and time resolution of geodetic parameters, the International VLBI Service for Geodesy and Astrometry (IVS) is forced to reconsider the design of their networks and observing strategies. Thus in October 2003 the IVS established Working Group 3 (WG3) `VLBI2010`. As a result of the final report of WG3 (Niell et al., 2005) the VLBI2010 committee (V2C) was established as a permanent institution of the IVS to push the investigations on hardware, software and observing strategies for the next generation VLBI system. The most important strategies proposed by WG3 are to significantly increase the observation density (i.e. the number of observations per 24 hours), densify the station network (especially on the southern hemisphere) and to reduce stochastic errors. This should significantly improve accuracy and will provide a position accuracy at sub-mm level. The high demands on accuracy of space geodetic techniques, such as VLBI or Global Navigation Satellite Systems (GNSS), come from the decision to establish a Global Geodetic Observing System (GGOS) in order to understand the interactions between the various components of System Earth more thoroughly. When reaching the goal of sub-mm accuracy, VLBI will play a key role within GGOS and will make it possible to observe such subtle effects as non-linear station motions or sea- level rise. The FWF project `Optimum design of geodetic VLBI networks and observing strategies` at the Institute of Geodesy and Geophysics of the University of Technology, Vienna, is contributing to the realization of the new generation of VLBI with the simulations of the VLBI2010 system and ensures that the goals of GGOS will be reached. Austria`s contribution to the V2C will strengthen the participation to IVS significantly and will importantly contribute to the new generation VLBI systems and GGOS.