SCHED2010: Next Generation Scheduling fpr VLBI2010

In 2003, the International Very Long Baseline Interferometry (VLBI) Service for Geodesy and Astrometry (IVS) tasked a Working Group (WG3) to begin a process of modernization with the major goal of bridging the gap from the current 5 to 7 mm level of accuracy for VLBI position determination to the new sub 1 mm requirement, nearly one order of magnitude improvement. Based on the final report of WG3 the IVS established the VLBI2010 Committee (V2C) as a permanent body which is supposed to design hardware, software and observing strategies of the next generation VLBI system. As a contribution to V2C, various simulations have been carried out at the Institute of Geodesy and Geophysics (IGG), Vienna. A Monte Carlo Simulator and a corresponding VLBI simulation software VV-SIM were developed to get as realistic answers as possible about the quality of a scheduled VLBI session. These simulations showed that more intensive investigations on scheduling the VLBI observation sessions have to be done to improve the future VLBI system in terms of important criteria such as accuracy, temporal resolution, real time capability, and reliability. In the project proposed here different VLBI scheduling strategies and algorithms will be carefully investigated, the resulting schedules will be thoroughly evaluated and a new scheduling software "SCHED2010" will be generated to exploit the full power of the future VLBI2010 system and to derive the best possible geodetic parameters. In VLBI each antenna observes only one radio source at a particular time and the number of observations and the accuracy of the estimated parameters are directly related to the sequence of the observed sources. Scheduling considerations which will be investigated and implemented in SCHED2010 refer either to the selection of the radio sources or to the specifications of the antennas. The new scheduling software will be mainly based on criteria related to the radio sources such as uniform sky coverage, different source effective flux densities (SEFD), and the use of source structure maps. Various other aspects are focused on the antennas, such as optimizing the schedule for fast moving antennas, combine existing and new antennas to a sound network, and make use of multiple antennas at one site. Also economical and ecological considerations are added to the investigations for SCHED2010 such as antenna maintenance and electric power saving. SCHED2010 will be directly linked to the existing simulation tools at IGG. Dynamical change of the schedule depending on the status of the antennas, radio sources, and record media will be considered for the final implementation of SCHED2010 within the operational tasks of the IVS. It is envisaged to link the new scheduling software to the existing VLBI system and to apply it for operational purposes.

Very Long Baseline Interferometry (VLBI) is a radio astronomical technique in which two or more antennas simultaneously observe the same source of radio emission. The VLBI technique has now been employed in geodesy for over 40 years and it plays an important role for the realization of global geodetic reference frames. In particular, it is a major contributor to the definition of a stable scale of the Terrestrial Reference Frame (TRF) and it has a unique capability to determine the orientation of the Earth in space, an essential element for all positioning and navigation tasks. The next generation VLBI system, called VLBI2010, has been developed with a major goal of bridging the gap from the current 5 mm level of accuracy for VLBI position determination to the new sub 1 mm requirement. With the high accuracy of the VLBI2010 system, subtle effects like nonlinear station motions related to geo-hazards such as earthquakes or sea level rise can be detected and studied. To reach the goals of VLBI2010, various new facets have been investigated within the International VLBI Service for Geodesy and Astrometry (IVS) including small fast-slewing antennas, broadband frequency observations (2-14 GHz), and two or more antennas at a site. In order to exploit the full power of the future VLBI2010 system and to derive the best possible geodetic parameters, a new scheduling package (VIE_SCHED) has been developed within this project, implemented and tested according to the requirements of the VLBI2010 system. Scheduling a VLBI sessions means to decide which radio telescopes record the signals of which radio sources at which time. Due to the multitude of technical parameters and geometrical scenarios these optimization problems does not have a unique solution. VIE_SCHED is part of the software package VieVS (Vienna VLBI Software) and is made available to all registered users. This research takes into consideration all present and future VLBI2010 requirements and specifications. Considering a more uniform network and fast moving antennas, one of the strategies is the so-called new source-based scheduling, which means that the scheduling program selects radio sources from the catalogue without regard for their direct impact on individual stations. The conventional strategy is station-based scheduling, i.e. the sky coverage is optimized in short intervals taking into account the rapid atmospheric variability, partly at the expense of the total number of observations. The research also includes radio source structure models, antenna models, and the considerations of antenna maintenance and power saving mode. Schedules of sites with multiple antennas are also considered. To validate the newly developed scheduling algorithms, thorough and realistic Monte Carlo simulations have been carried out. The comparisons with other VLBI scheduling packages (e.g. SKED software) also show a good agreement. In the last phase of the scheduling research, VIE_SCHED has been accepted by the IVS Coordinating Center for scheduling RD sessions successfully since June 2012.