MainWhy is Geodetic VLBI so important for high accuracy geodesy to science and society?High-precision geodesy is central to a broad variety of human activities, including private, commercial and governmental interests, as well as being of broad scientific interest. In everyday life the applications include monitoring of dam and bridge deformation; navigation for commercial airlines; and agriculture (fertilizing by tractor or crop duster). In not-so-every-day life high accuracy geodesy is fundamental to the evaluation of natural hazards with the goal of mitigating the suffering of individuals and reducing the cost of society. Among these applications are monitoring volcano inflation, measuring stress levels for earthquake hazard assessment, and refining our models of sea level change. For the advancement of science, precise geodetic measurements contribute to the fundamental understanding of many aspects of our Earth, including structure and deformations of the crust, mantle, and core, and the magnetic coupling between the inner and outer cores. Modern geodesy relies on space-based observing systems. The three techniques are Very Long Baseline Interferometry (VLBI), the Global Navigation Satellite System (GNSS), and Satellite Laser Ranging (SLR). All three systems provide the basic measurement of the positions of the instruments on the surface of the Earth. These positions define reference frame for the high-accuracy applications described above. In addition, each technique makes unique and complementary contributes to the overall framework. GNSS provides the high surface density and ready availability of reference positions needed for a practical system. SLR most accurately measures the center of mass of the Earth. VLBI provides the orientation of the Earth in intertial space and the celestial reference frame (CRF). Contribution of VLBIVLBI can accurately measure the geodetic parameters associated with the shape of the Earth and orientation in intertial space. This includes the positions and velocities of the sites occupied by VLBI antennas. UT1-UTC, polar motion, and nutation. In addition, of the three space geodetic techniques, VLBI provides the only access to the inertial reference system through observation of the extragalactic sources that form the CRF. The orientation of the Earth in inertial space, as given by UT1-UTC and nutation, is necessary for accurate satellite orbit determination. The scale of the Earth-fixed reference frame is accurately determined by VLBI measurements of the relative positions and velocities of the VLBI antennas on the surface of the Earth. The changes in position are due to the motion of the tectonic plates, to deformation of the crust near faults, to post-glacial rebound, and even to volcanic activity. The inertial frame is defined by quasars billions of light years away which form the volcanic activity. The inertial frame is defined by the two frames by measurements of the rotational position of the Earth (UT1-UTC) and measurement of changes in direction of the spin axis in the inertial frame. A further significant contribution of the VLBI technique is accurate positioning of planetary spacecraft relative to the CRF for interplanetary navigation. A recent application has been the measurement of the change in position of the Huygens's lander of the Cassini mission to determine the velocity of winds in the atmosphere of Saturn's moon Titan [SCtracking05]. Excerpt from: 'IVS Working Group 3 Final Report: "VLBI2010: Current and Future Requirements for the Geodetic VLBI Systems"'. The document is also available in PDF format. The VLBI correlator in BonnHistorical DevelopmentParticipation in astronomically motivated VLBI activities at the Max Planck Institut for Radioastronomy (MPIfR) dates back to the early seventies. The first VLBI processor (a copy of NRAO's 3-baseline MKII correlator) was installed at the institute in 1978. Aready in 1979 the first geodetic observation was correlated under the direction of the Geodetic Institute of the University of Bonn (now Institute for Geodesy and Geoinformation of Bonn University [IGG]). Routine correlation for the European VLBI Network (EVN) began in 1980. The MKII correlator was taken out of operation in 1992. In 1982 a MKIII correlator was aquired from MIT Haystack Obervatory; geodetic correlation switched to the MKIII system in Bonn in 1983. The first correlation of the IRIS series was performed in Bonn in 1984. In 1989 a copy of Haystack's MKIIIA correlator became operational with a capacity of up to 12 baselines. It had been implemented at MPIfR with the 'multi-wire' technique and was taken out of operation in mid-2000. The MKIII correlator remained operational until summer 1999. The first of the geodetic EUROPE observation series was correlated in 1990, the first O'Higgins (Antartika) observation in 1995. In the middle of the 90th the Institute for Applied Geodesy (IFAG - today named Federal Agency for Cartography and Geodesy [BKG]) agreed with the MPIfR to jointly acquire and operate the MKIV correlator. It was installed by Haystack at MPIfR in December 1999. In 1993 MPIfR and BKG signed an MoU to build and operate a MKIV correlator on a 50/50 basis. This fruitful cooperation meanwhile resulted in the new Distributed FX (DiFX) correlator (Deller et al.,2007) which was installed in 2007/2008. It is the first software correlator used at MPIfR. In 2015 it is planned to upgrade the current DiFX cluster to a brand-new system. PresentToday the correlator in Bonn is operated jointly by the Federal Agency for Cartography and Geodesy [BKG] in cooperation with the Institute for Geodesy and Geoinformation of Bonn University [IGG] and the Max Planck Institute for Radio Astronomy [MPIfR]. The correlator is a supercomputer able to compare the signals from a quasar (quasi stellar radio source) arriving at two telescopes and to measure the difference in arrival time of the signals at the telescopes. The difference in arrival time depends on the position of the quasar and the positions of the telescopes. This technique localizes telescopes separated by up to 10000 km with millimetric precision. These positions are used to study the Earth tectonic, the polar motion, Earth rotation, and other geophysical phenomena.
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The VLBI2010 Global Observing System (VGOS)Over the past several years the International VLBI Service for Geodesy and Astrometry (IVS) has been engaged in an effort to modernize all aspects of geodetic VLBI from observing systems and processes to correlation and analysis. The goals of the next generation system are to achieve: 1-mm position accuracy on global scales, continuous measurements of time series of station positions and Earth orientation parameters, and turnaround time to initial geodetic results of less than 24 hours. Strategies for achieving these goals include an increase in the number of observations per day, careful attention to reducing systematic errors, automation of operations and analysis, and increased use of eVLBI, a process whereby data are transmitted from antennas to the correlator electronically. The new VLBI2010 technology involves a complete reworking of the legacy S/X-band systems including the introduction of very fast slewing antennas, broadband observing systems, and a software correlator. At present a network of next generation stations is emerging which has been named the VLBI2010 Global Observing System (VGOS). New fast antennas have been built or are under construction in Australia (Hobart, Yarragadee, Katherine), New Zealand (Warkworth), Germany (Wettzell), Spain/Portugal (Yebes, Canary Islands, Azores), Japan (Tsukuba), and USA (Arecibo). Others have been funded but are not yet under construction. It is widely recognized that VLBI plays an essential role in defining the scale and orientation of global terrestrial reference frames.
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