VERY LONG BASELINE INTERFEROMETRY (VLBI)Quasars for measuring the EarthVLBI (Very Long Baseline Interferometry) is one of the most accurate methods used to measure the Earth and its orientation in space. With the help of large radio telescopes compact radio sources called quasars are observed and their signals used to estimate the distance between the telescopes.
Because of their great distance from the Earth, quasars form an almost ideal inertial system. With VLBI observations we can determine with great precision irregularities in the rotation of the earth and variations in the direction of the Earth's axis. The Very Long Baseline Radiointerferometer consists in principle of at least two radio telescopes and a central analysis unit called a correlator. At precisely timed epochs, the telescopes simultaneously record radio signals on special tapes which are sent to a central facility to be correlated. In this process the exact differences in the arrival of the signals at each telescope are determined. These time delays are the actual observations used to determine the length and direction of the baselines connecting the telescopes. There are more than 50 radio observatories world wide of which approximately 30 are regularly taking part in the VLBI observing programmes. In Germany, the main geodetic observing station is located at Wettzell in the Bavarian Forest. In addition, about 1 or 2 times a year, the 100m radio telescope of Effelsberg in the northern part of the Eifel also participates. Jointly run by the MPIfR Bonn and the BKG (Bundesamt fuer Kartographie und Geodaesie Frankfurt), the new MkIV correlator in its designed configuration, allows the simultaneous processing of data from 9 VLBI stations. The data analysis and scheduling of observation are mainly carried out by the Geodetic Institut of the University of Bonn. The coordination of the regular VLBI campaigns and the dissemination of the results measuring is carried out by the recently launched IVS (International VLBI Service). The parameters of the Earth rotation as well as the radio observation station coordinates and their temporal changes, are collected, combined and published here. The continuous measurement of the variations of the Earth's rotational speed and orientation of its axis are of great importance for the evolution of our climate, because up to 90% of the effects can be traced back to the influence of the global atmospheric circulation and the tidal currents of the oceans. Based on the outstanding quality of the results obtained in recent years, geodetic VLBI will continue to form the foundation for the long term monitoring of the system Earth and its changes.
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