Determination of Ionospheric Parameters by Geodetic VLBI

In geodetic VLBI the observations are performed at two frequencies (2.3 and 8.4 GHz) in order to correct for the ionospheric delay. This also allows to get information about the total electron content (TEC) of the ionosphere from VLBI observables. Due to the large frequency separation the accuracy of the individual VLBI TEC measurement is higher than that of GPS. Various VLBI sessions of the CORE-A network were used to determine the TEC. The results were compared with TEC measurements obtained from GPS within IGS. A very good agreement can be observed, in particular for baselines longer than 1000km. The TEC measurements allow to derive ionospheric maps for areas with dense VLBI networks, e.g. over Europe. For that purpose the EUROPE VLBI sessions were used with 7-10 stations observing simultaneously.