Precise measurements with dual frequency instruments have a long tradition in geodesy. Among several techniques Very Long Baseline Interferometry (VLBI) started its routine observations already in the late 80ies and has carried them out until today. The dual frequency receiving systems, operating at X- and S-band, were designed to correct ionospheric time delays within geodetic analysis. The potential for ionospheric research of this correction was disregarded until the community around the Global Position System (GPS) showed that parameters of the ionosphere can be deduced from dual frequency satellite measurements. With this thesis a method has been developed, which enables estimation of similar parameters from dual frequency VLBI measurements without any external information. Due to the fact that VLBI is a differential technique, the calculated ionospheric corrections depend on the differences of the propagation media over the stations. Additionally, an instrumental delay offset per station causes a bias of the ionospheric measurements. Within this thesis a method is presented, which is capable of estimating ionospheric parameters, i.e. values of vertical total electron content, from VLBI data. The obtained results are cross-validated against GPS, satellite altimetry data and theoretical models of the ionosphere. As VLBI observations cover more than two complete solar cycles, longer than all other space geodetic techniques using radio signals, the relation to space weather indices on long time-scales can be shown. Generally it can be stated that the overall agreement between VLBI and GPS is within the formal error of each technique and that both systems detect the same periods of ionospheric variations.
But only VLBI is able to reveal long period signals like the solar cycle, since it covers a sufficiently long time-span. Apart from explanations for small biases among the techniques also deficiencies of theoretical models are discussed. Instrumental biases, a by-product of ionospheric parameter estimation, demonstrate how receiving systems evolved with the time, as instrumental changes are absorbed in this parameter. The usage of (fringe) phase information from VLBI measurements is a new and challenging field of research, which can be utilized for the detection of short period variations (scintillations). A method for the extraction of such disturbances is discussed and by an example it is shown that such a short period variation can be detected very precisely. Therefore VLBI can be used to detect both, long-term trends and short period variations of the ionosphere and thus it can contribute to ionospheric research as a new independent technique.