Investigation of the ionosphere by geodetic VLBI

Goal of the project is the development of a sound and solid procedure for the determination of ionospheric parameters from Very Long Baseline Interferometry (VLBI) measurements. This procedure should be applicable at any time for any region provided that a sufficient number of VLBI measurements are available for that particular region and period. The ionospheric parameters are generated in terms of: - relative total electron content (TEC) values between two stations; - absolute TEC values at individual stations; - static ionospheric maps over continental regions; - dynamic ionospheric maps. The main steps to be carried out within project VLBIonos will be: - development of a method for the determination of ionospheric parameters from VLBI; - use of measurements done by several external techniques for absolute calibration; - thorough error analysis of the results; - investigation of the effective ionospheric frequencies in VLBI needed for the method; - comparison of the ionospheric results obtained by VLBI with other techniques; - establishment of a procedure for routine determination of ionospheric parameters by the Institute of Geodesy and Geophysics (IGG) VLBI Analysis Center within the International VLBI Service for Geodesy and Astrometry (IVS). Whereas the first five steps need purely scientific investigations the last one will prepare the generation of ionospheric parameters as operational products. This includes the combination and relative weighting of individual submissions. The results of the project will be useful for: - correction of any single-frequency measurements done by VLBI (e.g. for astrophysics), by GPS or other observing techniques that are using radiofrequencies; - comparison of ionospheric parameters obtained by other techniques; - studies of the physics of the upper atmosphere and the solar terrestrial environment.

Precise measurements with dual frequency instruments have a long tradition in geodesy. The Very Long Baseline Interferometry (VLBI) started its routine observations already in the late 70ies and has carried them out until today. The dual frequency receiving systems, operating at X- and S-band, were designed to correct the ionospheric time delays of the signals. In this project the effect of higher order ionospheric terms, which are usually neglected in geodetic analysis, was investigated and it was shown that these corrections are still accurate enough for present-day VLBI systems. In the late 90ies the community around the Global Position System (GPS) showed that parameters of the ionosphere can be deduced from dual frequency satellite measurements. Within this project a method has been developed, which enables estimation of ionosphere parameters in terms of vertical total electron content 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. The obtained results have been cross-validated against GPS, satellite altimetry data, and theoretical models of the ionosphere. In order to compare the findings, we developed routines for computation of local, regional, and global models of the ionosphere from GPS data. 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 reveal the same periods of ionospheric variations. Additionally, these algorithms are already prepared to assimilate VLBI measurements, beside other observation types, to a consistent model of the ionosphere on global scales. 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 investigated. Apart from giving explanations for small biases among the techniques also deficiencies of the theoretical models were discussed. Instrumental biases, a by-product of ionospheric parameter estimation, demonstrate how receiving systems evolved with time, as instrumental changes are absorbed in this parameter. The usage of (fringe) phase information from geodetic VLBI measurements is a new field of research, which can be utilized for the investigation of short period variations (scintillations). A method for the extraction of such disturbances was developed. Dedicated VLBI experiments were successfully carried out with colleagues from Japan, to observe these short period ionospheric disturbances. Therefore, VLBI can be used to monitor both, long-term trends and short period variations of the ionosphere.