Systematic differences between VTEC obtained by different space-geodetic techniques during CONT08

The ionosphere is a dispersive medium for microwaves, and most space-geodetic techniques using two or more signal frequencies can be applied to extract information on ionospheric parameters, including terrestrial as well as satellite-based GNSS, DORIS, altimetry, and VLBI. Because of their different sensitivity regarding ionization, their different spatial and temporal data distribution, and their different signal paths, a joint analysis of all observation types seems reasonable and promises the best results for ionosphere modeling. However, it has turned out that there exist offsets between ionospheric observations of the diverse techniques mainly caused by calibration uncertainties or model errors. Direct comparisons of the information from different data types are difficult because of the inhomogeneous measurement epochs and locations. In the approach presented here, all measurements are combined into one ionosphere model of vertical total electron content (VTEC). A variance component estimation is applied to take into account the different accuracy levels of the observations. In order to consider systematic offsets, a constant bias term is allowed for each observation group. The investigations have been performed for the time interval of the CONT08 campaign (2 weeks in August 2008) in a region around the Hawaiian Islands. Almost all analyzed observation techniques show good data sensitivity and are suitable for VTEC modeling in case the systematic offsets which can reach up to 5 TECU are taken into account. Only the Envisat DORIS data cannot provide reliable results.