An assessment of the Ionospheric Total Electron content estimated from VGOS

In the new VGOS system, the ionospheric delay is handled in a new way. Instead of removing it in the data analysis using an ionospheric free combination of two frequencies, as done for the legacy S/X system, the ionospheric total electron content (TEC) is estimated fringe-fitting along with the group delays and delay rates. This means that the group delays from VGOS are free from ionospheric delays. Nevertheless, it is still interesting to study the ionospheric TEC estimated by VGOS. Firstly, to evaluate if VGOS can be useful for studying the ionosphere. Secondly, because the quality of the VGOS TEC estimates are related to the quality of the other parameters estimated in the fringe-fitting, like the group delays. In this work the TEC from VGOS is investigated. This is done by first estimating station-wise timeseries of vertical TEC (VTEC) and TEC gradients for the VGOS stations. These are then compared to VTEC estimated from co-located GNSS receivers, from co-located legacy VLBI telescopes, and Global Ionospheric Models (GIM). The VGOS sessions from 2019-2025 are used in this study. This provides an interesting period for ionospheric studies, since it begins when the ionospheric activity is low, and ends at a solar maxima with high ionospheric activity.