Ionospheric corrections for single-frequency tracking of GNSS satellites by VLBI based on co-located GNSS

Tracking l-band signals of gnss satellites by radio telescopes became a new observation type in recent years and will be used to improve reference system realizations and links between earth- and space-fixed frames. first successful test observations were done, with the drawback of being single-frequency only. in order to correct the ionospheric delay by using gnss phase observations from co-located receivers, the l4r approach was developed. based on residuals derived by a least-squares processing of the gnss geometry-free linear combination corresponding corrections could be derived. as a first validation step l4r corrections were applied to gnss $$l_1$$ data analysis. station coordinate repeatibilities at the 1-cm level were obtained for baselines of a few thousand kilometers. comparing the derived delay corrections to vlbi ionospheric delays for quasars located in same directions, differences with a standard deviation of 2.2 tecu could be achieved.