At the Dourbes station, a digisonde 256 is colocated with a Turbo Rogue GPS receiver. Real time processing of the digisonde data gives the electron density profile and the ITEC value (SAO file) for each sounding. The GPS receiver produces data that are treated at the Royal Observatory in order to extract a vertical TEC. Running the wellknown NeQuick ionospheric model allows to compute vertical TEC values. Comparisons of the results obtained in 1996 and 2001 by these different approaches are shown.