Galileo Single Frequency Ionospheric Correction: Performances in Terms of Position

The ionospheric effect remains one of the main factors limiting GNSS accuracy. For GPS single frequency users, this contribution to the error budget is estimated thanks to the well-known Klobuchar algorithm. For Galileo, it will be mitigated by a global algorithm based on the NeQuick model. This algorithm relies on an optimisation procedure called ingestion. In this framework, an "effective ionisation level" Az plays the role of the solar activity information provided to the model in order to fit a specific dataset. For Galileo single frequency operation, daily Az values will be computed from slant Total Electron Content (sTEC) measurements performed within the ground segment and three coefficients will be broadcast to the users within the navigation message allowing them to run the model.
The performance specifications of these algorithms are respectively expressed in terms of delay and TEC but the users might find more interest in their impact on positioning. Hence we propose to investigate their performances in terms of positioning accuracy. To this extent we compare positions of permanent stations calculated with and without the ionospheric correction to the actual ones which are known at the sub-centimetre level. Our simulation uses sTEC generated from Global Ionospheric Maps to provide the effective ionization level coefficients and GPS single frequency code measurements to compute positions. We present results for Brussels station in Belgium (mid-latitudes) and for 2002 (high solar activity level). It gives an interesting insight in the situation we could observe when Galileo reaches its Initial Operation Capability, during the next solar maximum.
This study constitutes a first step in the development of a real-time service in the framework of the SWANS project of the University of Liège and the Royal Meteorological Institute of Belgium. As two Galileo receivers have been bought in this context, this service will be available for the In-Orbit Validation phase of Galileo.