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. Although the performance specifications of these algorithms are respectively expressed in terms of delay and TEC, the actual 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 Brussels permanent station in Belgium (mid-latitudes) calculated for the year 2002 (high solar activity level) with and without the ionospheric correction to the actual ones which are known at the sub-centimetre level. We obtain different conclusions for vertical and horizontal accuracies: on the one hand, the vertical errors decrease by 50 to 60% with the analysed ionospheric corrections; on the other hand, the horizontal errors decrease at most by 25%. We interpret these results using a fictitious symmetric satellite distribution highlighting the role of TEC gradients in residual errors. Hence we adopt an original point of view for futher investigation of potential alternative ionospheric corrections and we provide an interesting insight in the situation we could observe when Galileo reaches its Initial Operation Capability, during the next solar maximum1.