## 3 Practical Examination of the Quality of Ionosphere Models  
In order to test the quality of ionospheric models in difficult observational conditions, a weekly period of 6-12 September 2017 in which the last strong geomagnetic storm took place was selected for research and analysis. Ionospheric storms usually come along with geomagnetic storms, both influencing each other. Many applications relying on radio frequency transmission can be impacted by strong ionospheric perturbations [41]. The culmination of the analysed storm took place on 8 September 2017 and was caused by an X-ray solar flare [42]. This was the most powerful flare of the 24 solar cycle and the strongest flare in a decade. It also ranks within the top 15 flares in records \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Model** & **Used in System** & **Coverage** & **Update Rate** & **Ionospheric Error Elimination** \\ \hline Klobuchar & GPS & global & once a day & 50\%–70\% \\ Klobuchar & BeiDou & regional/global & 2 h & 50\%–70\% \\ NeQuick G & Galileo & regional/global & at least once a day & 55\%–75\% \\ EGNOS & EGNOS & local & 5 min & over 90\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of real-time ionospheric models used in satellite navigation. GPS: Global Positioning System. Figure 2: Example of European Geostationary Navigation Overlay Service (EGNOS) Ionospheric Grid Points mask, in green Ionospheric Grid Points (IGPs) monitored in a regular way. stretching back to the 1970s. In terms of intensity, this storm was categorised as a G4 level storm and was classified on site 20 on the list of the 50 strongest geomagnetic storms [43]. The Kp-index, the Ap-index, as well as the Auroral Electrojet Index (AE-index) are commonly used as the indicators of solar and geomagnetic activity level. During the analysed period, the Kp-index and Ap-index reached 8+106 [44], respectively, while the AE-index reached nearly 2700 nT on 8 September, which can be seen in Figure 3. The Kp-index is an indicator of disturbances in the Earth's magnetic field with an integer in the range 0-9, with 1 being calm and 5 or more indicating a geomagnetic storm. Meanwhile, the Ap-index provides a daily average level for geomagnetic activity and is obtained by averaging the eight 3-h values of the Kp-index for each day. The Auroral Electrojet Index (AE-index) is designed to provide a global, quantitative measure of auroral zone activity associated with solar wind-magnetosphere interactions. The practical research and in-depth analyses were carried out at three permanent IGS stations located in Europe at various latitudes (Figure 4). Due to the inclusion of the EGNOS ionospheric model in the research, all considered stations had to be within the service range of the EGNOS system. The examined stations were located near the Approach Procedures with Vertical guidance (APV-1) availability border. The APV procedures are in the group of the RNAV (Area Navigation) approaches and were designed to make use of both lateral and vertical guidance of an aircraft with the use of GNSS [18]. These locations of the stations allowed not only checking the quality of the considered ionospheric models at various latitudes, but also to check whether the EGNOS ionospheric model will meet the aviation positioning accuracy criteria for the APV-1 approach during a geomagnetic storm. The tests were performed at stations: KIRU (Kiruna, Finland), the northernmost station (Lat. 67.86\({}^{\circ}\), Lon. 20.97\({}^{\circ}\)); LAMA (Lamkowko, Poland), the station at medium latitude (Lat. 53.89\({}^{\circ}\), Lon. 20.67\({}^{\circ}\)), and ISTA (Istanbul, Turkey), the southernmost station (Lat. 41.10\({}^{\circ}\), Lon. 29.02\({}^{\circ}\)) (Figure 4). Figure 4: Location of the examined International GNSS Service (IGS) stations against the theoretical availability of EGNOS Approach with Vertical Guidance (APV-I) service in 2017 [18]. Figure 3: Kp-index, Ap-index, and Auroral Electrojet Index (AE-index) for the analysed periods [44].