## 3 Practical Examination of the Quality of Ionosphere Models
One of the parameters that help to estimate the ionosphere state change is the vertical total electron content (VTEC). Usually, during a geomagnetic storm, a considerable increase of VTEC can be observed. In-depth analyses of the response of low-mid latitude ionosphere to the large geomagnetic storm in the examined period have been already presented in [45]. In order to create the graphs of VTEC in the tested IGS stations, a special self-developed script in MATLAB was used. The IGS final ionosphere model in IONEX format was acquired from [46]. Although the analysed geomagnetic storm was classified in the G4 category, and the Kp-index reached 8+, Figure 5 clearly shows that the VTEC content over analysed stations was not extremely high, reaching maximum values from 15 (KIRU) to 26 (ISTA). However, the culmination of the VTEC value is clearly visible in Figure 5, and for every examined station, the maximum VTEC values were achieved on 7 September 2017. The next step of the study was to examine in detail the influence of the ionospheric models on the quality of GPS/EGNOS positioning. In all considered cases, the input for position determination was GPS L1 raw observation data with 1 s interval, which was augmented with EGNOS data. EGNOS messages include accurate information on the position of each GPS satellite, the accuracy of the atomic clocks on board the satellites, and up-to-date information about the ionosphere. The EGNOS clock corrections for GPS satellites as well as EGNOS ephemeris corrections were applied in every examined scenario. For the purposes of this research, the changes were only made with the ionosphere model. The Septentrio Post-Processing Software Development Kit (PP-SDK) software, which was used for the calculations, allows for manipulation with the ionospheric model while using the EGNOS clock and ephemeris corrections. Examined options included positioning without any ionospheric model (off) and positioning supported with ionospheric models such as Klobuchar (Klob), NeQuick G (NeQuick) and EGNOS (EGNOS). The analyses were made with the use of the above-mentioned PP-SDK and specialised PP_SBAS_Analyzer software developed by the authors. The self-developed application was created for the analysis of basic quality parameters (accuracy, availability, continuity, and integrity) related to the use of SBAS systems in aviation. In this study, it was utilised to perform data filtering, prepare input files, and analyse output files. PP-SDK is a professional set of tools allowing for detailed analysis of GNSS data collected from various SBAS and GBAS (Ground Based Augmentation System) systems, using official positioning models defined in minimum operational performance standards (MOPS) documents for aviation. This toolset uses only algorithms in accordance with the relevant Radio Technical Committee for Aeronautics (RTCA) guidelines [38], which ensures the user that the results are in line with official specifications. On the other hand, it is not possible to perform calculations that are not consistent with strict aviation requirements. Since using no model (off) or using a NeQuick G model by aeronautical GNSS receivers is not yet standardised, it was not possible to calculate parameters other than accuracy for all the examined options. This is the reason why other parameters that demonstrate the quality of positioning are not presented in this paper. Nevertheless, these parameters, including the integrity, continuity, and availability of the EGNOS model, have already been tested and presented for the considered period by [PERSON] [47]. Figure 5: The mean vertical total electron content (VTEC) over the examined locations during 6–12 September 2017. KIRU: (Kiruna, Finland), the northernmost station (Lat. 67.86, Lon. 20.97); LAMA: (Lamków, Poland), the station at medium latitude (Lat. 53.89, Lon. 20.67), and ISTA: (Istanbul, Turkey), the southernmost station (Lat. 41.10, Lon. 29.02).