## 5 Conclusions  
The article presents the results of GPS/EGNOS positioning using different ionosphere models at three IGS stations located at different latitudes during a strong ionospheric storm in September 2017. Figure 7: The vertical position error (VPE) in examined locations during 6–12 September 2017. Four different positioning versions--without any ionosphere model (off), with Klobuchar model (Klob), NeQuick G model (NeQuick) and EGNOS model (EGNOS)--were analysed in the study. Although there was a strong magnetic storm during the period under review, culminating on 8 September 2017, the obtained accuracy seems to meet successfully the GPS SPS standards of 9 m for horizontal positioning and 15 m for vertical positioning with 95% probability. During the test, there were some moments when the errors reached higher values (for off solution), but they lasted for very few epochs. One of the reasons for obtaining such high accuracy is the use of the EGNOS clock and ephemeris corrections in all the examined solutions. The second reason is undoubtedly the fact that despite this storm being categorised as a G4 level storm in a five-level scale used by the NOAA (National Oceanic and Atmospheric Administration) and the Kp-index reaching 8+, the VTEC content over the analysed stations was not extremely high, reaching maximum values from 15 (KIRU) to 26 (ISTA). It is estimated that a higher value of VTEC could lead to more significant positioning errors. As expected, the option with the EGNOS model came out the best, which for horizontal positioning was on average almost two times better than the other solutions. For vertical positioning, the EGNOS model proved to be two times better for the Klob and NeQuick models, while for off solution, it was more than three times better. The differences between Klob and NeQuick are not significantly distinct from each other; this was mainly caused by the similar (rare) update rate of these models. The rare update rate of these models does not take into account rapid ionosphere changes, which often happen during geomagnetic storms. It should be emphasised that the EGNOS model did not work as it was expected during the culmination of the storm at the beginning of 8 September. In this period, the EGNOS ionospheric model presented very large errors both in the horizontal and vertical directions. This anomaly was not observed for other examined models. This fact was caused by the very high magnetic activity produced by enhanced ionospheric currents flowing below and within the auroral oval (Figure 3). This caused the instability of the EGNOS ionospheric model in the high latitude area and proves that further research is needed to improve the SBAS solutions to exclude such dangerous anomalies of operation. Nevertheless, the obtained results prove that the EGNOS ionospheric model meets the aviation positioning accuracy criteria for the APV-1 approach during the studied geomagnetic storm. The graphical representation of the errors on charts suggests that the most stable GPS performance is at station located in mid-latitude: LAMA. The impact of auroral scintillations can be observed at northernmost station, KIRU, while at the southernmost station, ISTA, the influence of high solar flux that occurs at and near the equatorial crest level can be noticed. Nevertheless, the numerical results show that the latitude of the station does not have a severe impact on the overall accuracy of horizontal positioning; the average error values for four ionospheric variants range from 1.10 m (ISTA) to 1.26 m (LAMA). Much greater dependence of positioning accuracy due to latitude was recorded for vertical accuracy, whose average values for four ionospheric variants reached the values of 1.58 (ISTA), 1.72 m (LAMA), and 2.28 m (KIRU). In the future research, it is planned to compare the performance of different ionospheric models during two periods (one with storm and other without any storms) with similar VTEC values, as this will allow extending the achieved results into more general cases.