## 4 Results and Discussion  
The results of the horizontal position error (HPE) for the analysed stations and ionosphere models are presented in Figure 6 and Table 2 on the left. The HPE is the difference between the reference (true) values of the stations' coordinates and the coordinates calculated with the examined ionosphere models. The drawings clearly show that the largest fluctuations in horizontal positioning accuracy occurred at the northernmost station: KIRU. This seems to be mainly related with the localisation of KIRU site at high latitudes, which implicates two possible problems. First of all, there is no doubt that the positioning accuracy for such stations is affected by the much worse geometry of GNSS satellites and consequently higher values of DOP (Dilution of Precision) indices. This issue was analysed in detail by [PERSON] [48], which confirmed the significance of this factor. The second reason of the lower accuracy is a highly variable ionosphere. As is well recognised, the high-latitude scintillations, which occur also for quiet periods, may cause the GNSS signals' loss of lock and consequently affect the accuracy and integrity of navigation performance. Analysing the mean errors obtained for all the examined modes at the individual stations, the differences range from 0.57 to 1.78 m; surprisingly, the extreme values were observed at the KIRU station. The EGNOS ionosphere model was definitely the best and allowed achieving an accuracy that was almost two times better than the other options analysed. For the EGNOS model, the mean errors ranged from 0.57 m (KIRU) to 0.96 m (LAMA). For the other modes, mean errors of 1.01 m (ISTA--NeQuick) to 1.78 m (KIRU--NeQuick) were obtained. For the NeQuick model, the largest differences in accuracy related to the latitude of the stations were observed, ranging from 1.01 to 1.78 m; for the Klob model, these differences were not so significant and ranged from 1.23 to 1.33 m.  
Figure 6: The horizontal position error (HPE) in examined locations during 6–12 September 2017. The smallest impact of station location on the mean positioning error was obtained for the option without the ionosphere model (off), and it stretched from 1.39 to 1.45 m. The largest positioning errors were achieved on 8-9 September, during and the day after the climax of the storm, which is clearly visible at the KIRU and ISTA stations in the off, Klob, and NeQuick solutions. Considering the EGNOS model, it is hard to see differences due to time of day or ionospheric storm. At the LAMA station, in the off, Klob, NeQuick, and EGNOS solutions, the time of day and the impact of the storm are hardly visible. The maximum error was achieved for the off solution at KIRU, which reached as high as 14.43 m, while the rest of the maximum errors for various options/stations did not exceed 10 meters, and usually, these were short periods not exceeding a few dozen seconds. Sudden short decreases in accuracy were also observed for the EGNOS model at the KIRU station on 8 September. The lower accuracy in this case correlates well with the enhanced values of the AE-index (Figure 3) corresponding to the main phase of the geomagnetic storm. Taking into account the southwardly oriented Interplanetary Magnetic Field (IMF) for these periods, one can assume that this effect is related with the extreme ionospheric variability at the auroral zone [49]. It should be noted that although during these events the EGNOS model showed significant decreases in accuracy, the horizontal position errors did not exceed the limit values for APV-1 operations.