# Effects of the 12 May 2021 Geomagnetic Storm on Georeferencing Precision
## 4 Discussion and Main Conclusions
### Positioning Errors  
The manner in which we quantified the positioning errors in this work was through the statistics of perturbed stations around the world and in particular in the South American sector. Our RMS position values for the quiet days are in accordance with the results presented by [PERSON] et al. [79]. They gathered that the performances of the kinematic postprocessed PPP-AR method are \(\leqslant\)0.8 and \(\leqslant\)2 cm for the horizontal components and the vertical component, respectively. Table 2 shows that for the quiet days, 71% of the stations had an error less than 5 cm in the 3D estimation. Table 2 also shows that for the May 2021 geomagnetic storm the main increment suffered by the Up component, passing from 27% of the total stations (868) with perturbations over 5 cm to 48%. Although the impact over the North and East components was less affected, they jumped from 1% to 13% and from 2% to 15%, respectively, for the stations with errors over 5 cm. Figure 6 graphically shows this increment in the number of stations over certain level of positioning error for a period of time that include the main phase of the May 2021 storm, also showing that the Up component is the most affected. The TEC (Figure 3) and ROTI (Figure 5h) plots show that for the May 2021 storm the main geophysical activity concentrated on the poles and on the South America sector. However, Figure 5g shows that the positioning errors were perceived mainly in stations in the South American sector. Table 3 shows that in the Up component, we went from 45% of the total station over South America (325) with an error over 5 cm to 87%. The similar increment can be perceived for the other components. In the North component, we passed from 2% to 28% of stations over an error of 5 cm. Similarly in the East component, the variation went from 2% to 30% for the same error range. Only 1% of the stations had errors over 40 cm in the North and East components during the May 2021 storm, while for the Up component with errors over 40 cm, the stations percentage reached 4%. Furthermore, the persistence of the position errors caused by the 12 May 2021 geomagnetic storm lasted for 9.25 h [6.50-15.75 UT] (Figures 2d-f, 6 and 7 right panels). We compared the effects obtained over the positioning error due to the May 2021 storm, with the effects of the moderate geomagnetic storms that occurred on 27 March 2017 and 5 August 2019. Figure 7 compares the effects on the position in two different GNSS stations, one in Chile (South America sector) and one in Europe, where a previous storm produced strong ionospheric effects during the 5 August 2019 geomagnetic storm [16]. From the comparison, it is possible to notice that the perturbations in the position are much higher in the Vizcacha (Santiago, Chile) station and for the May 2021 storm. This is an intriguing result. Both storms, May 2021 and August 2019, are very similar, except for the duration of the main phases, but still, they produced very different effects on the position estimation (see Figure 7). Even more puzzling is the fact that the August 2019 storm has been reported to produce a strong ionospheric storm that affected the European sector [61]. For instance, the %DVTEC is almost double that in the Madrid station compared to the Santiago station (Vizcachas station), but we have found with our analysis on one station in the European sector that it did not impact the position estimation. Table 4 presents the positioning error and %DVTEC data for the three selected for comparison stations (one in Europe and two in South America) during three moderate geomagnetic storms. It is possible to notice the same pattern as that obtained between the  
May 2021 and August 2019 storms which was obtained between the March 2017 and the May 2021 storms. Thus, the effects over the position estimation were very severe during the May 2021 storm over the South America sector (Vega de Itala, vita-station), even though the %DVTEC was much higher for the March 2017 storm both for Spain (Madrid) and Chile (vita) stations.