# Effects of the 12 May 2021 Geomagnetic Storm on Georeferencing Precision
## 3 Results
Each component uses two contiguous columns in Table 2, one with the percentages obtained for the average of the previous quiet days (e.g., North) and another with the percentage measured during the day and period of the geomagnetic storm (e.g., North gs). In this table, the percentage of the stations per interval for East, North, Up, and 3D position errors were obtained using the data from the total 868 GNSS stations around the globe we had available. In Table 2, we present a snapshot of the errors focused on the geomagnetic storm time. Nevertheless, we also present a temporal evolution of the percentage of the stations with positioning errors over a certain threshold per component for the total of the available stations around the world in Figure 6. From this figure, it is possible to notice that the main errors are concentrated over the mid-day of the DoY 132, which is the geomagnetic storm period. It is possible to notice from the TEC (e.g., Figure 3) and the derived PPP-AR (Figures 4 (left panel), and 5 (left panel)) data that there is a region where the errors are more severe during the geomagnetic storm (DoY 132, 14.50-14.75 UT). This region is South America. In South America are located 325 stations of the 868 total available GNSS stations (\(\sim\)37%). In Table 3, we present similar results compared to in Table 2 but with the 325 stations of this region. However, there is no other localized area in which we can detect strong variations. The other perturbed stations are distributed around the world. For this reason, we concentrated this study in the South American sector. Figure 3: Ionospheric TEC maps using the Kriging interpolation method during the maximum apparent variation of position (between 14.50 and 14.75 UT). (**a**) VTEC at each IPP on the geomagnetic storm day. (**b**) \(\overline{VTEC}\) is obtained by averaging the VTEC values of DoYs 129, 130, and 131, for 2021. (**c**) VTEC of geomagnetic storm day. (**d**) DVTEC is VTEC of geomagnetic storm day minus \(\overline{VTEC}\). (**e**) The percentage changes in VTEC, %DVTEC. Figure 4: 3D position errors (**left panels**), and ROTI > 0.25 TECu/min (**right panels**) on (**a,b**) DoY 129; (**c,d**) DoY 130; (**e,f**) DoY 131; (**g,h**) DoY 132; (**i,j**) DoY 133; and (**k,l**) DoY 134 (between 14.50 and 14.75 UT). Figure 5: 3D position errors (**left panels**); and ROTI \(>\) 0.25 TECu/min on DoY 132 (**right panels**). (**a,b**) [5.00–5.25] UT; (**c,d**) [6.75–7.00] UT; (**e,f**) [9.25–9.50] UT; (**g,h**) [13.00–13.25] UT; (**i,j**) [14.50–14.75] UT; and (**k,l**) [17.75–18.00] UT. The ROT index (ROTI) was calculated as described in Section 2.2 to study the relation of the variation of TEC in the positioning error for the 12 May 2021 geomagnetic storm. The images in the Figure 4 (right panel) show six maps of ROTI presenting the stations that had ROTI over 0.25 TECu/min. Each map represents a day at the same time of the geomagnetic storm, DoVs 129, 130, 131, 133, and 134 at 14.50-14.75 UT. We also compared the results obtained for the 12 May 2021 geomagnetic storm with the effects on the positioning error obtained for other recent moderated geomagnetic storms, 27 March 2017 and 5 August 2019. The comparison was performed by using three particular stations: one station close to Madrid (mad, 40.43\({}^{\circ}\)N, 4.25\({}^{\circ}\)W) and two located in \begin{table}
\begin{tabular}{c c c c c c c c} \hline
**Error Intervals** & **East** & **East gs** & **North** & **North gs** & **Up** & **Up gs** & **3D** & **3D gs** \\
**[cm]** & **[\%]** & **[\%]** & **[\%]** & **[\%]** & **[\%]** & **[\%]** & **[\%]** & **[\%]** & **[\%]** \\ \hline \(<\)5 & 98 & 85 & 99 & 87 & 73 & 52 & 71 & 48 \\
5–10 & 2 & 8 & 1 & 8 & 24 & 23 & 26 & 25 \\
10–20 & 0 & 5 & 0 & 2 & 3 & 16 & 3 & 17 \\
20–40 & 0 & 1 & 0 & 2 & 0 & 6 & 0 & 7 \\
40–60 & 0 & 1 & 0 & 1 & 0 & 1 & 0 & 1 \\
60–100 & 0 & 0 & 0 & 0 & 1 & 0 & 1 \\ \(>\)100 & 0 & 0 & 0 & 0 & 1 & 0 & 1 \\ \hline \end{tabular}
\end{table}