# Effects of the 12 May 2021 Geomagnetic Storm on Georeferencing Precision
## 1 Introduction
Since the contribution to the TEC usually comes from low orbital altitudes (below 1000 km), then, vertical TEC (VTEC) is obtained from Slant TEC (STEC) at the ionospheric pierce point (IPP) at an altitude of 350 km. Another relevant ionospheric index used to study ionospheric variations is the rate of change of the TEC index (ROT). ROT is calculated in a temporal window. ROTI is defined as the standard deviation of ROT. It describes the small-scale variability of the line-of-sight electron content resulting from the ionosphere and plasmasphere. The total electron content (TEC) maps, together with other indexes derived from TEC, are used to estimate the locations and time where larger signal delays in GNSS receivers might produce higher positioning errors. Several TEC disturbances studies have been conducted in recent years [13; 32; 33; 34; 35; 36; 37; 38; 39]. However, they have mainly focused on TEC and in the northern hemisphere. Nevertheless, some studies of TEC have been conducted with special focus on the South Atlantic Anomaly area [40; 41; 42; 43; 44; 45; 46; 47; 48]. Besides these works and some other exceptions (e.g., [49]), there is a lack of studies in the southern hemisphere. In addition, all the abovementioned works did not study the effect of TEC on positioning error. Although the position estimation of a GNSS receiver depends on TEC, for several reasons (location, correction algorithms, etc. ), perturbations on TEC are not reflected in a simple manner over the position accuracy. Therefore, it is relevant to study the actual performance of the positioning estimation of GNSS receiver during geomagnetic storms. To evaluate the position error during geomagnetic storms, the precise point positioning (PPP) method is used. PPP is a method that performs efficient computation to determine high-quality coordinates. It uses a single receiver processing strategy for GNSS [50; 51]. PPP does not require any additional data from a reference station and can provide a solution from a centimeter to decimeter level of positional accuracy both in static and kinematic modes [50; 52]. For these reasons, PPP has become the predominant positioning technique [50]. Many free PPP online services are available, such as the Automatic Precise Positioning Service (APPS) of the Global Differential GPS (GDGPS) System ([[https://apps.gdgps.net/](https://apps.gdgps.net/)]([https://apps.gdgps.net/](https://apps.gdgps.net/)), accessed on 3 December 2021), the GNSS Analysis and Positioning Software (GAPS-PPP) ([[http://gaps.gge.unb.ca/](http://gaps.gge.unb.ca/)]([http://gaps.gge.unb.ca/](http://gaps.gge.unb.ca/)), accessed on 3 December 2021), the magicGNSS solution (magicGNSS) ([[https://magicgnss.gmv.com/](https://magicgnss.gmv.com/)]([https://magicgnss.gmv.com/](https://magicgnss.gmv.com/)), accessed on 3 December 2021), and the Precise Point Positioning of Canadian Spatial Reference System (CSRS-PPP) ([[https://webapp.geod.nrcan.gc.ca/geod/tools-outils/PPP.php](https://webapp.geod.nrcan.gc.ca/geod/tools-outils/PPP.php)]([https://webapp.geod.nrcan.gc.ca/geod/tools-outils/PPP.php](https://webapp.geod.nrcan.gc.ca/geod/tools-outils/PPP.php)), accessed on 3 December 2021) [53; 54; 55]. The CSRS-PPP service is one of the most used PPP online services in the field, and we use this service in this work to obtain the PPP estimation. More recent studies have presented TEC disturbances produced by geomagnetic storms including GNSS position errors (PPP). These latest studies of geomagnetic storms and errors in GNSS positioning have focused on storms of the Solar Cycle 24. A summary of these studies can be found in Table 1.  
Table 1 shows that there are not many studies related to the impact over the GNSS positioning accuracy of moderate geomagnetic storms. In a recent study, it was shown that the storm of 5 August 2019, which can be categorized as moderate (Dst peak = \(-\)53 nT, Kp = 5\({}^{+}\), AE\(\sim\)1000 nT), had strong effects on TEC [61]. However, this work did not study the positioning accuracy of GNSS stations.