Performance Analysis of DF-PPP under the Effects of Extreme Geomagnetic Storms: A Case Study of "May 2024 Solar Storm (10-15 May 2024)"

The strongest geomagnetic storm of solar cycle 25 (Dstmin < -412 nT) occurred from May 10 to15, 2024. Its source traces back to a solar event on May 8, 2024, when National Oceanic and Atmospheric Administration (NOAA) Active Region 13664 produced multiple solar flares and launched several coronal mass ejections (CMEs) directed toward Earth. Classified as a G4-G5 geomagnetic storm (Kp 8-9), it was the most intense storm of 21st century since the “Halloween” Storm of October 2003 (Dstmin < -383 nT). The resultant disturbances in the Earth's ionosphere-magnetosphere system, particularly variations in electron density, deteriorate radio wave propagation, potentially disrupting Global Navigation Satellite System (GNSS) operation and communication services. Such disruptions can compromise accuracy and induce positioning errors in GNSS-based high-quality methods, such as Precise Point Positioning (PPP), as previous studies on geomagnetic storms have indicated. This study evaluates the performance of GPS dual-frequency PPP (DF-PPP) under both quiet (Day of Year – DOY 002/2020) and severe (DOY 132/2024) ionospheric conditions. The analysis is based on GNSS data from 30 stations of the International GNSS Service (IGS) network, processed using RTKLIB software. The global root mean square error (RMSE) of GPS DF-PPP shows that performance degradation is most severe at high latitudes during May 2024 storm, with three-dimensional (3D) RMSE reaching 2.37 m, a significant increase from 0.40 m on the quiet day. In contrast, mid- and low-latitude stations experienced lesser impacts, achieving 3D RMSE of 1.69 m and 1.30 m, respectively, compared to 0.14 m and 0.17 m on the quiet day. The reduced GPS positioning performance during geomagnetic storms correlates strongly with intensified ionospheric irregularities, as conveyed by solar wind velocity (Vsw), Kp, and disturbance storm time (Dst) indices. These indices reflect heightened solar and geomagnetic activity, leading to greater fluctuations in electron density and, consequently, decreased accuracy. The impact is most pronounced at high latitudes on storm days, where ionospheric variability is driven by charged particles following the Earth’s magnetic field into the upper atmosphere at polar regions. On the other hand, PPP accuracy is compromised on quiet days, as low latitudes are affected by the enhanced equatorial ionization anomaly (EIA) and scintillation phenomena. Mid-latitudes, however, experience relatively stable ionospheric conditions under settled circumstances, confirming their lower susceptibility to irregularities. Overall, the analysis underscores the varying effects of ionospheric disturbances on GPS positioning accuracy across different magnetic latitudes.