Session B1: Atmospheric Effects, GNSS Remote Sensing, and Scientific Applications

The G4 Solar Storm of May 2024: Impact on the GNSS Carrier-Phase Measurements
Giovanni Cappello, International PhD Programme / UNESCO Chair “Environment, Resources and Sustainable Development”, Department of Science and Technologies, University of Naples “Parthenope”, Centro Direzionale Isola C4, 80143 Naples, Italy Ciro Gioia, European Commission, Joint Research Center, 21027 Ispra, Italy Antonio Angrisano, Department of Engineering, Messina University, 98166 Messina, Italy Gabriele Portelli, International PhD Programme / UNESCO Chair “Environment, Resources and Sustainable Development”, Department of Science and Technologies, University of Naples “Parthenope”, Centro Direzionale Isola C4, 80143 Naples, Italy Salvatore Gaglione, International PhD Programme / UNESCO Chair “Environment, Resources and Sustainable Development”, Department of Science and Technologies, University of Naples “Parthenope”, Centro Direzionale Isola C4, 80143 Naples, Italy
Location: Beacon B

Introduction
In the last decades, the Global Navigation Satellite Systems (GNSSs) became fundamental for several applications such as pedestrian, automotive, maritime, air and so on [1]. Despite the potentialities of satellite positioning systems, and the high-level performance they can reach with specific techniques, there are some intentional and unintentional sources of disruption that can deteriorate the accuracy, the continuity, the availability of the GNSS signals. Among these, space weather is one source of disruption of GNSS, indeed geomagnetic storms, intense solar activities can lead to tens or more meters of errors [2, 3].
Geomagnetic storms are phenomena of Earth magnetosphere caused by the solar wind effects. It consists in an energy exchange between the solar wind and the magnetosphere and its effects influence not only the terrestrial magnetic field, but also the ionosphere. The most intense geomagnetic storms are due to two main causes: the high-speed solar wind and solar Coronal Mass Ejection (CME) [4].
The main effect in the ionosphere is the increased number of Total Electron Content (TEC), i.e., the density of electrons along the satellite-receiver direction. That leads to an unmodeled error that affects the position estimation process. This happens mostly at high latitudes, but when the storms are intense, even the middle latitudes can be affected by this phenomenon [3, 5].
To describe the intensity of a geomagnetic storm, NOAA (National Oceanic and Atmospheric Administration) has established a scale that runs from G1, corresponding to a minor geomagnetic activity, to G5, referring to an extreme geomagnetic activity [6].
The most recent solar storm began on the 9 May 2024 and NOAA classified it as “severe”, i.e., G4 on the NOAA scale. As reported by NOAA, CMEs were expected to interact with the terrestrial magnetosphere on the 10 May 2024, persisting until the 12 May 2024.
In this study, the effects of that event on the carrier-phase measurements are analyzed from the 8 May 2024 to the 14 May 2024.
Literature review
In the GNSS community, the effect of geomagnetic storms is a recurrent topic: its periodicity, predictability, intensity-level and, therefore, the impact it can have on the GNSS signals makes this topic always interesting to explore.
One of the most explored parameters when a geomagnetic storm occurs is the ionospheric TEC. TEC is strongly impacted by geomagnetic storms [5] and several studies describe its variation during different events [7]. Four solar storms of different intensity among 2002-2004 were analyzed in [8], the solar storm in November 2004 was considered in [9, 10]; finally, [11] assessed the impact of the events in June 2015 and in May 2021.
Since the GNSS signals run from the satellite to the receiver, it naturally passes through the ionosphere, and under the effects of the geomagnetic storm, the PVT (Position, Velocity, Time) solution can be degraded. This has been shown in [12]: in that study, the authors analyzed the Precise Point Positioning (PPP) kinematic solution accuracy during a strong geomagnetic event on the 25-26 August 2018 and it was found that the mean error was 5 times higher than the usual values obtained on not-disturbed days.
Since these strong events can cause severe losses of lock [7] and cycle slips, in [13] a method for detecting cycle slips under ionospheric disturbance has been proposed and validated using a set of IGS stations dataset and considerable improvements in the PPP-solution accuracy have been obtained.
Materials and methods
In the proposed study, a set of 32 IGS stations in a world-wide configuration have been selected, one week of daily data at 1 Hz has been retrieved. These datasets run from the 8th to the 14th of May 2024, corresponding the Days Of Year (DOY) 129 to 135, and covering the geomagnetic event. The days before the geomagnetic storm are considered as nominal days.
The assessment is conducted using GPS L1 measurements and exploiting the TDCP (Time Differenced Carrier Phase) technique [14]. TDCP has been used to process carrier phase measurements in order to obtain receiver location information. The advantage of using TDCP is related to the difference of two consecutive carrier-phase measurements, that allows to remove the ambiguity unknown; such approach allows to estimate the position increment between two consecutive epochs. The TDCP approach has been used instead of the classical Precise Point Positioning (PPP), because its performance is not impacted by the tuning of the Kalman Filter. The adopted approach uses Weighted Least Squares (WLS) as estimation method and the estimated parameters are the coordinates increment and the receiver clock drifts, hence for the estimation a minimum of 4 TDCP measurements are required.
The analyses have been performed at two different levels: at first, on the measurement level exploiting the raw carrier-phase, and then after solution analyzing carrier-phase residuals as well as positioning error. For measurements level assessment, the number of cycle-slips, the C/N0, and the number of tracked satellites are considered. Different cycle slip detectors have been used including a single frequency cycle slip detector exploiting Doppler and Carrier-phase measurements collected at two consecutive epochs. At the post solution level, the carrier phase residual, and the positioning error (with respect to the reference coordinates of the stations from SINEX file) are evaluated.
Results
From the preliminary results, GNSS data belonging to one IGS stations (UCAL) have been inspected in terms of number of cycles slips per day, and a peak in the amount of cycle slips have been found in DOYs 131 and 132, corresponding to those days when the CMEs interacted with the terrestrial magnetosphere. During these days, the number of cycle slips was between three and five times higher than the quite days.
Furthermore, a more in-depth analysis on DOY 132 (11 May), corresponding to the peak of the geomagnetic storm (where the maximum number of cycle slips for the UCAL stations has been found), considering all the IGS stations has been conducted. Many of the cycle slips have been detected for four stations, i.e., ASCG, GAMB, JFNG, KRGG, in an order of magnitude of hundreds of thousands while, for the remaining stations, tens of thousands of cycle slips were present.
The inspection will be extended to all the DOYs and all the stations, even including the above-mentioned features. Generally, a strong difference between DOYs 131-132 has been noted, with respect to the quite days.
From the results, it emerged that the quality of carrier phase measurements was affected by the geomagnetic storm in May 2024. With respect to quite days, a larger number of cycle slips was detected for IGS several stations. The carrier phase residuals were influenced as well as the positioning errors.
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