The Hunga-Tonga volcanic eruption on 15 January 2022, produced significant atmospheric and ionospheric disturbances that may degrade global navigation satellite system (GNSS) and precise point positioning (PPP) accuracy. Using data from the GEONET GNSS network and Soratena barometric pressure sensors across Japan, we analyzed the eruption’s effects through the gradient ionospheric index (GIX) and the rate of TEC index (ROTI) to characterize the propagation and effects of these disturbances on ionospheric total electron content (TEC) gradients. Our analysis identified two separate ionospheric disturbance events. The first event, coinciding with the arrival of atmospheric Lamb waves, was characterized by wave-like pressure anomalies, differential TEC (dTEC) fluctuations, and modest horizontal gradients of vertical TEC (VTEC). In contrast, the second, more pronounced disturbance was driven by equatorial plasma bubbles (EPBs), which generated severe ionospheric irregularities and large TEC gradients. Further analysis revealed that these two disturbances had markedly different impacts on GNSS positioning accuracy. The Lamb wave–induced disturbance mainly caused moderate TEC fluctuations with limited effects on positioning accuracy, and mid-latitude stations maintained both average and 95th percentile positioning ($\mathrm{p}\mathrm{p}\mathrm{p},{\mathrm{P}}_{95}$) errors below 0.1 m throughout the event. In contrast, the EPB-driven disturbance had a substantial impact on low-latitude regions, where the average horizontal PPP error peaked at 0.5 m and the horizontal and vertical $\mathrm{p}\mathrm{p}\mathrm{p},{\mathrm{P}}_{95}$ errors exceeded 1 m. Our findings reveal two episodes of spatial-gradient enhancement and successfully estimate the propagation speed and direction of the Lamb waves, supporting the potential application of ionospheric gradient monitoring in forecasting GNSS performance degradation.

Geomagnetic storms induce ionospheric disturbances, significantly affecting Global Navigation Satellite System (GNSS) positioning accuracy. This study investigates how geomagnetic storm-induced ionospheric irregularities influence GNSS Precise Point Positioning (PPP), using data from approximately 260 GNSS stations across China during 15 storm events between 1 January and 30 June 2015. We applied two indices—the Gradient Ionosphere Index (GIX), representing spatial gradients of vertical total electron content (VTEC), and the Rate of TEC Index (ROTI), describing temporal TEC variations. The analysis identified the St. Patrick’s Day geomagnetic storm (17 March 2015) as causing the most pronounced ionospheric disruptions, with significant east–west TEC gradients (|GIXx,P95| > 50 mTECU/km) consistently associated with substantial PPP errors (>0.5 m). Spatial analyses further indicated that significant 3D PPP errors (PPP, P₉₅ > 0.4 m) closely overlapped with regions experiencing intense east–west TEC gradients, predominantly in the 20–35°N latitude band. Further analysis indicated notable pre-storm ionospheric enhancements driven by zonal electric fields, distinct ionospheric suppression associated with westward disturbance dynamo electric fields (DDEFs) on 18 March, and re-intensification due to eastward penetration electric fields (PEFs) on 19 March.

The spatial gradient of Total Electron Content (TEC) demonstrates a notable correlation with occurrences of ionospheric scintillation and degradation in Global Navigation Satellite Systems (GNSS) performance. To assess the degree of the ionosphere perturbation over China, we utilize two ionospheric disturbance indices, i.e., the Gradient Ionospheric Index (GIX) and Rate of Total Electron Content Index (ROTI). Analyzing data from over 230 GNSS stations in China, we investigated the consistency and differences in ionospheric irregularities as indicated by GIX and ROTI, focusing particularly on the impact of horizontal gradients of vertical TEC (VTEC) on GNSS positioning. Experimental results indicate that the occurrence of ionospheric irregularities and the degradation of kinematic Precise Point Positioning (PPP) performance are more closely aligned with the strong horizontal VTEC gradients indicated by GIX, both in evolutionary characteristics and temporal variations, compared to ROTI. Notably, severe position errors and strong horizontal gradient of VTEC were consistently observed throughout the entire duration of St. Patrick's Day storm, including the prestorm period (06-16 UT on 16th March), the main phase (06-13 UT on 17th March), and the late recovery phase (05-14 UT on March 19th). Furthermore, significant PPP degradation (3-D PPP>1m) occurred not only preceded the intensification of ROTI but also in periods of stable ionospheric conditions indicated by ROTI values below 0.4 TECU/min, particularly on March 19th. The degradation primarily occurred in regions with significant irregularities, especially when the horizontal VTEC gradient surpassed a specific threshold (GIXx, P95+ > 55 mTECU/km, 1 mTECU/km = 10−3 TECU/km). These findings indicate the potential of GIX in effectively capturing the effects of ionospheric disturbances and enhancing the safety and accuracy of GNSS navigation and positioning.