SBAS in Tropical Regions: A Study on GAGAN Availability and its Dependency on Ionospheric Metrics

Rosa Alòs I Florit, Dídac Rodríguez Solbas, and Ramón Sánchez-Verdejo Cidfuentes

Abstract:	The tropical ionosphere is very dynamic - it shows strong diurnal, seasonal, and solar-cycle variations, revealing phenomena like equatorial plasma bubbles, scintillations, and rapid TEC fluctuations. These disturbances pose challenges to the integrity and availability of Satellite-Based Augmentation Systems (SBAS), particularly for safety-of-life navigational services. This study examines the impact of ionospheric variability on GAGAN availability during approach with Vertical Guidance, Level I (APV-I), which require Vertical Protection Levels below 50?m. We analyze correlations between GAGAN availability and several ionospheric indicators: Along-Arc TEC Rate (AATR), global space weather indices (Kp, ap, F10.7), newly incorporated scintillation-derived stability indices, and residuals from the dedicated ionospheric model GILION. GILION uses dual frequency GNSS observations with a Kalman filter approach to estimate TEC and quantify the mismatch between modeled and observed TEC at ionospheric pierce points (TEC residuals) (Sardón et al., 1993). Our results show that GAGAN availability is negatively correlated with AATR and GILION residuals, particularly during disturbed geomagnetic conditions. In contrast, global indices and scintillation metrics were less effective in explaining availability variations within the tropical context examined. The GILION residual metric demonstrates promise as a localized, high resolution indicator of ionospheric mismodeling affecting SBAS protection levels. These findings help to better understand how the ionosphere impacts SBAS systems in tropical regions, and they show that using model residuals together with TEC rate indicators like AATR can be very useful to monitor ionospheric conditions in real time and to study their effect on SBAS performance.
Published in:	Proceedings of the 38th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2025) September 8 - 12, 2025 Hilton Baltimore Inner Harbor Baltimore, Maryland
Pages:	3399 - 3420
Cite this article:	Florit, Rosa Alòs I, Solbas, Dídac Rodríguez, Cidfuentes, Ramón Sánchez-Verdejo, "SBAS in Tropical Regions: A Study on GAGAN Availability and its Dependency on Ionospheric Metrics," Proceedings of the 38th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2025), Baltimore, Maryland, September 2025, pp. 3399-3420. https://doi.org/10.33012/2025.20394
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