QZSS CLAS: Performance Evaluation and Improvement against Severe Ionospheric Disturbances Under High Solar Activity

Natsuko Hayase, Susumu Saito, Ryosuke Saito, Masakazu Miya, Rui Hirokawa, Hiroyuki Mineyama, So Tajima, Seigo Fujita, Kazutoshi Sato

Abstract:	The Centimeter Level Augmentation Service (CLAS) of Quasi-Zenith Satellite System (QZSS) is the world's first open service for satellite based Precise Point Positioning Real-Time Kinematic (PPP-RTK) introducing centimeter-level accuracy across Japan (Hirokawa et al., 2023; Cabinet Office, 2025a). This study aims to evaluate CLAS performance and propose improvements to address severe ionospheric disturbances, particularly during high solar activity. While CLAS has consistently met performance specifications since its launch in November 2018, recent evaluations indicate performance degradation during periods of significant ionospheric disturbances. We analyze the correlation between CLAS performance and these disturbances, focusing on Medium Scale Traveling Ionospheric Disturbances (MSTIDs) and plasma bubbles. In response to these challenges, we propose targeted improvements in both the utilization and generation of augmentation information. For the utilization of augmentation information, parameter optimization of the CLAS Test Library (CLASLIB) revealed significant improvements in Time to First Fix (TTFF), especially during periods of active MSTIDs. For the generation of augmentation information, we introduce a service upgrade that allows for the transmission of two distinct CLAS Transmit Patterns, effectively increasing the number of augmented satellites available for users. A geometrically distributed increase of the augmented satellites is effective in mitigating the impact of regional ionospheric disturbances. The ongoing evolution of CLAS demonstrated in this study underscores the necessity of adaptive strategies to maintain the reliability of CLAS, contributing to future countermeasures and technological development.
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:	2470 - 2486
Cite this article:	Hayase, Natsuko, Saito, Susumu, Saito, Ryosuke, Miya, Masakazu, Hirokawa, Rui, Mineyama, Hiroyuki, Tajima, So, Fujita, Seigo, Sato, Kazutoshi, "QZSS CLAS: Performance Evaluation and Improvement against Severe Ionospheric Disturbances Under High Solar Activity," Proceedings of the 38th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2025), Baltimore, Maryland, September 2025, pp. 2470-2486. https://doi.org/10.33012/2025.20334
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