Navigating the Storm: A Diagnostic Analysis of QZSS CLAS Performance and Vulnerabilities Through the Solar Cycle 25 Maximum

This paper presents the first multi-year, nationwide diagnostic analysis of the Quasi-Zenith Satellite System Centimeter Level Augmentation Service (CLAS) precise point positioning (PPP) real-time kinematic (RTK) service, spanning the rise, peak (late 2024 to mid-2025), and initial decline of Solar Cycle 25. Using data from approximately 1,300 GNSS Earth Observation Network System (GEONET) stations, we validated our 30-s sampling methodology against official 1-Hz reports and analyzed nationwide performance trends. The analysis isolated two distinct degradation mechanisms: (1) unavoidable environmental stressors, such as plasma bubbles and medium-scale traveling ionospheric disturbances, driven by heightened solar activity, and (2) four structural vulnerabilities in the CLAS architecture that were exposed under stress. These vulnerabilities include persistent vertical bias from reference-frame misalignment (International Terrestrial Reference Frame [ITRF] 2014 vs. ITRF2020), error cross-contamination in bottom-up correction generation, user-level instability caused by ground facility (message generation facility) switching, and a single point of failure arising from dependence on continuous GEONET availability. We discuss the implications for system resilience and propose remedies for next-generation PPP/PPP-RTK services, including enhanced frame alignment and adaptive switching mechanisms.