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.