Real-time precise point positioning performance in Hong Kong during May 2024 space weather events

Space weather events are the primary drivers of ionospheric variations. These variations challenge conventional GNSS algorithms- ionospheric processing strategies, leading to degraded positioning accuracy. In low-latitude regions during extreme events, the coupling of space weather effects with inherent low-latitude ionospheric anomalies makes it challenging to understand the mechanisms behind the decline in Global Navigation Satellite System (GNSS) positioning accuracy, continuity, and availability. This poses a significant challenge for users relying on real-time Precise Point Positioning (PPP). To investigate the impact of the May 2024 space weather events on real-time PPP, we evaluated and analyzed the ionospheric responses, as well as the positioning performance of dual-frequency Ionosphere-Free PPP (IF-PPP) and Undifferenced Uncombined PPP (UDUC-PPP), using Quasi-Zenith Satellite System (QZSS) L6 band (1278.75 MHz) data in Hong Kong. The results demonstrate that, during this event, daily positioning accuracy degraded from centimeter to decimeter levels, and in severe instances, positioning results became unavailable. The degradation in positioning accuracy for both IF-PPP and UDUC-PPP showed a clear correlation with ionospheric conditions, particularly during scintillation, which led to the failure of cycle slip detection algorithms and deterioration of observation quality. L1 band (1575.42 MHz) pseudorange measurements showed better resistance to ionospheric scintillation than L2 band (1227.60 MHz). Additionally, we identified different mechanisms by which the ionosphere affects the IF-PPP and UDUC models during these periods, and the possible ionospheric phenomena associated with positioning accuracy in low latitudes.