A considerable level of correlation between SID-monitoring time series and GPS-derived tec observations taken during development of a massive ionospheric storm

Abstract

Space weather, geomagnetic, and ionospheric conditions are the most prominent causes of degradation in the positioning performance of the Global Navigation Satellite System (GNSS), through introduction of ionospheric delay in the GNSS signal. This affects numerous GNSS-based technologies and socio-economic systems and services. Analyses of case studies of GNSS positioning-performance degradation contribute to the characterization of GNSS positioning error, and support error-correction methods and model development. A case study of a rapidly developing ionospheric storm is examined here, with the aim of characterizing the event using a low-cost sudden ionospheric disturbance (SID) monitor observation of lower-ionospheric-level conditions through continuous reception of very-low-frequency (VLF) signal-strength values. Time series of observations, taken in Croatia during the St. Patrick 2015 event of the fast development of the large ionospheric storm, were compared with the time series of the dual-frequency GPS-derived observations of total electron content (TEC), a parameter linearly related to GNSS ionospheric delay. A comparison framework was developed in the open-source R programming framework for statistical computing. Time series of SID and GNSS-based TEC observations were examined for cross-correlation. The research revealed correspondences between the two time series. Although not linear, the correspondences identified may be used as an early warning for potential GNSS positioning-performance deterioration. Furthermore, this may serve as the foundation for understanding the lower-ionosphere contribution to the over-all TEC, and thus to the formation of the GNSS ionospheric delay. Our team intends to explore both research directions in forthcoming studies.