On correlation between SID monitor and GPS-derived TEC observations during a massive ionospheric storm development

Space weather, geomagnetic and ionospheric conditions are the most prominent single cause of the Global Navigation Satellite System (GNSS) positioning performance degradation, through introduction of the GNSS signal ionospheric delay. This affects numerous GNSS-based technology and socio-economic systems and services. Analyses of case-studies of GNSS positioning performance degradation contribute to characterization of the GNSS positioning error, and support error correction methods and models development. Here a case of rapidly developing ionospheric storm is examined, with the aim of the event characterization using a low-cost Sudden Ionospheric Disturbance (SID) monitor observations of lower ionospheric levels condition 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 fast development of the large ionospheric storm, were compared with time series of dual-frequency GPS-derived observations of Total Electronic Content (TEC), a parameter linearly related to GNSS ionospheric delay. A comparison framework has been 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 correspondence between two time series. Although not linear, the correspondence identified may be used for an early warning for potential GNSS positioning performance deterioration. Further to this, it may serve as the foundation for understanding of the lower ionosphere contribution to the over-all TEC, and thus to formation of the GNSS ionospheric delay. Our team intends to explore both research directions in forthcoming studies.