Comparison of GIX, SIDX and ROTI ionospheric indices and their relationships with GNSS positioning results

Ionospheric disturbances can negatively impact the accuracy, continuity, availability, and integrity of Global Navigation Satellite Systems (GNSS) based services. Therefore, reliable description and characterization of these disturbances are essential to guarantee an adequate level of safety for GNSS-based systems, susceptible to strong spatial gradients and rapid changes in electron density along the receiver-satellite path (slant total electron content, STEC).

This study aims to compare various indices that describe the current condition and intensity of ionospheric disturbances using GNSS observations from approximately 350 reference stations in Europe. The indices studied include the Gradient Ionospheric Index (GIX), the Sudden Ionospheric Disturbance Index (SIDX), and the Rate Of Tec Index (ROTI). The study focuses on selected geomagnetic storms and compares the results for different latitude zones (30-45°N, 45-60°N, and 60-75°N). The results show that the behaviors of the investigated indices differ in each case and are strongly dependent on the ionospheric storm propagation mechanism and associated with actual space weather and geo-physical conditions. The study also examines the relationships between the analyzed indices and GNSS positioning results using absolute and differential approaches and code pseudorange and carrier phase-based results. In addition, a comparison with vertical protection error (VPE) and vertical protection level (VPL) for selected EGNOS Ranging and Integrity Monitoring Stations is included.