Estimation of Spatial Gradients and Temporal Variations of the Total Electron Content Using Ground-Based GNSS Measurements

Precision and safety of life applications of Global Navigation Satellite Systems require key information on space weather conditions in particular on the perturbation degree of the ionosphere. Such systems are particularly vulnerable against severe spatial gradients and rapid changes of the total electron content (TEC) measured along different satellite-receiver links. To estimate spatial gradients and rapid temporal variations of ionospheric TEC, two approaches are discussed. The Gradient Ionosphere indeX (GIX) and the Sudden Ionospheric Disturbance indeX are able to estimate the perturbation degree of the ionosphere instantaneously without taking into account previous measurements. The capabilities and accuracy of the index approaches are demonstrated by simulations using a 3-D electron density model of the ionosphere and plasmasphere in conjunction with realistic Global Navigation Satellite Signal constellations. Real data tests confirm the applicability of GIX and the related standard deviation GIX Sigma to monitor spatial gradients. Sudden Ionospheric Disturbance indeX is able to monitor rapid temporal variations of TEC as exemplified by using Global Navigation Satellite Signal measurements carried out during solar flare events. Both approaches could identify enhanced space weather impacts on precise point positioning and the European Geostationary Navigation Overlay Service. More comprehensive studies analyzing ionospheric storms in close dialogue with potential customers are needed to fully utilize the potential of these approaches to serve as objective ionospheric indices for scaling horizontal TEC gradients and rapid TEC variations in space weather services. Plain Language Summary Modern telecommunication and navigation systems may be heavily disturbed by charged particles of the Earth's ionosphere like electrons and ions. These important technological systems may fail due to degradation or even loss of signals during severe perturbations of the ionospheric plasma initiated by severe space weather events. In this paper two methods are described which provide instantaneous information on the perturbation degree of the ionosphere after analyzing continuously transmitted signals from Global Navigation Satellite Signals (GNSS). The data are available via numerous national and international GNSS receiver networks and serve to derive the Total Electron Content of the ionosphere. Using this quantity, spatial gradients and rapid temporal variations of ionospheric Total Electron Content can be estimated in two different approaches over a predefined region. The Gradient Ionosphere indeX and the Sudden Ionospheric Disturbance indeX are able to characterize the spatial and temporal perturbation degree of the ionosphere instantaneously without taking into account previous measurements. The applicability of these indices has been simulated and tested by using GNSS data obtained over Europe. The proposed indices which could easily be distributed via space weather services should be well suited to reducing safety and accuracy risks in GNSS navigation and positioning.