Various natural and man-made hazards, including tsunamis, earthquakes or explosions, produce atmospheric internal gravity waves (IGWs), acoustic waves, and acoustic-gravity waves that could vertically propagate to the E and F regions of the ionosphere thereby generating spatial and temporal electron density disturbances known as Travelling Ionospheric Disturbances (TIDs). These ionospheric disturbances are investigated in detail using ionospheric total electron content (TEC) measurements collected by continuously operating ground-based Global Navigation Satellite Systems (GNSS) receivers. The GNSS satellites are usually located in Medium Earth Orbits (MEO) at an approximate altitude of 20,200 kilometers, which yields an orbital period of about 12 hours (in case of the GPS satellites).
The aim of the research is to leverage the VARION (Variometric Approach for Real-Time Ionosphere Observation) algorithm which is able to estimate TEC variations in real-time and already successfully used with MEOs, for geostationary satellites (GEOs). The main advantages of applying the VARION algorithm with GEO observations are the removal of all geometry effects, which may help the TIDs detection, and the ability to provide continuous time series, useful to monitor the ionosphere activity.
Nonetheless, GEOs have lower spatial coverage, indeed they are few GEOs satellites in orbit, even if the increasing of private companies operating in space business and the new Beidou geostationary satellites denote a positive trend in this direction.
In conclusion, VARION application to GEO satellites can improve real-time tsunami TIDs detection and help to separate time/space ionospheric variability, representing at the same time a new challenge for the VARION algorithm.
As stated before, the introduction of the VARION algorithm represented a new step in the ionosphere monitoring, since, before its birth, sTEC variations were routinely estimated and geolocated with post-processing elaborations [1]. Indeed, VARION is able to estimate sTEC variations in real-time and this feature made it suitable for the real-time ionosphere monitoring. For this reason, it was implemented in the GDGPS (Global Differential GPS System) system, managed by the Jet Propulsion Laboratory (JPL) of NASA. The VARION-GDGPS is a real-time interface that may be accessed at https://iono2la.gdgps.net/. This new web-based system is currently capable of processing real-time GNSS data streams using satellites from GPS, Galileo, GLONASS and BeiDou systems and as observed at more than 30 permanent stations located in the Pacific region often devastated by major earthquakes and tsunamis [2].
Nowadays, GEO satellites for the ionosphere monitoring are not yet used and as a consequence, there is not a scientific literature about this topic.
GEOs are the satellites belonging to Satellite-based augmentation systems (SBAS), such as those of Wide Area Augmentation System (WAAS), operated by the United States Federal Aviation Administration (FAA) or of the European Geostationary Navigation Overlay Service (EGNOS), operated by the ESSP (on behalf of EU's GSA). Recently, BeiDou navigation satellite system also launched six GEO satellites as part of their constellation. These satellites follow a geostationary orbit, a circular geosynchronous orbit 35786 km above Earth's equator and following the direction of Earth's rotation.
In this outlook, the introduction of geostationary satellites in the VARION algorithm will represent an ulterior step for the real-time ionosphere monitoring. Indeed, the preliminary analyses (sTEC Perturbations Induced by FORMOSAT-5 Falcon 9 Launch and by GRACE- FO Falcon 9 Launch using GEOs) show a significant improvement in reducing the observed background noise due to the fact that geostationary satellites are almost motionless relative to a point on Earth, and as a result the IPPs (ionospheric pierce points where a notional ionosphere intercepts a receiver to satellite line-of-sight) may be assumed to be stationary in the sky [2]. This feature, hence, guarantees the removal of all geometry effects in the model used to define sTEC variations.
Furthermore, for all these characteristics, GEOs provide continuous sTEC time-series, which are of extremely importance in the monitoring of the ionosphere activity.
In conclusion, the integration of geostationary satellites can be considered as a crucial next step towards increasing the reliability of real-time tsunami detection systems using ground-based GNSS receivers as an augmentation to existing tsunami early warning systems [2].
[1] Savastano, G., Komjathy, A., Verkhoglyadova, O., Mazzoni, A., Crespi, M., Wei, Y., & Mannucci, A. J, Real-Time Detection of Tsunami Ionospheric Disturbances with a Stand-Alone GNSS Receiver: A Preliminary Feasibility Demonstration, Scientific Reports (2017).
[2] Savastano, G., Shume, E., Komjathy, A., Meng, X., Verkhoglyadova, O., Bar-Sever, Y., Mannucci, A. J., Wang, D., Ravanelli, M., Mazzoni, A. and Crespi, M., Real-Time Detection of Meteorological Tsunami-Generated Ionospheric Disturbances Using Stand-Alone GNSS Receivers Enhanced by Use of Geostationary Satellites, ION GNSS+ 2018 meeting