Assessment of the orbital variations of GNSS GEO and IGSO satellites for monitoring ionospheric TEC

The geostationary orbit (GEO) satellites provide a great opportunity to continuously monitor the earth system, which has shown a powerful application in ionospheric total electron content (TEC) studies. As the GEO satellites revolve the earth over the equator synchronously with the earth's rotation, the GEO satellites appear motionless in the sky. In TEC studies, the position of the GEO satellites is normally assumed to be fixed, but the actual position changes slightly. We assess the effects of the orbital variations of the GEO satellites on monitoring ionospheric TEC based on precise satellite orbit solutions or broadcast ephemeris. The results indicate that the sub-satellite points of the GEO satellites change slightly and could induce certain variations of ionospheric pierce point (IPP). The orbital inclination and the receiver location with respect to the GEO satellite mainly determine the magnitude of IPP drift. Based on simulations with an empirical ionospheric model, the TEC differences due to the fixed GEO IPP assumption are not completely negligible. The day-to-day variations of the daily GEO IPP trajectories are not stable over a long-term time scale. The daily and yearly variations of the inclined geosynchronous orbit (IGSO) are also examined. The IGSO-based TEC could be used in the day-to-day comparison for a short period, though it is not suitable for long-term TEC studies. This work provides additional clues to improve the ionospheric studies based on GEO and IGSO TECs.