The Impact of Ionospheric Delay on GEO GNSS Receivers and a New Perspective for TEC Estimation

The global navigation satellite systems (GNSS) signals experiment a delay when crossing the Ionosphere. This effect is particularly relevant for a satellite in a geostationary orbit (GEO) because the geometry of the orbit implies that the line of sight between the GNSS transmitter (Tx) and the receiver (Rx) crosses an earth-tangent section of the ionosphere. The antenna pattern main lobe of a GNSS transmitter is oriented towards the Earth, which implies that the strongest signals observed by a GEO satellite are those also with a larger bias due to the ionosphere, generating a degradation of the position. In this study, we use data from a real, in flight, geostationary single-frequency GNSS receivers to determine the pseudorange errors induced by the Ionospheric delay. We study several cases for exclusion of GNSS signals as a function of the elevation above the Earth level. As a reference case, we consider a 250 [Km] exclusion zone above earth level, and an enhanced exclusion zone at 1000 [Km], leading to a 97 % improvement on the position accuracy. We assess that the decreasing on the mean number of tracked satellites and a mean loss of 1.2 [dB-Hz] in the strongest measured CN0, does not threaten the quality of the positioning. The increased accuracy of the position-velocity-time (PVT) solution has a direct impact on the on-board on-orbit propagator (OOP) robustness. In fact, GNSS Rx measurements allow to periodically update the OOP, and to increase the autonomy of the propagator itself. With a new generation of satellites using GNSS to determine their position in GEO orbit, we want to emphasize the originality and uniqueness of this data to improve a tomography of the total electron counter (TEC). This information is an augmentation of the ground-based estimates of the Ionosphere.