Zenith Total Electron Content from the EUMETSAT Metops/GRAS instruments

The GRAS (GNSS Receiver for Atmospheric Sounding) instrument onboard the EUMETSAT Polar System satellites (EPS) Metop-A and Metop-B has a zenith antenna used for obtaining a precise orbit of the satellite which is needed for RO processing. But GNSS signals acquired through this antenna cross solely the ionosphere, and can thus be used to monitor the electron content characterizing the top side region above the LEO satellite. In fact, each tracked dual frequency GNSS signal is sensitive to the Total Electron Content in the LEO-GNSS link, the so called slant Total Electron Content (sTEC), which can be routinely estimated by processing such data with the knowledge of the precise positions of the LEO and GNSS satellites.