Determination of resonant geopotential terms using optical observations of geostationary satellites

The ellipticity of the Earth's equator causes a resonant librational motion of geostationary satellites. Combining astrometric observations of Meteosat satellites from Zimmerwald (Switzerland) and Graz-Lustbuhel (Austria), the resonant geopotential terms C ₂₂ and S ₂₂ were determined. The accuracy of the parameters provided by the limited campaign is close to that achieved in JGM-3. The comparatively simple observation and reduction technique offers a useful alternative method to monitor the values of the resonant geopotential terms C ₂₂ and S ₂₂ in order to determine possible time variations or to assess upper limits for variations.