Determining the orbits of EGNOS satellites based on optical or microwave observations

The satellites of geostationary navigation overlay systems such as EGNOS (European Geostationary Navigation Overlay System) are equipped with single-frequency microwave transponders. The tracking data contain a GPS-like signal corresponding to the GPS C/A-code (Clear Access code) in the GPS L ₁ -band of the electromagnetic spectrum. This signal is tracked by (some of the) commercially available GPS receivers and may be used for orbit determination and for the estimation of EGNOS clock corrections. GPS and EGNOS observations, acquired by eight combined GPS/EGNOS receivers in a time interval of five days in spring 2004, were used (a) to precisely position the GPS/EGNOS tracking sites (using the conventional GPS observables) and (b) to determine the EGNOS orbits and clock corrections using the EGNOS C/A-code observable. The resulting orbit determination scheme proved to be robust and accurate - in particular in view of the fact that only L ₁ C/A-code was analyzed. The quality of the procedure, which might be easily transformed into a routine EGNOS orbit determination scheme, is discussed in our article. The EGNOS satellites can also be observed with optical telescopes (situated in the appropriate geographical longitude sector). The observed astrometric places may be used to determine EGNOS orbits, as well. In spring 2004 an observation campaign using the 1-m telescope in Zimmerwald (near Bern in Switzerland) was organized to acquire observations for two EGNOS satellites. Based on these optical observations orbits were determined for one of the EGNOS spacecrafts. The quality of this complementary and independent orbit determination procedure is discussed, as well. Accurate EGNOS orbits may thus be derived with two independent methods. For the current study, however, no observations of the two types could be made available and orbits resulting from the two methods could not be compared directly. This step, as well as a combined processing of the two observation types, is planned for the future.