Determination and Maintenance of Navigation Systems Reference Frames: The EGNOS Example
Charlot, B.¹; Brocard, D.²; Bockmann, L.³; Berenguer, X.¹; Chambre, E.^1
1Thales Alenia Space; ²CNES member of ESA GNSS1 (EGNOS) Project Office; ³Norvegian Mapping Agency

Satellite Based Augmentation Systems (SBAS) like EGNOS provide users with data allowing an accurate determination of their position and integrity parameters. In EGNOS the GPS and GLONASS observations done by Ranging and Integrity Monitoring stations (RIMS) are transferred to the Central Processing Facilities (CPFs) which computes the data transmitted to the EGNOS users through geostationary satellites.

To be able to perform these computations, the EGNOS CPFs needs accurate and reliable ground station antenna coordinates. The EGNOS reference frame is determined through the RIMS coordinates. Therefore a key issue is the explicit knowledge of the reference frame for the EGNOS coordinates. Considering aviation as one of the main applications for EGNOS, it can be pointed out that the ICAO SARPS mandatorily require coordinates to be referred to WGS84. Consequently, users can expect EGNOS and thus RIMS coordinates to be in WGS84 within a certain accuracy limit, 5 cm being the EGNOS requirement. In principle , this requires the RIMS coordinates to be time dependent, thus needing frequent updates during EGNOS lifetime of 15 years.

EGNOS being a safety critical element to be used in Civil aviation, integrity is a key requirement and leads to the need to get a high level of confidence in the determination of these coordinates.

It is clear that the determination of coordinates in a global reference frame such as WGS84 or ITRF2000 with that accuracy and reliability and the maintenance of this accuracy over a time span of 15 years are not straight forward tasks.

The paper will details the needs of a navigation system like Egnos for such accurate coordinates . It will present the way this determination has been primarily done and how difficulties were overcome. The paper will then concentrates on the optimization of the maintenance of these coordinates including operational constraints of this activity. Ultimately the paper will show how this task has contributed to the excellent performances of EGNOS system.