Operational Composite Clock for SBAS Systems

Accurate and robust time synchronization is a crucial part in the operation of any global navigation satellite system like the American Global Positioning System (GPS) and the future European system Galileo as well as of Satellite based Augmentation Systems (SBAS) (e.g. the European Geostationary Navigation Overlay System (EGNOS) or the American Wide Area Augmentation System (WAAS)). Using the calculated time parameters the navigation satellite clocks and the ground system clocks are synchronized relative to the system time providing the means for an accurate and robust positioning and timing service. Focusing on the reliability and availability of the system, it is to be mentioned, that any failure or mal-functioning of the system time generation affects the synchronization and the related service performance of the whole system. Therefore, a high robustness of the system time generation has to be ensured, which is achieved by the application of an operational composite clock. A further evolutionary step, envisaged in this paper, is to include the satellite clocks of the augmented Global Navigation Satellite System (GNSS) (e.g. GPS and Galileo) within the operational composite clock. The satellite clocks participate by their assigned weights to the generated system time and provide the opportunity to relax the stability requirements of the monitor station clocks. It is argued that the system time stability resulting from the satellite clocks is still sufficient to fulfil the needs of the SBAS application. As a consequence, the ground stations can be equipped with less stable clocks such as internal receiver clocks or low cost Rubidium clocks. Additionally, the replacement of the conventional atomic frequency standards (e.g. standard or high-performance Caesium clocks) leads to a cost-reduction of the infrastructure. The evolutionary concept of a satellite clock based composite clock is studied in this paper. Its performance is evaluated with synthetic and real-data from the International GNSS service network (IGS). It is to be emphasized that there are of course other timing architectures and evolution options for SBAS and GNSS available. The above defined option is regarded as one such opportunity