Design of the Precise Time Facility for Galileo

Under contract to the European Space Agency (ESA), an expert team lead by Kayser-Threde GmbH has elaborated a design concept for the Precise Time Facility (PTF) for Europe’s satellite positioning system Galileo. The major purpose of the PTF is to generate, maintain, and distribute Galileo System Time (GST). The PTF is represented by an ensemble of atomic clocks (active H-maser, cesium) with appropriate measurement equipment and time algorithms. In addition, Two-Way Satellite Time and Frequency Transfer (TWSTFT) and GNSS Common-View (CV) equipment is included, since GST will be steered to TAI by linking the PTF master clock to selected European National Metrological Institutes (NMI). This process shall be managed by the yet to be established external Galileo Time Service Provider (GTSP). During Galileo In-Orbit Verification (IOV), one PTF is planned to be physically implemented, whereas for Final Operational Capability (FOC) two identical and redundant PTFs at two different sites in Europe are foreseen. A preliminary PTF turn-key architecture has been proposed in 2003 by the team. This architecture has been further detailed during the 2004 Phase C0 study. The architecture is entirely based on the results of the previous Galileo system studies and the requirements derived thereof. In addition, the proposed turn-key design covers a number of features which are deemed key to successful and timely procurement, installation, and operations of the PTF during IOV and FOC. Concerning the PTF design baseline, the redundancy mechanisms and a reliable 36th Annual Precise Time and Time Interval (PTTI) Meeting 294 connection between the PTF and the GTSP are considered to be more critical than the GST generation and steering algorithms, where valuable experience exists worldwide at the NMIs. The critical issues mainly refer to parametrization of the GST algorithm and its operational implementation (e.g. weighting of each clock in the ensemble, redundancy switching). Moreover, reliable determination of the GPS/Galileo Time Offset (GGTO), which is planned to be broadcast by both GPS and Galileo to the users, is considered to be a critical element. The paper reports about the team’s PTF design concept and the latest results of the PTF algorithm prototyping activities.