The Use of Galileo for Time Dissemination
Achkar, J.1; Bauch, A.2
1LNE-SYRTE, Observatoire de Paris; 2PTB

The operation of a satellite navigation system requires accurate synchronization among the various elements in the ground and space segment. The core navigation function of Galileo will be based on Galileo System Time GST as realized in the Precise Timing Facilities which are part of the Ground Mission Segment. The use of Galileo as a time dissemination means requires that the relation between GST and the international time reference UTC, maintained by the BIPM with input from IERS, is well defined and broadcast in the Galileo Signal in Space (SIS). The required support for "metrological time-keeping" shall be outsourced to the so-called Galileo Time Service Provider TSP. For the IOV phase a prototype TSP is currently being developed under contract of the GNSS Supervising Authority by the "Fidelity" consortium directed by Helios Technology Ltd., UK.

In this contribution we describe the means by which the TSP is going to ensure that the following GST performance requirements are fulfilled:

  • The time offset UTC − GST (modulo 1 s) shall be less than 50 ns during 95% of the time
  • The uncertainty of the UTC − GST time offset as reported in the Galileo SIS shall be less than 26 ns (95% probability)
  • The normalized frequency offset between UTC and GST shall not exceed 5.4×10-14 at an averaging time of one day.

    Fidelity has in addition obtained the task to support the calibration of the time comparison equipment installed in the Galileo PTFs, the United States Naval Observatory, and associated national timing institutes in Europe. The involvement of USNO is related to the determination of the GPS to Galileo Time Offset which shall support inter-operability of the two GNSS. The technology in use and the planned calibration missions will be briefly described.

    The authors acknowledge the fruitful cooperation with their colleagues in the Fidelity consortium consisting of Helios Technology Ltd (Chamberlain House, Bagshot, Surrey, UK), National Physical Laboratory (Teddington, Middlesex, UK), Istituto Nazionale di Ricerca Metrologica (Torino, Italy), Physikalisch-Technische Bundesanstalt (Braunschweig, Germany), LNE-SYRTE, Observatoire de Paris (Paris, France), Astrogeodynamical Observatory (Borowiec, Poland), Centre National d’Etudes Spatiales (Toulouse, France), Kayser-Threde GmbH (Munich, Germany), Thales Research and Technology (Worton Drive, Reading, UK).