Recommendation 5 of the 18th CCTF (2009)

Alignment of geodetic references and synchronization of time references to international standards

The Consultative Committee for Time and Frequency,

considering that

  • the International Terrestrial Reference System (ITRS) has been recommended by the International Astronomical Union (IAU) and the International Union of Geodesy and Geophysics (IUGG) for applications in space and Earth sciences;
  • access to the ITRS is primarily achieved through the International Terrestrial Reference Frame (ITRF), and with an uncertainty of between 3 cm and 40 cm by WGS84, PZ-90, the Galileo Terrestrial Reference Frame (GTRF), the China Geodetic System 2000 (CGS’2000), and regional densifications;
  • the time scale endorsed by the 15th General Conference of Weights and Measures (1975) for world-wide time coordination and dissemination is Coordinated Universal Time (UTC);
  • the BIPM provides coordination for the maintenance and dissemination of UTC;
  • GPS time is steered to UTC(USNO) (modulo 1 s), GLONASS time is steered to UTC(SU), and Galileo time will be steered to an ensemble of European realizations of UTC, keeping the same number of seconds as GPS time;
  • the BIPM participates as observer in the International Committee on GNSS (ICG);

aware that

  • new global navigation satellite systems (GNSS) are being designed and developed;
  • interoperability of the various GNSS would be facilitated by the adoption of international geodetic and time references in the various GNSS;
  • civil and scientific activities world-wide need to refer to common internationally recognized geodetic and time references;

recognizes that

  • the ICG is a unique structure to enable GNSS Service Providers to align their geodetic and time references to the ITRS and UTC for the operation of their systems;

recommends that

  • the geodetic references for GNSSs be aligned as closely as possible to the ITRS;
  • the internal system times (ST) of the GNSS be synchronized as closely as possible to UTC (modulo 1 s);
  • the GNSS broadcast, in addition to their own ST:

    the difference between the ST and a real-time realization of UTC,

    the differences between the time scales of the various GNSS;



  • the BIPM to coordinate actions within the ICG for the accomplishment of this recommendation.

DOI : 10.59161/CCTF2009REC5E

The reader should note that the official version of this Resolution is the French text

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