Resolution 2 of the 26th CGPM (2018)

On the definition of time scales

The General Conference on Weights and Measures (CGPM), at its 26th meeting,

considering that

• Resolution 1 adopted by the CGPM at its 14th meeting (1971) requested the CIPM to define International Atomic Time (TAI),
• no complete self-contained definition of TAI has been provided officially by the CIPM,
• the Consultative Committee for the Definition of the Second (CCDS) proposed in its Recommendation S2 (1970) a definition which was extended by a Declaration of the CCDS in 1980,
• the CGPM at its 15th meeting (1975) noted that Coordinated Universal Time (UTC), derived from TAI, provides the basis of civil time, and strongly endorsed this usage,

recognizing that

• the mission of the BIPM is to ensure and promote the global comparability of measurements, including the provision of a coherent international system of units,
• the International Astronomical Union (IAU) and the International Union of Geodesy and Geophysics (IUGG) with the International Association of Geodesy (IAG) are responsible for defining reference systems for Earth and space applications,
• the International Telecommunication Union Radiocommunication Sector (ITU-R) is responsible for coordinating the dissemination of time and frequency signals and making relevant recommendations,
• the International Earth Rotation and Reference Systems Service (IERS), a service of the IAU and IUGG, is responsible for providing information required to relate terrestrial and celestial reference systems, including time-varying measurements of the Earth's rotation angle, UT1 - UTC, the low-precision prediction of UT1 - UTC for time signal broadcasts, DUT1, and for deciding and announcing leap second insertions,

noting that

• Resolution A4 (1991) of the IAU defined, in Recommendations I and II, the Geocentric Reference System as a system of space-time coordinates for the Earth within the framework of general relativity, and, in Recommendation III, named the time coordinate of that reference system "Geocentric Coordinate Time" (TCG),
• Resolution A4 (1991) of the IAU further defined, in Recommendation IV, Terrestrial Time (TT) as another time coordinate in the Geocentric Reference System, differing from TCG by a constant rate; the unit of measurement of TT being chosen to agree with the SI second on the geoid,
• Resolution B1.9 (2000) of the IAU redefined TT to be a time scale differing from TCG by a constant rate: dTT/dTCG = 1 – L_G, where L_G = 6.969 290 134 × 10^–10 is a defining constant (the numerical value of L_G was chosen to conform to the value W₀ = 62 636 856.0 m²s^–2 for the gravity potential on the geoid as recommended by Special Commission 3 of the IAG in 1999),
• the redefinition of TT in 2000 introduced an ambiguity between TT and TAI as the CCDS had stated in 1980 that TAI was to have "the SI second as realized on the rotating geoid as the scale unit" while the definition of TT does not refer to the geoid,

states that

• TAI is a continuous time scale produced by the BIPM based on the best realizations of the SI second, and is a realization of TT as defined by IAU Resolution B1.9 (2000),
• in the transformation from the proper time of a clock to TAI, the relativistic rate shift is computed with respect to the conventionally adopted equipotential W₀ = 62 636 856.0 m²s^–2 of the Earth's gravity potential, which conforms to the constant L_G defining the rate of TT,
• as stated in the IAU Resolution A4 (1991), TT - TAI = 32.184 s exactly at 1 January 1977, 0h TAI at the geocentre, in order to ensure continuity of TT with Ephemeris Time,
• UTC produced by the BIPM, based on TAI, is the only recommended time scale for international reference and the basis of civil time in most countries,
• UTC differs from TAI only by an integral number of seconds as published by the BIPM,
• users can derive the rotation angle of the Earth by applying to UTC the observed or predicted values of UT1 - UTC, as provided by the IERS,
• UTC provides a means to measure time intervals and to disseminate the standard of frequency during intervals in which leap seconds do not occur,
• traceability to UTC is obtained through local real-time realizations "UTC(k)" maintained by laboratories contributing data to the calculation of UTC, identified by "k",

confirms that

• International Atomic Time (TAI) is a continuous time scale produced by the BIPM based on the best realizations of the SI second. TAI is a realization of Terrestrial Time (TT) with the same rate as that of TT, as defined by the IAU Resolution B1.9 (2000),
• Coordinated Universal Time (UTC) is a time scale produced by the BIPM with the same rate as TAI, but differing from TAI only by an integral number of seconds,

and recommends that

• all relevant unions and organizations consider these definitions and work together to develop a common understanding on reference time scales, their realization and dissemination with a view to consider the present limitation on the maximum magnitude of UT1 - UTC so as to meet the needs of the current and future user communities,
• all relevant unions and organizations work together to improve further the accuracy of the prediction of UT1 - UTC and the method for its dissemination to satisfy the future requirements of users.

DOI : 10.59161/CGPM2018RES2E