What future for UTC ? Włodzimierz Lewandowski Bureau International des Poids et Mesures Sèvres

1. 43rd CGSIC - Timing subcommittee

2. 43rd CGSIC - Timing subcommittee

3. What future for UTC ? Włodzimierz Lewandowski Bureau International des Poids et Mesures Sèvres 43rd CGSIC - Timing subcommittee

4. Unification of time • October 1884 - adoption of a prime meridian and of an associated time • Greenwich meridian (origin of longitudes) • universal time, based on the rotation of the Earth • International Astronomical Union, 1948 • recommends the use of universal time (UT) • International Astronomical Union, 1971; International Telecommunications Union; Conférence Générale des Poids et Mesures • recommend the use of coordinated universal time (UTC) 43rd CGSIC - Timing subcommittee

5. Atomic time (1971) • Based on an atomic transition. • Unit is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom. • The second of atomic time (SI second) is shorter in 1.4 x 10-8 s than the second of ET (averaged for year 1960). • Clocks have no more the solely role of time keepers, they also produce the frequency that realises the second of the SI. 43rd CGSIC - Timing subcommittee

6. Coordinated Universal Time (UTC) • UTC is the reference time scale for world wide time coordination. • It serves as the basis of legal times in the different countries. • UTC is calculated at the BIPM on the basis of readings of clocks in the national laboratories. • Local realizations of UTC named UTC(k) are broadcast by time signals. 43rd CGSIC - Timing subcommittee

7. Coordinated Universal Time (UTC) • Defined to fulfil mainly the need of a time scale somehow related to the rotation of the earth. • The validity of this need is under discussion at present (ITU, IAU, URSI) • Conceptually identical to TAI but suffering from 1 second time steps (TAI - UTC = 32s today). • The definition of UTC is under revision. 43rd CGSIC - Timing subcommittee

8. International Atomic Time (TAI) • Uniform time scale. • High stability in the long term(0.6 x 10-15, ~ 40 days). • Accuracy conferred by using the reported measurements of the PFS. • Calculated in differed time on the basis of monthly blocks of data. 43rd CGSIC - Timing subcommittee

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10. Colloquium on the UTC Time Scale 28 - 29 May 2003 IEN Galileo Ferraris, Torino, Italy 43rd CGSIC - Timing subcommittee

11. Conclusions 1. Any change should slowly evolve from the current UTC by transition to a uniform time scale. 2. The final time scale should have a new name to distinguish it from Universal Time, since Universal Time is generally understood to be linked to Solar Time, and the earth rotation. A new name suggested was International Time, Temps International (TI). 3. A suggested date inaugurating any change would be 2022, the 50th anniversary of the UTC time scale. The date suggested was influenced by the beyond the lifetime of currently existing systems that would expensive to change. 43rd CGSIC - Timing subcommittee

12. Conclusions (cont.) 4. TI should be a continuous atomic time scale without Leap Seconds that is synchronized with UTC at the time of transition. 5. UT1 information would continue to be made available under the sole responsibility of the IERS. 43rd CGSIC - Timing subcommittee

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14. Thanks! 43rd CGSIC - Timing subcommittee

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