InternationalTelecommunicationUnion

APT INFORMATION SESSIONON WRC-15 Agenda Item 1.14(Bangkok, 10 February 2015)

Vincent MeensChairman of ITU-R SG 7

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 2

to consider the feasibility of achieving a continuous reference time-scale, whether by the modification of coordinated universal time (UTC) or some other method, and take appropriate action, in accordance with Resolution 653 (WRC-12)

What is time and why do we have a time scale that is not continuous

A day is defined as the duration between two solar passes at the meridian

However because Earth’s orbit is elliptical each day has a different duration

Agenda Item 1.14

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 3

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Agenda Item 1.14

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 4

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APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 5

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Agenda Item 1.14

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok

Agenda Item 1.14

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 7

to consider the feasibility of achieving a continuous reference time-scale, whether by the modification of coordinated universal time (UTC) or some other method, and take appropriate action, in accordance with Resolution 653 (WRC-12)

What is time and why do we have a time scale that is not continuous

A day is defined as the duration between two solar passes at the meridian

However because Earth’s orbit is elliptical each day has a different duration

From astronomical observation it is possible to define the mean duration of the day from where the second is defined as the 86400th part of a mean solar day

Agenda Item 1.14

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 8

Since 1967, the second is defined as :

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 SI second was based on the ephemeris second of 1952 and time is now based on the SI second as determined by atomic clocks;

Universal Coordinated Time (UTC) coordinates the difference between atomic time and ephemeris time by the insertion or deletion of leap seconds;

The differences are due to :

• Slowing down of the Earth rotation due to interactions with the Moon (about 2.3 ms/century),

• Irregularities in this value because of mantle convection and redistribution of mass,

• The ephemeris second of 1952 was chosen as the mean value over the 19th century.

Agenda Item 1.14

Situation avant 1967 Situation après 1967

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 9

Agenda Item 1.14

APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 10

Agenda Item 1.14Results of studies

Digital systems are highly dependent on keeping very precise time synchronization and thus can be disrupted when a leap second is introduced. Internet timing, RNSS systems and financial transactions suffered serious disruptions during leap second insertions the last being in June 2012;

Some systems requiring knowledge of the Earth’s orientation which is closely related to mean solar time (UT1) will need to be updated;

With leap seconds the UTC and UT1 difference is kept less than 0.9 s, without leap seconds UTC will gradually diverge from UT1. Since the implementation of UTC in 1972 the divergence has been of 25 s. The highest value foreseen today would be in the ordre of 10 mn in 300 years.

Today many systems use continuous GNSS system times to prevent discontinuities, however they are not time scales and can lead to serious consequences in case of time maintenance or failure;

When a leap second is inserted, repeating the last second of the day can lead to time tagging confusion;

In case of broadcasting the current UTC time scale and a new continuous time scale, operational considerations on differentiating the two time scales still need to be assessed.

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APT information session on WRC-15 Agenda Item 1.14, 10 February 2015, Bangkok 11

Agenda Item 1.14

How to satisfy the Agenda Item

Method AIntroduce a continuous reference time-scale by stopping the insertion of leap seconds :• Method A1 : stop the insertion of leap seconds in UTC to be effective no less than five years after the date of entry into force of the Final Acts of the WRC-15;• Method A2 : similar to A1 but also change the name of UTC.

Method BRetain UTC as currently defined, introduce a continuous atomic time-scale based on TAI (Temps Atomique International) and broadcast these two reference time-scales (Current UTC and Continuous atomic time scale).

Method CNo change to UTC but modify Rec. ITU-R TF.460-6 to include correction and information to recover :• Method C1 : TAI;• Method C2 : a continuous time scale with a constant offset with TAI