Update from�A continuous UTC: �may we get rid of Leap Seconds?��Feb 23, 2022�

Patrizia Tavella

Director of the Time Department

Time Appliances Project

The future of UTC and the agreement with UT1�

March 27, 2024

Two main messages:�� thanks!�� give your view!

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�

27^th General Conference on Weights and Measures (CGPM), Nov 2022

draft Resolution D – On the use and future development of UTC

recognizing that the use of UTC as the unique reference time scale for all applications, including advanced digital networks and satellite systems, calls for its clear and unambiguous specification as a continuous time scale, with a well-understood traceability chain,

decides that the maximum value for the difference (UT1-UTC) will be increased by or before 2035,

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requests that the CIPM consult with the ITU, and other organizations that may be impacted by this decision in order to

• propose a new maximum value for the difference (UT1-UTC) that will ensure the continuity of UTC for at least a century,
• prepare a plan to implement by, or before, 2035 the proposed new maximum value for the difference (UT1-UTC),
• propose a time period for the review by the CGPM of the new maximum value following its implementation, so that it can maintain control on the applicability and acceptability of the value implemented,
• draft a resolution including these proposals for agreement at the 28th meeting of the CGPM (2026),

A success from

• the deepest collaboration and confidence from national metrology institutions
• other international organization as URSI, ITU-Radio and Telecommunication, IGS
• the support from digital companies as Google, Meta, OPC-TAP, Timing manufacturers

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Actions at international level

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ENDORSED by

Nov 20- Dec 15, 2023

The International Telecom Union – R is responsible for setting standards concerning time signals and frequency standard emission by radio systems

https://www.itu.int/en/itunews/Documents/2023/2023-02/2023_ITUNews02-en.pdf

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Let’s summarize

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Timekeeping is related to the rotation of the Earth.

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The real time timekeeping is based on atomic clocks and, since 1972, UTC is obtained from the International Atomic Time (TAI) plus leap seconds.

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When the difference between the Earth rotational angle UT1 and UTC reaches 0.9 second, an integer second is inserted to UTC to keep it within 1 s of UT1.

UTC = TAI + leap seconds

|UTC - UT1| < 1 second

23:59:59

23:59:60

00:00:00

The process to insert the leap second and the code to transmit DUT1= UT1-UTC are described in Rec ITU-R TF 460-6

UTC = TAI + n seconds

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UTC is supported by the work of:

• BIPM and the 85 time laboratories providing data and realizing real-time traceability

under the authority of the General Conference of Weights and Measures,

where all states are represented

• the International Earth Rotation and Reference Systems Service - IERS

computing and publishing the difference versus the Earth rotation angle UT1-UTC, and announcing leap seconds

• the International Telecommunication Union, ITU-R, to ensure it is used and correctly transmitted

(ITU-R TF.460-6 (2002): Standard-frequency and time-signal emissions)

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• But, users underpinning critical infrastructures, need a continuous and unique timescale.
• Increasingly UTC is not being used by:
• most of the GNSSs (not adding leap seconds),
• the digital network giants (eg GAFA and Alibaba) (smearing in a different way),
• the most widely used Internet time synchronization protocols as NTP and PTP, (repeating the same second twice)

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Coordinated Universal Time UTC is the international time standard

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Different “alternative methods

• Ignore leap seconds after an initial synchronization
• GPS, Galileo, BeiDou system times.
• Most current versions of Windows

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• Stop clock for 2 seconds at 23:59:59 or 00:00:00
• Network Time Protocol, Posix time on many computers
• NIST Network Time Services
• NPLtime service in the UK
• Two seconds have same name
• Problems with causality, time ordering, time intervals
• Leap second has no indicator

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• Reduce frequency of clock over some interval
• Google (24 h before), Microsoft, Facebook (18 h after), Alibaba (12 h before – 12 h after) …

Details can be found for example here:

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https://en.wikipedia.org/wiki/Unix_time

https://developers.redhat.com/blog/2015/06/01/five-different-ways-handle-leap-seconds-ntp/

https://googleblog.blogspot.com/2011/09/time-technology-and-leaping-seconds.html

https://www.theregister.co.uk/2016/12/02/google_public_ntp_servers/

https://spectrum.ieee.org/tech-talk/telecom/internet/facebook-new-time-keeping-service

https://engineering.fb.com/production-engineering/ntp-service/

https://en.wikipedia.org/wiki/Precision_Time_Protocol

https://blog.meinbergglobal.com/2019/04/15/ptp-timescale-and-what-the-heck-is-arb-time/

https://mp.weixin.qq.com/s/bBecGJrTn5HCx7_92a9uWQ    

All the alternative methods are not in agreement with UTC on the leap second day, and many disagree with each other

Users cannot tell which method is used by a time source, especially a posteriori

Leap second and the alternative methods threatens the resilience of the synchronization

The leap second process in UTC needs to be revised

Increase the tolerance in|UT1 - UTC| to a new limit (e.g. 1 min reached after ≈1 century or 1 hr reached after ≈ 5000 yrs) or to an unlimited value (= the difference UT1 – UTC will be let growing with no limit).

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🡪 UTC remains linked to UT1, the Earth’s rotation angle, whose origin is the reference meridian of Greenwich.

In the daily life, there is no change for the general public since the evolution of |UT1 - UTC| will remain negligible compared to the +/- 15 min seasonal day variations, for centuries. The general perception of conformity to astronomical phenomena is not challenged.

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Users needing the knowledge of UT1-UTC find accurate and real time estimations by the services of IERS, NASA, GNSS, ITU-R broadcast signals

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Solution to progress towards a continuous UTC

In the ‘70s UTC was used as approximation to UT1 mostly for navigation with traditional optical instruments.

Approximation UTC ≈ UT1 corresponds to an uncertainty in the position up to 400 m (at the equator). It is used only in low accuracy applications (as amateurs telescope pointing).

But it is not adapted for high precision applications (as high accuracy astronomy and space applications) that are already using the IERS and NASA estimates with 10 microsecond uncertainty, corresponding to about 0.3 cm uncertainty in the position

Metrologia paper on « Towards a consensus on a continuous coordinated universal time”

https://iopscience.iop.org/article/10.1088/1681-7575/ac9da5

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decides that the maximum value for the difference (UT1-UTC) will be increased in, or before, 2035,

requests that the CIPM consult with the ITU, and other organizations that may be impacted by

this decision in order to

• propose a new maximum value for the difference (UT1-UTC) that will ensure the

continuity of UTC for at least a century,

• prepare a plan to implement by, or before, 2035 the proposed new maximum value for the

difference (UT1-UTC),

• propose a time period for the review by the CGPM of the new maximum value following

its implementation, so that it can maintain control on the applicability and acceptability of

the value implemented,

• draft a resolution including these proposals for agreement at the 28th meeting of the

CGPM (2026),

encourages the BIPM to work with relevant organizations to identify the need for updates in the

different services that disseminate the value of the difference (UT1-UTC) and to ensure the correct

understanding and use of the new maximum value.

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CGPM 2022 Resolution 4 - On the use and future development of Universal Coordinated Time (UTC)

CCTF Task Group on Continuos UTC

Following Resolution 4 of the GCPM 2022,

the Consultative Committee in Time and Frequency is working

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with the all UTC labs, other Organizations, GNSS Providers, and stakeholders, to prepare the draft resolution for the CGPM 2026 which should contain

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• The extended tolerance value of UT1-UTC
• The procedure to align UTC to UT1 when the new tolerance is reached
• The periodicity to revise this decision at the CGPM
• The exact implementation date

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Give your view

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Extended tolerance: 3 options

• 1 minute (or a few minutes)
• 1 hour (connected to daylight saving time)
• no limit yet (next generations will decide when to align UTC to UT1)

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Pros and Cons analysis and related questions:

• Relationship to time zone
• Shall we define the procedure for alignment far in the future?
• The real time value of UT1-UTC needs to be available in robust and reliable mode (IERS web page, GNSS, radio systems, …)

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When?

• ITU Radio Conference recommend in 2035, some member states and user communities are urging the change
• Ready for a negative leap second?

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• to prepare the draft resolution for the CGPM 2026 with the extended tolerance value of UT1-UTC, etc

https://eoc.obspm.fr/index.php?index=realtime&lang=en

Possibility for the first negative leap second ?

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Dec 2016 last leap second

Is the Earth deciding for us?

We receive requests from BIPM member states and stakeholders NOT to allow a negative leap second

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A negative leap second coming soon?

No reliable prediction is possible in the long term, let’s observe the Earth rotation and let’s the future generations decide

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L. Zotov, C. Bizouard, C.K. Shum, C. Zhang, N. Sidorenkov, V. Yushkin, “Analysis of Earth’s polar motion and length of day trends in comparison with estimates using second degree stokes coefficients from satellite gravimetry”, Advances in Space Research 69, 308–318 (2022). https://doi.org/10.1016/j.asr.2021.09.010

almost as likely as not to experience a negative leap second in the next 12 years

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When implementing the change to UTC

• is still an open question with a preference for 2035

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Which new tolerance?

• Proposals are under discussion

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A Proposal to change the leap-second method �Judah Levine NIST, Boulder �Judah.levine@nist.gov

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Increase maximum tolerance to about 1 minute (< 64 s)

Tolerance reached 1 or 2 times per century at current rate of change of UT1-UTC

UTC would be consistent with everyday solar time – no discernible difference

GPS, BeiDou, and upgraded GLONASS broadcast formats support this change

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Make smaller, periodic, algorithmic adjustments instead of waiting until maximum tolerance is reached

10% of tolerance every 10 years

Method can be programmed once

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Suppose maximum tolerance had been set to 60 s in 1972

•Adjust UTC by 6s (10%) every 10 years on 1 January

5 adjustments: 1980, 1990, 2000, 2010, 2020

30 s total

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Every 50 – 100 year a readjustment may be necessary

Presented at the ION PTTI meeting Jan 2024

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The alternative –1 hour or more

No adjustment for a very long time –probably several centuries

No need to do anything now

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Problems with this solution

Very small probability of getting it right when it happens

Another Y2K and negative leap-second concern

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Visible offset between UTC and solar time

One-hour adjustment very large and disruptive

Much worse than current 1 s adjustment

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One-hour adjustment by changing time zones is not practical

Time zones are defined by local administrations

One-hour time-steps very unpopular in US

�A Proposal to change the leap-second method �Judah Levine NIST, Boulder

Judah.levine@nist.gov

Presented at the ION PTTI meeting Jan 2024

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Other ideas

Increase maximum tolerance to max 1 hour (to set number of bits)

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Do not do anything for 100 years – Readjust every 100 years

Let’s monitor and consider this decision every 20-30 years at the General Conference of Weights and Measures – CGPM

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No need of adjustment for 100 years (achieved offset could be 1 minute)

Adjustment may be “ad hoc” with risk of failure

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Difficulty of getting it right when it happens

Another Y2K event

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Small and not perceivable offset between UTC and solar time

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Write the next page

24 hours timezone allow one hour offet

Not to decide for the future generation

UT1-UTC can be disseminated by several systems

Who really needs UT1 as proxy of UTC

GNSS in 100 years will be different, modernized, with different services

Do nothing

Suggestions can be given to future generations

Keep the knowledge

Review the decision in future

Make something every 10 years to keep memory, but then reajsut periodically ……………..

We have two clocks and they do not agree,

one in Mother Earth

the other one underpins modern life

patrizia.tavella@bipm.org

Thanks for your attention