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TIME & FREQUENCY

Overview from artefact to current definition

& Realisation UTC

Frank Coutereel

Legal Time in Belgium

Past: based on GMT or UT (observations in sky) Today: based on UTC (working atomic clocks)

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• Time interval • Frequency • Time of the day

Section Time & Frequency - Part of National Standards - Realization of UTC(SMD) with 5 atomic clocks - Traceability to UTC (internal and external calibrations) - Timeservers for dissemination of UTC - Project in cooperation with VSL (Time transfert in transnational optic fiber)

Definition of the second before 1960

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• Based on 1/86400th of mean solar day • Mean solar day based on a true solar day • Apply equation of time to get mean solar day • Mean Solar day base for time scale UT1 • UT1 is proportional to earth rotation

VLBI: measurement Earth rotation speed

Between 1960 and 1967: Ephemeris Time

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Solar system orbital motion Newtonian equation of motion The tropical year is the interval between two successive passages of the "mean sun" through the "mean spring equinox". Ephemeris second : 1/31 556 925,9747 of tropical year at epoch 1900

In practice the ephemeris second is determined by the position of the moon.

Initial phase: ET agreed UT1 in the year 1900 Difference ET & UT1: about 56 seconds in 1988

After 1967: Atomic Time

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The second 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 cesium 133 atom.

Affirmed by the CIPM in 1997 that this definition refers to a cesium atom in its ground state at a temperature of 0 K):

- Use of microwaves - Energy levels are defined by the

laws of quantum mechanics - Two quantum states (F3 & F4) - Slightly different energy levels due

to this nuclear spin interaction

Time Scales - Astronomical

UT (Universal Time) – Based on Earth’s rotation – 1884: accepted as world standard

time – UT0, UT1, UT2 – UT1: correction wandering poles – Reference: zero meridian through

Greenwich – Not stable (daily and long term) – 400 solar days in a year 400

million years ago – Still in use (IERS)

ET (Ephemeris Time) – Position of astronomical

objects – Adopted as standard in 1952 – Based on earth rotation around

sun – More stable that UT1 – Calculation based on Moon

orbit – Abolished in 1984 and

replaced by dynamical time scales

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Time Scales - Atomic International Atomic Time

- TAI - Established by BIPM - Initial phase : coincides with UT1

at January 1 1958 at 00:00:00 - Coordinated Time scale - Corrections due to gravity - Paper Clock - Continuous time scale (not

affected by rotation of earth

Coordinated Universal Time

- UTC - Established by BIPM - Start: 1961 - Frequency adjusted to keep

aligned with UT1 - Coincides with UT1 ( +/- 0,9 s) - 1972: UTC seconds = TAI

seconds - Leap seconds (+10 in 1972) - UTC = TAI + leap seconds - Paper time scale - Local realizations: UTC(k)

UTC realisation BIPM

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BIPM (Bureau International Poids

et Mesures)l

National Timing

Institutes

Time Labs

Free Atomic Time (EAL)

International Atomic

Time (TAI)

Coordinated Universal

Time (UTC)

BIPM Circular T

Local time scales

300 to 500 Free running

commercial Atomic Clocks

Primary Frequency

Standards (PFS)

IERS (International Earth Rotational service)

Earth Rotation Measurements

Universal Time (UT1)

Leap seconds

Calibration

Dissemination by broadcasting or

network (NTP, PTP, …)

Corrections to local time scales (UTC(k)

UTC(k)

Atomic Clock (commercial)

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- Cesium atoms heated into gas - Separation F3 and F4 state in magnetic Gate - Exposed to EM radiation near 9 192 631 771 Hz - Atoms in superposition - Passing of second µWave cavity - Completion of transition - Second separation of atoms (only F4 state) - Detector, feedback coupling

Fontain Cesium Clock

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Difference between the normalized frequencies of EAL and TAI. Steering correction 6.501x10-13 (2018 MAR 27 - 2018 APR 26) New correction 6.501x10-13 (2018 APR 26 - 2018 MAY 31) New correction foreseen 6.501x10-13 (2018 MAY 31 - 2018 JUN 30)

Duration of the TAI scale interval d. Estimate of d by the BIPM based on all PSFS measurements Period of estimation (2018 MAR 27 - 2018 APR 26) d = -0.03x10-15 +/- 0.19x10-15

TimeLabs WorldWide

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Time & Frequency Transfert - Two way (Communication satellite)

- Time transfer : 1 -10 ns) - Frequency transfer : 10-15

- One way (GPS,GLONASS,GALLILEO)

- Time transfer : 5 -20 ns - Frequency transfer: 10-15 (24 h)

- Optical fiber (50 km)

- Time transfer: 10 – 50 ps - Frequency transfer: 10-17

- Optical fiber (2000 km) - Time transfer: 100 ns - Frequency transfer: 10-14 (24 h)

Clocks and SI-second in the future

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• Optical clocks are the next generation of atomic clocks

• Still in development phase

• Optical frequencies vs microwave frequencies

• Uncertainties in range 10-18

• Will lead to new definition of second

• Easily transport of optical signals using fiber links