TIME & FREQUENCY. Overview from artefact to current definition & Realisation UTC. Frank Coutereel.

Transcription

1 TIME & FREQUENCY Overview from artefact to current definition & Realisation UTC Frank Coutereel

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

3 Definition of the second before 1960 Based on 1/86400 th 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

4 Between 1960 and 1967: Ephemeris Time 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/ ,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

5 After 1967: Atomic Time The second is the duration of 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

6 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

7 Time Scales - Atomic International Atomic Time - TAI - Established by BIPM - Initial phase : coincides with UT1 at January 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: Frequency adjusted to keep aligned with UT1 - Coincides with UT1 ( +/- 0,9 s) : UTC seconds = TAI seconds - Leap seconds (+10 in 1972) - UTC = TAI + leap seconds - Paper time scale - Local realizations: UTC(k)

8 UTC realisation BIPM UTC(k) Dissemination by broadcasting or network (NTP, PTP, ) Local time scales 300 to 500 Free running commercial Atomic Clocks Primary Frequency Standards (PFS) National Timing Institutes Calibration Free Atomic Time (EAL) International Atomic Time (TAI) Earth Rotation Measurements Time Labs Coordinated Universal Time (UTC) Leap seconds Universal Time (UT1) Corrections to local time scales (UTC(k) BIPM Circular T IERS (International Earth Rotational service) BIPM (Bureau International Poids et Mesures)l

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

10 Fontain Cesium Clock Difference between the normalized frequencies of EAL and TAI. Steering correction 6.501x10-13 (2018 MAR APR 26) New correction 6.501x10-13 (2018 APR MAY 31) New correction foreseen 6.501x10-13 (2018 MAY 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 APR 26) d = -0.03x /- 0.19x10-15

11 TimeLabs WorldWide Time & Frequency Transfert - Two way (Communication satellite) - Time transfer : 1-10 ns) - Frequency transfer : One way (GPS,GLONASS,GALLILEO) - Time transfer : 5-20 ns - Frequency transfer: (24 h) - Optical fiber (50 km) - Time transfer: ps - Frequency transfer: Optical fiber (2000 km) - Time transfer: 100 ns - Frequency transfer: (24 h)

12 Clocks and SI-second in the future Optical clocks are the next generation of atomic clocks Still in development phase Optical frequencies vs microwave frequencies Uncertainties in range Will lead to new definition of second Easily transport of optical signals using fiber links