Time Scales and Time Keeping in the 21st Century Peter Whibberley NPL Time & Frequency User Club 3 December 2008 Wednesday, 24 December 2008 Overview • How we got where we are – from GMT to UTC • How UTC is produced • Dissemination of UTC • Future developments 2 Wednesday, 24 December 2008 Astronomers’ time • Based on Sun’s movement across the sky • GMT is mean solar time on the Greenwich meridian • “clock time” adjusted to keep in step with the Sun – The second is 1/86400 of mean solar day 3 Wednesday, 24 December 2008 Greenwich Mean Time > Universal Time • 1884 – GMT adopted as worldwide reference – Prime meridian at Greenwich – Time zone system adopted • 1928 – IAU adopted the name “Universal Time” to designate the mean solar day starting at Greenwich mean midnight – The term “GMT” remained in use in some countries 4 Wednesday, 24 December 2008 Earth rotation • 1940s – evidence for variations in Earth’s rotation Plot shows excess length of day (LOD) Source USNO: http://tycho.usno.navy.mil/leapsec.html 5 Wednesday, 24 December 2008 UT1 • Several versions of Universal Time introduced: – UT0 is observed mean solar time – UT1 is UT0 corrected for polar wander – UT2 is UT1 corrected for seasonal variations • UT1 provides the most useful form of mean solar time – Since 2003, UT1 has been a measure of Earth rotation angle – Used by navigators, astronomers and Earth scientists • UT1 is determined and published by the International Earth Rotation and Reference Systems Service (IERS) • UT1 plays an important part in UTC 6 Wednesday, 24 December 2008 Atomic time • First caesium clock operational at NPL in 1955 7 Wednesday, 24 December 2008 The “atomic” second • 1955-1958 - Frequency of NPL caesium standard compared with second of Ephemeris Time (ET) determined by US Naval Observatory • Measurements led to re-definition of the second in 1967: ‘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 caesium-133 atom.’ 8 Wednesday, 24 December 2008 International atomic time • More caesium clocks developed – NBS (USA), ON (Switzerland), etc • 1958 - Combined atomic time scale begins • International Atomic Time (TAI) – (UT – TAI) ~ 0 on 1 January 1958 – Scale interval is the SI second at mean sea level – Conventionally counted using hh:mm:ss • Not regarded by BIPM as a disseminated time scale 9 Wednesday, 24 December 2008 Coordinated Universal Time (UTC) • UTC originated ~1960 as reference time scale coordinated between countries operating broadcast time signals – Name formally adopted ~1967 • UTC provides a time scale with stability of atomic clocks but steered to UT • UTC forms the basis of civil timekeeping world-wide • UTC defined in ITU-R Recommendation TF.460-6 10 Wednesday, 24 December 2008 How UTC is formed • Computed by the Bureau International des Poids et Mesures (BIPM), Paris • Based on 3 types of data: – Time transfer measurements between the BIPM and realisations of UTC (known as UTC(k)) at ~60 institutes – Offset measurements between each UTC(k) and other clocks in that institute or country, ~260 clocks in total contributing – Realisations of the SI second by primary frequency standards • Processing is carried out in monthly blocks • The results are published in BIPM Circular T 11 Wednesday, 24 December 2008 Institutes contributing to UTC Source BIPM: www.bipm.org/en/scientific/tai/tai.html 12 Wednesday, 24 December 2008 International time Local UTC(k) time scales BIPM ~260 commercial Free Atomic Time atomic clocks (EAL) IERS ations calibr Earth rotation Primary International measurements Frequency Atomic Time Standards (TAI) (PFS) leap seconds Universal Time (UT1) National Timing Institutes Coordinated Universal Time (UTC) corrections to local UTC time scales BIPM Circular T 13 Wednesday, 24 December 2008 Steering UTC to UT • Since 1 January 1972: – (UTC – TAI) is integer number of SI seconds – Leap second adjustments are applied to UTC to keep |UT1 – UTC| < 0.9 s • The International Earth Rotation and Reference Systems Service (IERS): – Monitors Earth rotation data (VLBI, GPS, SLR) – Announces in Bulletin C whether a leap second is required at the end of the following December or June, ~ 6 months in advance 14 Wednesday, 24 December 2008 Leap seconds • Current offset is TAI – UTC = 33 s – All leap seconds so far have been positive • Next leap second will be applied to UTC at end of 2008- 12-31 • Positive leap second insertion: 23:59:57 23:59:58 23:59:59 =>23:59:60 00:00:00 00:00:01 15 Wednesday, 24 December 2008 UT1 - UTC Source IERS: www.iers.org/MainDisp.csl?pid=36-1100231 16 Wednesday, 24 December 2008 BIPM Circular T CIRCULAR T 249 ISSN 1143-1393 2008 OCTOBER 10, 12h UTC BUREAU INTERNATIONAL DES POIDS ET MESURES ORGANISATION INTERGOUVERNEMENTALE DE LA CONVENTION DU METRE PAVILLON DE BRETEUIL F-92312 SEVRES CEDEX TEL. +33 1 45 07 70 70 FAX. +33 1 45 34 20 21 [email protected] 1 - Coordinated Universal Time UTC and its local realizations UTC(k). Computed values of [UTC-UTC(k)] and uncertainties valid for the period of this Circular. From 2006 January 1, 0h UTC, TAI-UTC = 33 s. From 2009 January 1, 0h UTC, TAI-UTC = 34 s. Date 2008 0h UTC AUG 31 SEP 5 SEP 10 SEP 15 SEP 20 SEP 25 SEP 30 Uncertainty/ns Notes MJD 54709 54714 54719 54724 54729 54734 54739 uA uB u Laboratory k [UTC-UTC(k)]/ns AUS (Sydney) 322.4 342.1 353.3 364.5 387.6 408.9 429.2 1.5 5.1 5.3 BIRM (Beijing) -5999.2 -6030.5 -6063.4 -6100.5 -6130.7 -6163.8 -6210.1 2.0 20.0 20.1 DLR (Oberpfaffenhofen) - - -10.0 -18.4 -13.7 -10.8 -8.3 0.7 5.1 5.2 IT (Torino) 12.8 16.4 17.2 22.8 22.8 23.9 20.7 0.6 1.5 1.6 NIST (Boulder) -1.4 -0.1 -1.2 -1.0 -2.0 -2.7 -4.7 0.5 4.9 4.9 NMIJ (Tsukuba) -0.3 10.9 21.5 30.0 39.4 46.8 53.5 0.7 5.1 5.1 NPL (Teddington) -38.4 -40.3 -40.7 -43.6 -45.1 -41.8 -41.2 1.5 5.1 5.3 SIQ (Ljubljana) 14.9 16.9 -380.1 -409.5 -461.1 -472.0 -490.0 5.0 20.0 20.7 SU (Moskva) -4.5 1.5 4.4 7.8 7.9 10.5 12.7 3.0 5.1 5.9 USNO (Washington DC) 3.3 3.7 4.9 6.9 7.2 7.0 7.2 0.4 1.8 1.8 VMI (Ha Noi) -80.9 -75.6 -66.1 -51.8 -45.6 -62.8 -69.2 1.0 20.0 20.1 Available from: www.bipm.org/jsp/en/TimeFtp.jsp 17 Wednesday, 24 December 2008 UTC(k) offsets from UTC 18 Wednesday, 24 December 2008 Properties of UTC • Not available in real time – Latency of 12-45 days • Traceability is via one of the ~60 UTC(k) • Provides the reference for ALL civil and broadcast time scales, eg: – GPS Time (via UTC(USNO)) – Internet time protocols, eg. NTP – Radio time signals, eg. MSF, DCF77, WWVB – Speaking clock • Provides an approximation to Universal Time – To within 1 s offset – UTC sometimes incorrectly called GMT 19 Wednesday, 24 December 2008 The future of UTC (1) • Proposals to cease applying leap seconds to UTC being debated in ITU-R WP 7A • Issue first raised in 1999 • International conference held in Turin in 2003 – Recommended long implementation period – Recommended new name: International Time (TI) – No overall consensus • Only 2 options currently being considered: – Continue the present system – Apply no further leap seconds to UTC after a specified date 20 Wednesday, 24 December 2008 The future of UTC (2) • Arguments for the change: – Leap seconds require manual intervention – Computer systems have difficulty coping with an additional second – Current situation is resulting in proliferation of time scales without leap seconds • Arguments against the change: – Problems identified are relatively minor – Equipment problems are amenable to technical solutions – Present system works so no urgency to change • Debate is currently deadlocked – UK, China opposed to change at present – BIPM and several other countries in favour 21 Wednesday, 24 December 2008 Improvements to UTC • Expect steady improvement in stability – More H-masers contributing, in better environments – Improvements to time transfer links – More rapid processing • Incorporation of optical frequency standards – Next generation of primary frequency standards – Optical frequency & time transfer methods: fibre, laser link (T2L2) – Future re-definition of the SI second? 22 Wednesday, 24 December 2008 Summary • UTC is the reference time scale in use world-wide • UTC provides the stability of atomic time (SI second) and a realisation of Earth rotation time (UT1) • Access to UTC is through disseminated time signals linked to UTC through the national time scales UTC(k) 23.