Realization and maintenance of UTC

Transcription

1 Realzaton and mantenance of UTC Elsa Felctas Aras Radocommuncaton development n lght of WRC-2 decson St. Petersburg, 6-8 JUNE 202. Tmescales mantaned at the BIPM Internatonal Atomc Tme (TAI) Coordnated Unversal Tme (UTC) Terrestral Tme (TT(BIPM)) Contnuous Interval unt s the SI second Calculated monthly at BIPM No clock representaton, no broadcast -second dscontnutes Interval unt s the SI second Calculated monthly at BIPM, derved from TAI Clock representatons UTC(k), broadcast Contnuous Interval unt s the SI second Calculated annually at BIPM Monthly predctons No clock representaton, no broadcast

2 UTC Coordnated Unversal Tme Coordnated broadcast of tme sgnals (ITU-R) Related to the Earth s rotaton; Unversal Tme (UT) was the name of the tme scale derved from the apparent daly moton of the Sun TAI dffers from UTC n an ntegral number of seconds Leap second announcement s the responsblty of the IERS GMT (Greenwch Mean Tme) was replaced by UT n 948 (IAU) UT was replaced by UTC n 972 (CGPM) 3 Each month: 68 partcpants ~400 clocks, one measurement / 5 days, t&f corr. monthly ftp server 3 prmary freq. standard data monthly ~80 tme transfer fles daly, weekly, monthly Data submsson deadlne 4/mm/yy/ Dataton s n «Modfed Julan Date», a contuous count of days snce an arbtrary orgn 4 2

3 Elaboraton of TAI and UTC - ALGOS 400 atomc clocks n 70 laboratores average EAL Echelle Atomque Lbre 3 prmary frequency standards (8 labs) steerng TAI freq stablty 0.4 x days Temps Atomque Internatonal Measurement of Earth s rotaton (IERS) leap seconds freq accuracy ~0-5 UTC(k) Crcular T UTC Temps Unversel Coordonné Role of the BIPM 5 Frst step: from clock readngs to clock dfferences - Remote clock comparson by tme transfer technques Global Navgaton Satellte Systems GPS (currently) GLONASS (currently) Galleo, BeDou (future) Two-way satellte tme and frequency transfer 3

4 Measure wth each clock a common external sgnal: GNSS Each staton measures (Local clock Satellte clock) Then two solutons Common-vew UTC(PTB)-UTC(NICT) = [UTC(PTB)-Sat] - [UTC(NICT)-Sat] PTB NICT Sat All-n-vew Sat UTC(PTB)-UTC(AUS) = [UTC(PTB)-Sat] - [UTC(AUS)-Sat2] + [Sat-Sat2] PTB wth [Sat-Sat2] provded by external global analyss AUS Sat2 GNSS tme transfer Calbrated recevers Common-vews / all-n-vew Correctons for the orbtal moton of the satellte Sgnal propagaton delays Ionosphere Troposphere Quas contnuous observatons Envronmental condtons lmt the performance of multchannel recevers sngle-channel, sngle frequency mult-channel, sngle frequency mult-channel, dual frequency Uncertanty mproves 8 4

5 Tme comparsons by correlaton: example of GNSS code Typcal uncertanty σ C k/(b/snr) where B s the transmtted bandwdth.e. /B s the chp length GPS: B = /0 MHz => σ C 0/ ns Remote sgnal Local sgnal GPS/GLONASS SC, MC, P3 Frequency comparsons: example of GNSS phase Contnuous mxng of remote sgnal vs. local sgnal Uncertanty on phase σ φ /(f/snr) where f s the transmtted frequency GPS: f =.5 GHz => σ φ ps GPS PPP Remote sgnal Local sgnal 5

6 TWSTFT smply computaton of clock offsets drect lnk of clocks no models necessary clock effects can be separated from others Calbrated recevng/emttng statons Telecommuncatons satellte No clock on board used Geostatonary TWSTFT, TWPPP A measurement technque used to compare two clocks or oscllators at remote locatons. Partcpatng laboratores and ther tme transfer equpment 6

7 Tme transfer technques n UTC (October 20) GPS/ GPS GLO SF TW/ SF GPS DF GPS DF GPS SF GPS DF Uncertanty of tme transfer n UTC 7 u B /CA/GNSS 6 Uncertanty Type A Type B /ns u A u B /C P/GNSS/TW 2 0.3ns u B / TW 0 PPP+TW GPS SA ON Off MC/CV MC/AV P3/AV PPP/TW

8 σ= 0.43 ns TWSTFT σ= 0.84 ns GPS P3 σ= 0.06 ns GPS PPP σ= 0.04 ns TW PPP Clocks n UTC Stablty 7x0-6 ( day) Stablty algorthm Stablty x0-4 ( day) Accuracy algorthm Accuracy 5 x0-3 Accuracy some

9 Stablty algorthm In an deal stuaton, Smultaneous clock readngs x, Fx set of contrbutng clocks, Contnuously partcpatng clocks So, EAL (t) = {x } (t) (weghted average) In the real stuaton, Weghts change, Clocks are nterrupted, clock frequences suffer changes New clocks arrve So, EAL (t) = {x } (t) + A + B(t-t 0 ) 7 EAL: Weghtng Algorthm The weght attrbuted to a clock reflects ts long-term stablty, snce the objectve s to obtan a weghted average that s more stable n the long term than any of the contrbutng elements. In the tme scale algorthms clock weghts are generally chosen as the recprocals of a statstcal quantty whch characterzes ther frequency stablty, such as a frequency varance (classcal varance, Allan varance...) EAL: Predcton Algorthm In the generaton of a tme scale, the predcton of the atomc clock behavor plays an mportant role; The predcton s useful to avod or mnmze the frequency jumps of the tme scale when a clock s added or removed from the ensemble or when ts weght changes. 8 9

10 The system solved by ALGOS: where x The soluton s: x ( t) = EAL( t) h ( t) j...eal (cont.) N N w x = x N ( t) = w h ( t) = ( t) x ( t) = x ( t) N s the number of atomc clocks N w the relatve weght of the clock H. h (t) s the readng of clock H at tme t w = = h '(t) s the predcton of the readng of clock H [ ] ( t) = EAL h = w h ( t) x ( t) j = Wegth j, j, j Predcton 9...Weghtng Algorthm The weght attrbuted to clock H s the recprocal of the ndvdual classcal varance σ 2 ω = N = σ 2 σ 2 Upper Lmt ω = MAX Two partcular stuatons are checked:. Clock H shows abnormal behavour 2. The weght s bgger then the upper lmt fxed to avod that a clock has a predomnant role. A N A=2.5 emprcal constant N w = = The weght attrbuted to clock H s computed from the frequences of the clock, relatve to EAL, estmated over the corrent 30 day nterval and over the past fve consecutve 30 days perod. The weght determnaton thus uses clock measurement coverng one year. 20 0

11 Weghtng procedure for EAL An teratve process, ncludng 4 teratons, s used n ALGOS: The system s solved (x ) wth normalzed weghts ω calculated n the prevous nterval. The frequency y are estmated for each clock usng x Indvdual varances are estmated 2 σ ) Abnormale behavour 2) The weght s bgger then the upper lmt The relatve weght ω TEMP of each clock s obtaned ω TEMP = N = σ 2 σ 2 Weght test 2...Predcton Algorthm t t t + 30 days EAL(t -t - ) 30 days EAL(t + -t ) In two dfferent ntervals the clock ensemble can change The consequences: EAL EAL t t t + Tme step t t t + Frequency step 22

12 Clock behavour The atomc clocks are caracterzed by dfferent behavour: ) Determnstc behavour Lnear cesum clocks Quadratc Hmaser clocks 2) Stochastc behavour A statstcal method used to determne nstabltes gven by the stochastc component s the: Allan Varance 23 Predcton Algorthm on EAL The correcton term h (t) for clock H s : Tme correcton frequency frequency drft h ' ( t) = a ( t ) + B ( t)( t t ) p 24 2

13 EAL was affected by a drft relatve to TT(BIPM) of about 4 x 0-6 /month. The new frequency predcton model s stoppng the drft and mproves the stablty of EAL. y(eal)-y(tt) 7 Lnear Predcton Quad. Pred Normalzed Frequency MJD Improvement of the frequency stablty of EAL wth the new clock frequency predcton model (snce July 20) 0-4 Frequency Stablty Lnear Predcton Quad. Pred Overlappng Allan Devaton Averangng Tme, Seconds 3

14 From EAL to TAI EAL s a free-runnng atomc tme scale optmzed to be a tme scale stable a long term. We evaluate the EAL frequency (f(eal)) by means the prmary frequency standards (PFS). TAI s expected to be stable (from EAL) and accurate (from PFS). Accuracy s obtaned by frequency steerng: f(tai) = f(eal) + steerng frequency 27 Prmary frequency standards Prmary frequency standards 3 n the last fve years (KRISS, INRIM, LNE-SYRTE, NICT, NIST, NMIJ, NPL, PTB), are Cs fountans Normalzed Frequency f(eal)-f(pfs) only fountans NIST-F PTB-CSF PTB-CSF2 SYRTE-FO SYRTE-FO2 SYRTE-FOM IT-CSF NPL-CsF NPL-CsF2 NICT-CsF NMIJ-F MJD (Modfed Julan Date) 4

15 TT(BIPM) The BIPM computes n deferred tme TT(BIPM), whch s based on a weghted average of the evaluatons of TAI frequency by the PFS. TT(BIPM) s computed n deferred tme and updated every year. Predctons of TT(BIPM) are computed monthly. It s the same algorthm used to evaluate f(eal) but n post processng. We consder TT(BIPM) the frequency reference to evaluate:.f(eal) performance 2.f(TAI) performance 3.PFS performance 29 f(tai) f(tt(bipm)) The goal! f(tai)-f(tt(bipm)) Year TAI s close to ts defnton (< 5x0-5 over last 2 years), but stll off 30 5

16 Stablty of TAI respect to TT(BIPM) The long-term nstablty of TAI s between x0-5 and 2x0-5, a factor two or three worse than the value for TT(BIPM). 3 Traceablty of UTC(k) to UTC 6

17 Improvng UTC for the 2 st century applcatons Better clocks (n labs) New pfs (n labs) Optcal frequency standards (n labs) Accurate tme and frequency transfer (labs, BIPM) Improved clock comparsons by refnng tme transfer (labs, BIPM) Improved algorthms (labs for UTC(k), BIPM for UTC) Provdng UTC more frequently (BIPM Rapd UTC project) Impact on UTC(k) Impact on steerng GNSS tmes to a representaton of UTC Renderng UTC contnuous (ITU) Benefts dsplayed at ths meetng Many thanks for your attenton! 7