Clock Comparisons: Present and Future Approaches

Transcription

1 Clock Comparisons: Present and Future Approaches Introduction I. Dissemination of Legal Time II. Comparisons of Time Scales III. Comparisons of Primary Clocks MicrowaveTime & Frequency Comparisons GPS P3 GPS carrier TWSTFT Future Possibilities Improved Microwave Links (Reports by Ch. Salomon, W. Schäfer) Time Transfer by Laser Link (T2L2) Frequency Transfer by Fibre Links Fritz Riehle (A. Bauch, D. Piester, H. Schnatz,...) Physikalisch-Technische Bundesanstalt, Braunschweig Germany Workshop on an Optical Clock Mission in ESA s Cosmic Vision Program, Düsseldorf, March 8, 2007

2 I. Time for the Public (PTB) Telephone time service (1000 calls per day) Internet Service (> 30 million accesses per day DCF77 (> 20 million receivers in Europe) 1 ms everywhere in Germany, Synchronization of phone networks, power plants, aviation security,... According to a market report (2001): The VAT from DCF77 related products exceeds the provisions for DCF 77 by 14 times

3 II. Comparison of National Time Scales [UTC UTC(k) ] / ns PTB NIST OP IEN NPL USNO UTC UTC(k) July 1999 July 2004 MJD

4 III. Comparisons of Primary Clocks FOUNTAIN u B TWSTFT GPS CP GPS TAI P3? FOUNTAIN u B H-Maser H-Maser Campaign: MJD ( ) (Oct. / Nov. 2004) R. Dach (AIUB), G. Petit (BIPM), L. Lorini (IMGC), P. Whibberley, R.Hlavac (NPL), T. Parker (NIST), J. Achkar, P. Uhrich (OP), A. Bauch (PTB): Time and Frequency Comparisons between four European Timing Institutes and NIST Using Multiple Techniques: EFTF 2005, Besancon

5 Time Transfer Technique I: GPS TAI P3 Input data: dual-frequency P1 and P2 code observables from geodetic GPS receivers Processing: Linear combination to provide ionosphere-free P3 observable Transfer to CGGTTS format: track length 16 minutes, non-continuous observation Corrections applied: precise satellite orbits, satellite clocks provided by IGS station displacements (solid Earth tides) Evaluation of links in common view (CV) mode, 16 minute-averages formed by weighting individual CV data according to satellite elevation. Reference: Defraigne and Petit, Metrologia 40 (2003), p. 184

6 Time Transfer Technique II: Carrier Phase Contributions of the Centre for Orbit Determination in Europe to IGS, Bernese GPS Software, Version 5.1 newalgorithmforgps CP: ambiguities are connected at the day boundaries, consequently no discontinuities anymore only carrier phase measurements analyzed (no code data used) applied fot the first time during this campaign GPS CP provides: consistent receiver clock estimates for each epoch (once every 30 s) no assumptions for the clocks from epoch to epoch Use of estimates once every hour for GPS CP comparison between hydrogen masers

7 Time Transfer Technique III: TWSTFT TWSTFT sessions co-ordinated by the CCTF Working Group on TWSTFT TWSTFT using PRN-modulated signals (Mitrex or Satre modems) in Ku-band Satellite transponder on IS-903 provided free of charge by Intelsat Links between pairs of stations lasting 120 s once every two hours (12 sessions per day) Availability: some links 3 to 5 missing points out of 320 (25 days) some links up to 20 points missing Evaluation and report of data according to ITU-R Recommendation ITU-R TF The operational use of two-way satellite time and frequency transfer employing PN time codes, ITU, Geneva, last update 2003.

8 Comparison of techniques (Link: NPL OP) Residuals after removing quadratic trend individually TAI P3 GPS CP TWSTFT

9 Comparison of techniques (Link: NPL OP) Instability of the relative frequency difference H-Maser(OP) UTC(NPL) TW GPS TAI P3 GPS CP GPS CP - TW

10 Summary (Instability achievable in 1 day) TWSTFT: modσ y < 1x10-15 collecting 12 points per day GPS CP: modσ y 1x10-15 apparently in reach, as seen from double-differences GPS TAI P3: apparently the same instability for tau >= 2 days R. Dach (AIUB), G. Petit (BIPM), L. Lorini (IMGC), P. Whibberley, R.Hlavac (NPL), T. Parker (NIST), J. Achkar, P. Uhrich (OP), A. Bauch (PTB): Time and Frequency Comparisons between four European Timing Institutes and NIST Using Multiple Techniques: EFTF 2005, Besancon

11 Time links existing between USNO and PTB GPS-related links multi-channel C/A code geodetic time-transfer (TAI-P3 and IGS) two-way links via telecom satellites Ku-band (11-14 GHz) X-band (7-8 GHz) USNO PTB D. Piester, A. Bauch, T. Polewka, J. Becker (PTB) A. McKinley, L. Breakiron, A. Smith, B. Fonville, D. Matsakis (USNO)

12 Motivation North America Europe Asia NRC ORB DLR IFAG CH LDS JV SP SMU KRIS NIMT NMLS BIRM OP NMC APL NIM SG IEN UME USNO NAO CRL PTB NPLI CNMP CNM NIST NPL TP OMH INPL NMIJ VSL CAO NTSC JATC TL ROA DTAG BEV AOS PL LT SU NIMB SCL ONBA TCC IGMA ONRJ CSIR AUS MSL South America Africa Oceania ORGANIZATION OF THE INTERNATIONAL TIME LINKS March 2004 Laboratory equipped with TWSTFT TWSTFT TWSTFT by Ku band with X band back-up Laboratory equipped with dual frequency reception GPS CV dual frequency link GPS CV dual frequency back-up link GPS CV single-channel link GPS CV single-channel back-up link GPS CV multi-channel link GPS CV multi-channel back-up link The link between USNO and PTB connects almost one half of the clocks contributing to TAI

13 Stability of the double differences TWSTFT KU-band X-band over 120 days in 2006

14 Long-term link characteristics Difference between daily data of Ku-band and X-band best-fit sine function period 370 days amplitude 0.6 ns

15 Calibration of Time Link USNO PTB Results of repeated calibrations using travelling X-band station differential calibration (ns) 4 3 Ku-band X-band date

16 Results of USNO PTB link 1) Link Characteristics - variations of 0.6 ns with one year period - investigation of correlation with temperature variations 2) Calibration - combined uncertainties below 1 ns achievable - operation of two low-noise links in parallel sufficient

17 Possible improvements with microwave links Use higher chip-rate (bandwidth limitation) Use dedicated transponders Present performance is not limited by microwave technology -but by not implementing the best possible existing technology However... 4x10-15 Stability of an Yb + single ion clock at PTB E. Peik, T. Schneider, Chr. Tamm, J. Phys. B: At. Mol. Opt. Phys 39, 145 (2006) σ y of difference frequency 2x x x x x T [s]

18 Time Transfer by Laser Link Laser station emits asynchronous, short light pulses (~ 20 ps FWHM) towards the satellite. Retro-reflecting corner-cubes returns a fraction of the received photons back to the stations. The station records the start (t S ) and return (t R ) time of each light pulse.

19 Time Transfer by Laser Link 1. Lasso: proposed in 1972 and launched in 1988 on Meteosat P2. A first optical time transfer had been successfully achieved in 1992 betw. OCA, France and Mac Donald, USA. 2. T2L2: first proposed in 1996 to fly on the Russian space station MIR later accepted by ESA in the framework of the ACES program scheduled on the ISS; descoped in 2001 now payload on the Jason-2 space vehicle, which will be launched in mid 2008 for a 5 years long mission. Expected performances are in the 100ps range for accuracy, with an ultimate stability better than 1ps over 1000s (one pass) and than 10ps over one day.

20 Frequency Transfer via Telecommunication Fibre Network Frequency standard Optical frequency comparison Frequency standard Frequency comb Frequency comb Transfer laser 1,5 µm Fibre network Transfer laser 1,5 µm

21 10-15 Two-way Fibre Link ALLAN STANDARD DEVIATION of FREQUENCY 86 km User End cw Laser Link 86 km LLink Modulated Laser ( with Scrambler) σ y ( 2, τ ) k 10k 100ks integration time G. Santarelli, H. Schnatz, B. Lipphardt, G. Grosche, Nov. SYRTE

22 Vision: European Fibre Network Sr, Yb + Mg Uni Berlin NPL London Uni Hannover PTB Braunschweig Sr, Yb + Sr, Yb SYRTE- LPL Paris Telecom Fibre Network MPQ Garching H ESA INRIM Turin

23 Conclusions TWSTFT on transcontinental links approaches a fractional instability of per day With repeated calibrations < 1ns have been demonstrated Better technology is in general available, that could improve stability by an order of magnitude The microwave technology does not seem capable to support the requirements of the optical clocks Optical satellite links are in an experimental status Terrestrial optical fibre links are going to be explored Help from A. Bauch, D.Piester, H. Schnatz, E. Peik is gratefully acknowledged