Clock Comparisons: Present and Future Approaches

IntroductionI. Dissemination of Legal TimeII. Comparisons of Time ScalesIII. Comparisons of Primary Clocks

MicrowaveTime & Frequency ComparisonsGPS P3 GPS carrierTWSTFT

Future PossibilitiesImproved 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-TechnischeBundesanstalt, Braunschweig Germany

Workshop on an Optical Clock Mission in ESA’s Cosmic Vision Program, Düsseldorf, March 8, 2007

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 productsexceeds the provisions for DCF 77 by 14 times

II. Comparison of National Time Scales

51384 51744 52104 52464 52824 53184

-100

-80

-60

-40

-20

0

20

40

60

80

100

PTB NIST OP IEN NPL USNO

[UTC

–U

TC(k

) ] /

ns

MJDUTC – UTC(k) July 1999 – July 2004

III. Comparisons of Primary Clocks

FOUNTAIN

uB ≤ 10-15

H-Maser

FOUNTAIN

uB ≤ 10-15

H-Maser

TWSTFT

GPS CP

GPS TAI P3

?

Campaign: MJD 53304 – 53324 ( .. 53330) (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 fourEuropean Timing Institutes and NIST Using Multiple Techniques: EFTF 2005, Besancon

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 observableTransfer to CGGTTS format:

track length 16 minutes, non-continuous observationCorrections 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 formedby weighting individual CV data according to satellite elevation.

Reference: Defraigne and Petit, Metrologia 40 (2003), p. 184

Time Transfer Technique II: Carrier Phase

• Contributions of the Centre for Orbit Determination in Europe to IGS, Bernese GPS Software, Version 5.1

• new algorithm for GPS 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 betweenhydrogen masers

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.1153-1“The operational use of two-way satellite time and frequency transfer employingPN time codes”, ITU, Geneva, last update 2003.

Comparison of techniques (Link: NPL – OP)

Residuals after removing quadratic trend individually

TAI P3

GPS CP

TWSTFT

Comparison of techniques (Link: NPL – OP)

Instability of the relative frequency difference H-Maser(OP) – UTC(NPL)

GPS TAI P3

GPS CP

TW

GPS CP - TW

Summary (Instability achievable in 1 day)

TWSTFT: modσy < 1x10-15 collecting 12 points per day

GPS CP: modσy 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 fourEuropean Timing Institutes and NIST Using Multiple Techniques:EFTF 2005, Besancon

Time links existing between USNO and PTB

USNO PTB

GPS-related linkstwo-way links viatelecom satellites

multi-channel C/A codegeodetic time-transfer

(TAI-P3 and IGS) Ku-band (11-14 GHz)

X-band (7-8 GHz)

D. Piester, A. Bauch, T. Polewka, J. Becker (PTB)

A. McKinley, L. Breakiron, A. Smith, B. Fonville, D. Matsakis (USNO)

Motivation

Laboratory equipped with TWSTFT GPS CV single-channel linkTWSTFT GPS CV single-channel back-up linkTWSTFT by Ku band with X band back-up GPS CV multi-channel linkLaboratory equipped with dual frequency reception GPS CV multi-channel back-up linkGPS CV dual frequency linkGPS CV dual frequency back-up link

North America Europe Asia

South America Africa Oceania

USNO

ONBA TCC IGMA ONRJ CSIR

CNMP CNM

NRC

APL

ROA DTAG BEV AOS PL LT SU NIMB

CRL

AUS

PTB

IEN

OP

NPL

VSL

ORB DLR IFAG CH LDS JV SP

NPLI

INPL

SMU

NMC

UME

TP

OMH

CAO

NIST

NTSCJATC

MSL

SCL

NMIJ

TL

KRIS

SG

NIMT NMLS BIRM

NIM

NAO

ORGANIZATION OF THE INTERNATIONAL TIME LINKS March 2004

The link between USNO and PTB connects almost one half of theclocks contributing to TAI

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

Long-term link characteristics

Differencebetweendaily data of Ku-band and X-band

best-fitsine function

period370 days

amplitude0.6 ns

Calibration of Time Link USNO – PTB

Results of repeated calibrations using travelling X-band station

1.1.2002 1.1.2003 1.1.2004 1.1.2005 1.1.2006-4-3-2-101234

-4-3-2-101234

diff

eren

tial c

alib

ratio

n (n

s)

date

X-band

Ku-band

Results of USNO – PTB link

1) Link Characteristics - variations of 0.6 ns with one year period

2) Calibration - combined uncertainties below 1 ns achievable

- investigation of correlation with temperature variations

- operation of two low-noise links in parallel sufficient

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 ...

T [s]

1 10 100 1000

σ y o

f diff

eren

ce fr

eque

ncy

2x10-16

4x10-16

6x10-168x10-16

10-15

2x10-15

4x10-15

E. Peik, T. Schneider, Chr. Tamm,J. Phys. B: At. Mol. Opt. Phys 39,145 (2006)

Stability of an Yb+

single ion clock at PTB

Time Transfer by Laser Link

Laser station emits asynchronous, short light pulses (~ 20 ps FWHM)towards the satellite. Retro-reflecting corner-cubesreturns a fraction of thereceived photons back to thestations. The station records the start (tS) and return (tR) time of each light pulse.

Time Transfer by Laser Link

1. Lasso:proposed in 1972 and launched in 1988 on Meteosat P2.A first optical time transfer had been successfullyachieved 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 programscheduled on the ISS; descoped in 2001

• now payload on the Jason-2 space vehicle, whichwill 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 (onepass) and than 10ps over one day.

Frequency Transfer via TelecommunicationFibre Network

Frequency standard

Frequency standard

Fibrenetwork

Transfer laser1,5 µm

Transfer laser1,5 µm

Frequency comb Frequency comb

Opticalfrequency

comparison

1 10 100 1k 10k 100ks10-18

10-17

10-16

10-15

10-14 ALLAN STANDARD DEVIATION of FREQUENCY

86 km User End cw Laser Link 86 km LLink Modulated Laser ( with Scrambler)

σy( 2

, τ )

integration time

Two-way Fibre Link

G. Santarelli, H. Schnatz, B. Lipphardt, G. Grosche, Nov. 2006 @ SYRTE

Vision: European Fibre Network

NPLLondon

UniHannover

PTBBraunschweig

UniBerlin

MPQGarching

INRIM Turin

Telecom Fibre Network

Sr, Yb

Mg

H

Sr, Yb+

SYRTE- LPLParis

Sr, Yb+

ESA

Conclusions

• TWSTFT on transcontinental links approaches a fractionalinstability of 10-15 per day

• With repeated calibrations < 1ns have been demonstrated

• Better technology is in general available, that could improvestability by an order of magnitude

• The microwave technology does not seem capable to supportthe requirements of the optical clocks

• Optical satellite links are in an experimental status

• Terrestrial optical fibre links are going to be explored

Help fromA. Bauch, D.Piester, H. Schnatz, E. Peikis gratefully acknowledged