Two-Way Time Transfer

A Universal Tool for Metrology and Wide-Area Time Synchronisation

NPL, Teddington, UK
17 September 2002

Wolfgang Schäfer

e-mail: wolfgang.schaefer@timetech.de
web: www.timetech.de
Phone: 0049-711-678 08-0
Fax: 0049-711-678 08 99
TimeTech GmbH
Curiestrasse 2
D-70563 Stuttgart
Germany

Contents

 1. Two-Way Time & Frequency Transfer via Satellite (TWSTFT) 3
 2. TWSTFT: Main Characteristics 4
 3. TWSTFT: Network and Operations5
 4. TWSTFT Long-Term Performance between Primary Laboratories 6
 5. TWSTFT: Comparison to GPS-Links 7
 6. TWSTFT: Modified Allan Deviation (Link USNO – NIST) 2.5 MChip/s 8
 7. TWSTFT: USNO – NIST, 20 MChip/s (Experimental) 9
 8. Frequency Transfer using 2-Way Carrier Phase (Experimental) 10
 9. Time Synchronisation using TWSTFT 11
10. TWSTFT: Ground-to-Ground, Space-to-Ground, Space-to-Space12
11. TWSTFT: Ground-to-Ground13
12. GALILEO: Time Service Augmentation14
13. GALILEO: Support of Navigation Service (Ranging) 15
14. Future of TWSTFT: MWL for ACES (Atomic Clock Ensemble in Space16
15. Summary and Outlook (Atomic Clock Ensemble in Space) 17
16. Conclusions 18


1. Two-Way Time & Frequency Transfer via Satellite (TWSTFT)



2. TWSTFT: Main Characteristics

Main Benefits:

  • Measure clock difference, DO NOT MODEL!
  • TWSTFT Link eliminates all 1st-order errors by SYMMETRY
  • Multiple links via same transponder using CDMA

    Independent from:

  • Ground: Location, height, tides, tectonic movement
  • Satellite: Location, velocity vector, transponder delay
  • Propagation: Path geometry, Troposphere, Ionosphere (dispersive)

    Asymmetry:

  • Relativistics: Sagnac
  • Propagation: Different up-down frequencies, Ionosphere
  • Instrumental and Calibration Errors

    3. TWSTFT: Network and Operations

  • Time-Transfer data contribute to generation of TAI by the BIPM
  • Regular operation in Europe, U.S. and Asia
  • Trans-Atlantic Links, US-Asia, Asia-Australia
  • Laboratories include Australia, France, Germany, Italy Japan, Netherlands,
        Spain, Sweden (2002), UK, Taiwan, USNO, Washingtion (GPS) incl. AMC,
        Colorado Springs, NIST, Boulder

    Performance:

  • Repeatability: 200 .. 300 ps
  • Calibration Error: 600 ps .. 3 µs

    4. TWSTFT Long-Term Performance between Primary Laboratories



    5. TWSTFT: Comparison to GPS-Links



    6. TWSTFT: Modified Allan Deviation (Link USNO – NIST) 2.5 MChip/s



    7. TWSTFT: USNO – NIST, 20 MChip/s (Experimental)



    8. Frequency Transfer using 2-Way Carrier Phase (Experimental)



    9. Time Synchronisation using TWSTFT



    10. TWSTFT: Ground-to-Ground, Space-to-Ground, Space-to-Space




    11. TWSTFT: Ground-to-Ground




    12. GALILEO: Time Service Augmentation



    13. GALILEO: Support of Navigation Service (Ranging)



    14. Future of TWSTFT: MWL for ACES (Atomic Clock Ensemble in Space)



    15. Summary and Outlook (Atomic Clock Ensemble in Space)



    16. Conclusions

  • Method designed to achieve specified end-to-end accuracy
  • TWSTFT relies mostly on direct measurements
  • Residual errors can be minimised with simple models
  • Time-proven and established operational reliability
  • Fully recognised by national timing laboratories, incl. USNO (GPS) and BIPM
  • Long-term experience and extensive performance characterisation available
  • Covering: G/S equipment, real hardware, system operations, data evaluation etc.

    Applications

  • Wide-Area (Computer) Time Synchronisation using Return-Channel Links
  • Augmentation / Contribution to GALILEO's Time- and Navigation Services
  • Time-Scale in space for worldwide use: (ACES, PARCS)
  • Ideal means for Harmonisation of Time in Europe
        –» Goal: Accuracy + Reliability comparable to GPS