31st Annual Precise Time and Time Interval Systems and Applications Meeting

New Trends in Two-Way Time and Frequency Transfer via Satellite

Dana Point, California
December 7-9, 1999

Wolfgang Schaefer, Alexander Pawlitzki, Torge Kuhn

e-mail: wolfgang.schaefer@timetech.de
alexander.pawlitzki@timetech.de
torge.kuhn@timetech.de
web: www.timetech.de
Phone: 0049-711-678 08-0
Fax: 0049-711-678 08 99
TimeTech GmbH
Nobelstrasse 15
D-70569 Stuttgart
Germany

Table of contents

 1. Monitoring and Control, Operational Aspects3
 2. Real-Time Data Generation4
 3. Co-Existence with Other Satellite Users5
 3.1. Higher Chipping Rates (up to 20 MChip/s) 5
 3.2. Spread-Spectrum with Tone Modulation 6
 3.3. Combination with Satellite Services9
 4. Two-Way Time Synchronisation via Satellite 10
 5. Two-Way Frequency Transfer using Carrier Phase12
 6. MDEV, PN-code 2.5 MChip/s, USNO <-> NIST 13
 7. MDEV, PN-code 20 MChip/s, USNO <-> NIST 14
 8. MDEV, Carrier Phase, Signal 2.5 MChip/s, USNO <-> NIST15
 9. MDEV, Carrier Phase, Signal 5 MChip/s, PTB <-> DLR, Spring 199916
10. MDEV, Residual between Receiver Channels, at USNO17
11. TDEV, Carrier Phase, Signal 2.5 MChip/s, USNO <-> NIST18
12. Frequency Transfer, PN versus Carrier Phase19
13. Frequency Transfer, PN 2,5 and 20 MChip versus Carrier Phase20
14. Frequency Transfer, PN 20 MChip/s versus Carrier Phase, Detail 21
15. Frequency Transfer, Carrier Phase Residuals, 20 MChip signal 22
16. Summary, Frequency Transfer 23
17. Conclusions 24
18. Acknowledgements 25




1. Monitoring and Control, Operational Aspects

  •  Automatic Operation of TWSTFT modems
  •  Automatic schedule to cover multiple signal links
  •  Remote control via local area network or Internet
  •  Centralised monitoring and control facility
  •  Real-time data- and performance evaluation at control centre
  •  Unattended stations
  •  Remote transmitter shut-down
  •  Warnings, error reporting and error logging
  •  Redundancy, automatic fail-over, automatic LAN re-connect
  •  Compact dedicated ground station controlled by modem
  •  Support instrumentation controlled by modem

    2. Real-Time Data Generation

  •  Data Exchange between stations via RF
  •  Real-time evaluation of TWSTFT formula: on-screen data display
  •  Data Reduction: 1st, 2nd and 3rd order regression
  •  Built-in Database and Data Storage
  •  Fast Time-to-Alarm for continuous operating systems:

    2.5 MChip/s
     
      3 ns @ 1 s   
    1 ns @ 10 s 
    20 MChip/s
     
      300 ps @ 1 s   
    100 ps @ 10 s 
    Carrier Phase  15 ps @ 1 s   


    3. Co-Existence with Other Satellite Users

    3.1. Higher Chipping Rates (up to 20 MChip/s)

  •  Max chip-rate adjusted to standard transponder bandwidth (~ 30 MHz)
  •  NO visible interference to Direct TV emissions at 20 MChip/s
  •  Direct TV Analogue FM:                                                          S/N   -  30 dB
  •  Direct TV Digital:                                                                      S/N   -  23 dB
  •  NO interference to digital satellite services at 20 MChip/s  S/N < - 20 dB

    3.2. Spread-Spectrum with Tone Modulation

  •  New modulation scheme: PN-modulated Tone

    Spread Spectrum with Tone Modulation (continued)

  •  Tone: high precision, poor ambiguity: 30 ..300 MHz . 30 .. 3ns
  •  PN:    full ambiguity resolution                   1 MChip/s . 1µs
  •  Minimise spectrum requirements, optimise performance

    Spread Spectrum with Tone Modulation (continued)

  •  PN-modulated tones may be located in guard bands between transponders

    3.3. Combination with Satellite Services

  •  Ranging and Orbit Determination:                         Benefit to Sat-Operator
  •  Ground Segment and Network Synchronisation: Benefit to Sat-Users

  •  Time Labs shall NOT pay for transponder use
  •  Detailed negotiations with Satellite Operators and Satellite Owners required

    4. Two-Way Time Synchronisation via Satellite

    Two-Way Time Synchronisation (continued)

  •  Real-time application of TWSTFT method
  •  Clock at slave station may be internal or external to modem
  •  Reduce clock costs at slave station (crystal instead of atomic clocks)
  •  Maximise knowledge about clock at slave station
  •  Continuous / frequent operation maximises performance
  •  Time to alarm: 1 .. 10 s

    5. Two-Way Frequency Transfer using Carrier Phase



    6. MDEV, PN-code 2.5 MChip/s, USNO <-> NIST



    7. MDEV, PN-code 20 MChip/s, USNO <-> NIST



    8. MDEV, Carrier Phase, Signal 2.5 MChip/s, USNO <-> NIST



    9. MDEV, Carrier Phase, Signal 5 MChip/s, PTB <-> DLR, Spring 1999



    10. MDEV, Residual between Receiver Channels, at USNO



    11. TDEV, Carrier Phase, Signal 2.5 MChip/s, USNO <-> NIST



    12. Frequency Transfer, PN versus Carrier Phase



    13. Frequency Transfer, PN 2,5 and 20 MChip versus Carrier Phase



    14. Frequency Transfer, PN 20 MChip/s versus Carrier Phase, Detail



    15. Frequency Transfer, Carrier Phase Residuals, 20 MChip signal



    16. Summary, Frequency Transfer

       MDEV1s100s
    PN @ 2.5 MChip/s4 E-104 E-13
    PN @ 20 MChip/s6 E-111 E -13
    Carrier Phase2 E-121 E -14

  •  2-Way Carrier Phase Frequency Transfer
         rough accuracy: ~ 2 E – 14 @ 1200 s TWSTFT
         TDEV < 0.5 ps @ 2..100s
  •  Ground station and distribution systems become crucial for system performance

    17. Conclusions

  •  TWSTFT ready to provide self-contained means for T&F transfer
  •  Operation and data handling fully automatic
  •  TWSTFT may be integrated into larger operational systems
  •  Satellite operators and time laboratories may benefit from each other
  •  Real-time operation allows very short time-to-alarm
  •  New modulation schemes reduce / eliminate transponder costs
  •  Accuracy may be improved by advanced modulation schemes
  •  2-Way Carrier Phase: improvement of 10 w.r.t. code phase @ 20 MChip/s
  •  Satellite systems with ‘return-channel’ lead to very low-cost ground stations

    18. Acknowledgements

    This work was only possible due to invaluable support and continuous contributions by:

  •  USNO, United States Naval Observatory, Washington
  •  NIST, National Institute of Standards and Technology, Boulder
  •  SES, Société Européenne des Satellites, Luxembourg, Link PTB <-> DLR
  •  PTB, Physikalisch-Technische Bundesanstalt, Braunschweig
  •  DLR, Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen