European Frequency and Time Forum, EFTF, 13/16 April 1999, Besancon, France

Extension of Two-Way Satellite Time- and Frequency Transfer Method to Real-Time Operation and Carrier Phase Measurements

Wolfgang SCHÄFER,
Alexander PAWLITZKI,
Torge KUHN

1. Status and Future Potential of Two-Way Time Transfer

  • Two-Way Satellite Time- and Frequency Transfer links major national time laboratories in US, Far East and EU
  • Regular operation in US and EU since more than one year
  • Availability of new modem hardware
  • New operational features with emphasis on ease of use and real-time operation
  • New operational modes
  • Low-cost VSAT equipment for communication use
  • New VSAT concepts being introduced for return-link operation via satellite
  • Advanced wide-bandwidth (500 MHz) satellite transponders in Ka-band
  • Bi-directional satellite links become reality in near future with tens of thousands of users
  • Low cost VSAT equipment ready for mass production

    2. Modem Operation Features

  • SATRE (Satellite Time- and Ranging Equipment) modems continuously improved
  • Few external equipment required
  • Automated operation
  • On-line data evaluation
  • Remote monitoring and control
  • Direct connection to IP networks
  • Real-time data exchange in between partner stations
  • GPS receiver provides positioning for automated correction of SAGNAC effect
  • Two- and three channel versions for multi-link applications and G/S delay monitoring

    3. Performance Aspects

  • Compatibility to MITREX systems
  • User selectable chip-rates up to 20 MChips/s
  • Up to 32 different PN codes
  • Well controlled transmitter spectrum
  • Transmitter and receiver frequency agility: +/- 18 MHz
  • Tx carrier steps: 1 mHz, Rx: continuous tuning
  • Fully coherent design

    4. Advanced operational modes based on TWSTFT measurements

  • Real-time TWSTFT
  • Monitoring of remote clocks
  • Two-Way Time-Synchronisation via Satellite
  • Two-Way Carrier Phase measurements

    5. Verification Test Campaign

  • A 10 day test campaign using 5 stations at 4 locations was conducted
  • Tests have been performed on Ku-Band via ASTRA 1G
  • A free transponder has been provided at no cost by Societé Européenne des Satellites (SES)

    6. Conclusions

  • Advanced Two-Way methods have been implemented and verified by test
  • It is believed, that a proof-of-concept for Two-Way carrier phase measurements has been achieved
  • But several imperfections in hard- and software remain to be resolved
  • Carrier Phase measurements show good short term stability as expected, but long term stability does not yet fully match code-phase measurements
  • "Two-way Time Synchronisation" is a very promising mode for users without access to precise clocks
  • Full performance is achieved already after 4-6 hours of operation
  • Short term stability exceeds that of TWSTFT data
  • Long-term stability compares favourably to TWSTFT data, making it an attractive solution to disseminate UTC-traceable signals via satellite
  • Real-Time operation allows clock monitoring and failure detection with latency of 4s to 6s

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

  • SES, Société Européenne des Satellites, Luxembourg
  • PTB, Physikalisch-Technische Bundesanstalt, Braunschweig
  • DLR, Zentrum für Luft- und Raumfahrt, Oberpfaffenhofen