32nd Annual Precise Time and Time Interval (PTTI) Systems and Applications Meeting


Two-Way Frequency Transfer via Satellite Using Carrier Phase

Reston, Virginia
November 28-30, 2000
Wolfgang Schaefer, Alexander Pawlitzki, Torge Kuhn
e-mail: wolfgang.schaefer@timetech.de alexander.pawlitzki@timetech.de torge.kuhn@timetech.de
web: www.timetech.de TimeTech GmbH
Phone: 0049-711-678 08-0 Fax: 0049-711-678 08 99
Curiestrasse 2
D-70563 Stuttgart
Germany

Table of contents

 1. Motivation to use Carrier Phase in TWSTFT Applications3
 2. Typical Operational Scenario of TWSTFT4
 3. 'GEO-STATIONARY' satellite: Similarity to Dual Mixer System5
 4. Error Sources and unknowns6
 5. Measurement configuration7
 6. Estimation of Satellite Local Oscillator Frequency8
 7. Error budget Satellite LO frequency by doppler compensation using range-rate9
 8. Summary and Outline of Algorithm10
 9. Frequency Transfer Uncertainties11
10. Proof of ncept12
11. Ground Station Considerations13
12. MDEV, Carrier Phase, Signal 2.5 MChip/s, USNO <-> NIST14
13. Frequency Transfer, PN 2,5 and 20 MChip versus Carrier Phase15
14. Satellite LO Characterisation16
15. Conclusions17


1. Motivation to use Carrier Phase in TWSTFT Applications

  • 2.5 MChip/s PN coded signal used traditionally
  • Higher chip-rates (20 .. 100 MChip/s) would improve precision and accuracy
  • Existing installations may be limited due to available satellite transponder bandwidth
  • Very encouraging results from GPS carrier phase and advanced processing software
  • Competition factor: TWSTFT may have been seen at the 'performance limits'
  • Design goal and expectation:
        1. Retain the proven and accepted properties of TWSTFT
        2. Improve frequency transfer precision (and accuracy?)
            by factor of carrier-frequency / chip-rate . 400 … 4000

    2. Typical Operational Scenario of TWSTFT

  • VSAT Ground stations, transmitting and receiving at both ends
  • Ku-band: 14 GHz uplink, 11.5 GHz downlink
  • Chip-rate: 1...2.5...20 MChip/s
  • Power: 1...5 W RF / 1.8 m antenna
  • C/No: 40...60 dBHz ->noise 500ps (PN 2.5 MChip/s, t = 1s)
  • Session duration: some minutes to reach noise floor
  • Satellite properties
  • Both stations see common transponder within footprint
  • Different east-west transponders for large distances (Ku-band)
  • But: Same local oscillator for all transponders, frequency: typ. 2 .. 2.5 GHz

    3. 'GEO-STATIONARY' satellite: Similarity to Dual Mixer System



    4. Error Sources and unknowns



    5. Measurement configuration



    6. Estimation of Satellite Local Oscillator Frequency

       

    1. Receiver carrier measurement:
        Continuous counter referenced to local clock

    2. Measure satellite LO frequency
        Perform code-based ranging to satellite
        Use same signals for code & carrier
        Determine satellite velocity (range-rate)
        Compensate down-link doppler



    7. Error budget Satellite LO frequency by doppler compensation using range-rate

    Carrier counter precision: 0.001 Hz (ô = 1s)
    Thermal noise carrier: 0.003 Hz (ô = 1s)
    Ranging noise: 3 .. 10 cm/s (ô = 1s)
    Satellite LO uncertainty: 0.1 Hz or 4E-11
    Measurement unaffected by

  • Satellite location / direction of movement
  • Transponder delay
  • Ionosphere
  • Troposphere

    8. Summary and Outline of Algorithm

    Solve for
    1. Satellite LO frequency
    2. Relative velocity to station 1
    3. Relative velocity to station 2
    4. Frequency error between ground clocks

  • 4 independent measurements required (receive 2* remote signal, 2* own signal)
  • But: no convergence (so far) on solution using the 4 carrier readings only
  • Estimate satellite LO frequency
  • Solve for the remaining 3 unknowns
  • Show, that Satellite LO uncertainty has minor effect on result
  • Far more complicated with respect to the straight forward 2-way formula

    9. Frequency Transfer Uncertainties

    Omission of higher order clock-error terms: (error)² or typ. 1 E-28
    Thermal receiver noise (0.003 Hz) 4 E – 14, ô = 1s, slope: sqrt(ô)
    Satellite LO uncertainty: 4 E – 18
    Sat. Location: none
    Sat Velocity: 4 E - 18 (same as LO uncertainty)
    Transponder: none
    Troposphere: none (frequency independent)
    Ionosphere: <300 ps / day (4 E-15), under pessimistic assumptions
    asymmetry effect assuming absolute ion. delay of 50 cm in Ku band

    10. Proof of concept

  • Use different satellites with different LO (total 3 so far)
  • Use different links between same clocks (USNO – NIST)
  • Compare with known clock parameters
  • Compare results with other time transfer means (TWSTFT, GPS CV etc)
         Results from USNO – NIST test (2 active H-masers)
         Frequency uncertainty few parts in 10-14 , ô = 20 min (PTTI 1999)
         Noise 2*10-12, ô = 1s
         Noise (PTB – DLR) 1*10-12, ô = 1s (EFTF 1999, 1 passive H-maser)

    11. Ground Station Considerations



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



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



    14. Satellite LO Characterisation



    15. Conclusions

  • Clear improvement of short term stability wrt code-phase by approx. 400 (ô = 1s)
  • Short term stability (10 s < ô < 300 s) comes into the vicinity of H-masers
  • Short term stability probably limited by station carrier frequency generation scheme
  • Long-term stability (ô = 1 day) affected by link asymmetries (variation of ionosphere)
  • Rigorous error assessment of TWSTFT and 2-way carrier phase
  • Determination and elimination of errors due to link asymmetry
  • Determination of satellite LO without transmission of a satellite-generated signal
  • More in-depth analysis required, convergence of algorithm, real-time calculations
  • USNO, NPL & PTB are setting up installations