High Precision GPS Carrier Phase Time Transfer

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High Precision GPS Carrier Phase Time Transfer

• Kristine M. Larson and John Plumb
• Department of Aerospace Engineering Sciences
• University of Colorado

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Outline

• Brief background
• Recent results
• Future work

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Carrier Phase Time Frequency Transfer

• Uses GPS constellation/Ashtech Z12T
• L1/L2 pseudorange and carrier phase observables
• Orbits determined by the IGS network (3 cm radial
  RMS)
• Least squares estimation (GIPSY)

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Why Carrier Phase Time Frequency Transfer?

• More precise and accurate than two-way or
  common-view.
• Delivers precision needed to compare cesium
  fountains.
• Dont have to pay for satellite time -)
• Nearly continuous receivers are cheap.

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Past Results

• Larson and Levine, IEEE UFFC, 1999
• Larson, Levine, Nelson, Powers, IEEE UFFC, 2000.

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Temperature Errors, USNO
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John Plumbs Dissertation

• Develop GPS carrier phase time transfer as an
  accurate and reliable method.
• Development of an error budget for GPS carrier
  phase time transfer using an absolutely
  calibrated system.
• Assessment of the most stable estimation
  strategies.

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Calibration Error Budget

• Antenna
• Cable
• Connectors
• Simulator
• Pseudorange/carrier phase bias
• GIPSYanalysis

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Cesium Fountains

• stability 100 ps/day
• located at NIST and at PTB (Germany)
• Good news Z-12Ts at both sites
• Bad news fountains dont seem to be up for very
  long and/or the same time.

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Time Transfer Comparison

• Compare cesium fountains (Tom Parker)
• Compare PTB-NIST(UTC) with TWSTFT
• Compare USNO-NIST-AMC2 with common-view and
  TWSTFT.

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Network Sites
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CP vs. TWSTFT, 7500km Baseline
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CP TW ( offset)
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CP(offset) vs. hourly TWSTFT 2400 km baseline
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CP hourly TW (offset)
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CV, CP, and Circular T (2400 km)
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CP CV (2400 km)
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Future Work

• Longer experiment.
• Analysis of Allan deviation and time deviation
  statistics.
• Re-check calibrations
• Comparisons with other timing sites.
• Receiver at NIST is being removed PTB receiver
  is not operating optimally.

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Acknowledgements

• NRL Joe White, Ken Senior, Ron Beard IGS orbits
  and data archives
• NIST Judah Levine and Tom Parker
• USNO Ed Powers
• JPL GIPSY
• CU Penny Axelrad