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GFO Cal/Val Timing Issues

GFO Cal/Val Timing Issues. John Lillibridge NOAA Laboratory for Satellite Altimetry. Overview of Mission Status. Launch on February 10, 1998 CPU resets fixed in Rev. E software: November, 1999 GPS system failure Utilize operational Doppler and precision laser orbits

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GFO Cal/Val Timing Issues

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  1. GFO Cal/Val Timing Issues John Lillibridge NOAA Laboratory for Satellite Altimetry

  2. Overview of Mission Status • Launch on February 10, 1998 • CPU resets fixed in Rev. E software: November, 1999 • GPS system failure • Utilize operational Doppler and precision laser orbits • Time tagging via ground-segment hardware: June, 1999 GFO Operational Evaluation Workshop

  3. CPU Resets & Data availability GFO Operational Evaluation Workshop

  4. Precision Timing Issues • Time-tag stability & uniformity • Spacing between records ~ 0.98 s • Ultra-Stable-Oscillator clock drift • Changes in clock frequency alter range travel-time counts • Timing biases between Range & Orbit • msec level timing biases yield cm level errors at twice-per-rev GFO Operational Evaluation Workshop

  5. GFO Timing Stability GFO Operational Evaluation Workshop

  6. Geosat Timing Stability GFO Operational Evaluation Workshop

  7. Example of non-uniform spacing GFO Operational Evaluation Workshop

  8. Summary of Timing Stability • GFO time-tagging is noisy at the 10 µsec level, compared to 1 µsec for Geosat (implies height error at the 0.1 mm level) • GFO exhibits 10 Hz frame “slips”, but this only occurs in ~ 0.1% of the data • The uneven spacing of GFO’s records does NOT imply an error in time-tagging GFO Operational Evaluation Workshop

  9. USO Height Correction - GFO GFO Operational Evaluation Workshop

  10. USO Height Correction - Geosat GFO Operational Evaluation Workshop

  11. Summary of Clock Drift • GFO’s Doppler beacon oscillator is drifting at a rate 10x worse than Geosat • The USO height correction is large, but is implicitly corrected during SDR production • The estimation of the USO height correction can suffer from large bookkeeping errors GFO Operational Evaluation Workshop

  12. Timing Biases - Laser Orbits GFO Operational Evaluation Workshop

  13. Height Variability - with orbit error GFO Operational Evaluation Workshop

  14. Height Variability - no orbit error GFO Operational Evaluation Workshop

  15. Summary of Timing Biases • Before installation of the time-tagging unit timing biases were on the order of seconds • Timing biases of 1± 1 msec are seen after time-tagging unit installed - acceptable • Removal of one and two cycle-per-rev orbit error yields expected sea level variability GFO Operational Evaluation Workshop

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