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Estimates of the precision of LEO orbit determination and GPS radio occultations from the FORMOSAT-3/COSMIC mission Bill Schreiner,Chris Rocken, Sergey Sokolovskiy, Stig Syndergaard, Doug Hunt UCAR COSMIC Project Office. Outline. POD overview Current POD results from COSMIC

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  1. Estimates of the precision of LEO orbit determination and GPS radio occultations from the FORMOSAT-3/COSMIC missionBill Schreiner,Chris Rocken, Sergey Sokolovskiy,Stig Syndergaard, Doug HuntUCAR COSMIC Project Office

  2. Outline • POD overview • Current POD results from COSMIC • RO retrieval errors - Previous results • RO precision from COSMIC • Summary

  3. Impact of Velocity Errors on RO Retrievals • Kursinski et al. (1997) • ~0.05% error in N at 40km due to 0.05 mm/s velocity error • UCAR simulation • ~0.1% in N at 40km due to 0.1 mm/s velocity error

  4. - GPS Orbits/EOP’s (Final/IGU) - IGS Weekly Station Coordinates - 30-sec Ground GPS Observations Estimate Ground Station ZTD’s and Station Coordinates Estimate 30-sec GPS Clocks • 30-sec LEO GPS Observations • - LEO Attitude (quaternian) data Estimate LEO Orbit And Clocks - 1-Hz Ground GPS Observations - 50-Hz LEO Occultation GPS Obs. Single/Double Difference Occultation Processing Excess Phase Data LEO POD at CDAAC with Bernese v5.0 LEO POD • Zero-Difference Ionosphere-free carrier phase observables with reduced-dynamic processing (fully automated in CDAAC) • Real-Time (~50 ground stations) and Post-Processed (~100 stations) Soln’s • Dynamic Model: Gravity - EIGEN1S, Tides - (3rd body, solid Earth, ocean) • Model State: • 6 initial conditions (Keplerian elements) • 9 solar radiation pressure parameters (bias and 1 cycle per orbital revolution accelerations in radial, transverse, and normal directions) • pseudo-stochastic velocity pulses in R-T-N directions every 12 minutes • Real ambiguities • Quality Control • Post-fit residuals • Internal overlaps

  5. Current POD Results - Near Real-Time • Internal overlaps for 2006.200-280 • Average: ~24 cm 3D RMS • Median: ~16 cm 3D RMS • External overlap with NCTU post-processed orbit (courtesy of Cheinway Hwang) • ~ 20 cm 3D RMS FM1 on July 8, 2006

  6. Post-Processed External Overlaps: UCAR-NCTU Radial Along-track Cross-track Position Velocity UCAR - NCTU 2006.216-218 FM1-6

  7. Post-Processed POD Results (cont) • External Orbit Overlaps with initial orbits from Univ. of Texas-CSR for FM1-3 on 2006.216-217 (courtesy Rick Pastor). Some orbits show mean cross-track and along-track differences. Under investigation • External Orbit Overlaps with JPL orbits for FM1-6 on 2006.216-218 (courtesy Da Kuang). Some orbits show mean along track differences. Under investigation • Internal Orbit Overlaps for 2006.216-218 (27-hr arcs) • Average: ~9 cm (~0.1 mm/s) 3D RMS for 12 overlaps

  8. COSMIC POD Issues • Attitude errors • Phase center offsets and variations, ~1 cm variation • Local spacecraft multipath • Changing center of mass, ~1-3 cm variation • Tune stochastic velocity pulses • Estimate stochastic accelerations instead of stochastic velocity pulses • Data gaps and latency will improve

  9. RO Retrieval Errors - Previous Results • First estimates: Yunck et al. [1988] and Hardy et al. [1994] • Detailed analysis: Kursinski et al. [1997] • ~0.2 % error in N at 20 km (horizontal along track variations) • ~1 % at surface and ~1 % at 40 km • Experimental validation: Kuo et al. [2004] • Errors slightly larger than Kursinski et al. [1997] • Experimental precision estimates: Hajj et al. [2004] • ~0.4 % fractional error (0.86K) between 5 and 15 km

  10. COSMIC Collocated Occultations Occultation map of atmPhs.C002.2006.157.04.30.G13.0001.0001.nc Occultation map of atmPhs.C003.2006.157.04.30.G13.0001.0001.nc

  11. Precision from Collocated Soundings • Only precision (not accuracy) can be estimated from collocated soundings • Thermal noise (uncorrelated for any two occultations) affects precision and accuracy • Horizontally inhomogeneous irregularities whose correlation radii are less than TP separation affect precision and accuracy • Errors due to calibration of excess phase affect precision and accuracy • Insufficient tracking depth (including loss of L2) degrades accuracy • Different tracking depths for a pair of occultations degrades precision

  12. Collocated Retrievals Inversions of pairs of collocated COSMIC occultations with horizontal separation of ray TP < 10 km. Upper panel: tropical soundings, 2006, DOY 154, 15:23 UTC, 22.7S, 102.9W. Lower panel: polar soundings: 2006, DOY 157, 13:14 UTC, 72.6S, 83.5W.

  13. Statistical Comparison of Refractivity for FM3-FM4 Mean STD # matches

  14. Statistical Comparisons of Refractivity, 2006.111-277 • Setting Occultations with Firmware > v4.2 • Tangent Point separations < 10km • Same QC for all retrievals • One outlier removed • Near real-time products used ALL Collocated pairs Pairs with similar straight-line tracking depths Schreiner, W.S., C. Rocken, S. Sokolovskiy, S. Syndergaard, and D. Hunt, Estimates of the precision of GPS radio occultations from the COSMIC/FORMOSAT-3 mission, GRL (in review), 2006

  15. Statistical Comparisons for 1DVar Retrievals Kinetic Temperature Water vapor pressure < 0.4K between 10 and 20 km < 0.7mb [K] [mb]

  16. Impact of Tangent Point Separation, 2006.111-277

  17. Impact of Latitude for TPs < 10km , 2006.111-277

  18. Real-Time vs Post-Processed Results

  19. Summary • Continue assessment of COSMIC POD quality and investigate methods to minimize error sources • Estimates of RO precision from COSMIC are close to theoretical estimates • Sufficient straight-line tracking depth (~ -150 km) important for lower troposphere retrievals

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