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Estimates of the precision of GPS radio occultations from the FORMOSAT-3/COSMIC mission Bill Schreiner, Chris Rocken, Sergey Sokolovskiy, Stig Syndergaard, Doug Hunt UCAR COSMIC Project Office. Outline. Processing overview RO retrieval errors - Previous results RO precision from COSMIC

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

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

  3. CDAAC Processing Overview Estimate Ground Station ZTD’s and Station Coordinates - GPS Orbits/EOP’s (Final/IGU) - IGS Weekly Station Coordinates - 30-sec Ground GPS Observations • LEO POD • Zero-Difference Ionosphere-free carrier phase observables with reduced-dynamic processing (fully automated in CDAAC) • Real-Time (IGU, ~50 ground stations) and Post-Processed (IGS, ~100 stations) Soln’s • Excess phase calibration • Single-Difference processing with CDAAC 30-sec GPS clock estimates • Abel Retrieval • Statistical optimization of bending angle with NCAR climatology • Full Spectrum Inversion radioholographic processing • Extrapolation of ionospheric bending below L2 cutoff • Temperature initialization (Pressure = 0 at 120km) • 1D-Var • Optimal combination of RO and model (Real-time: NCEP-GFS, Post-Processed, ECMWF) profiles Estimate 30-sec GPS Clocks • 30-sec LEO GPS Observations • - LEO Attitude (quaternian) data Estimate LEO Orbit And Clocks Single/Double Difference Occultation Processing - 1-Hz Ground GPS Observations - 50-Hz LEO Occultation GPS Obs. Abel Retrieval 1D-Var Retrieval Retrieved Profiles

  4. 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 • ROSE inter-agency (GFZ, JPL, UCAR) comparison [Ao et al., 2003; Wickert et al., 2004] and GFZ-UCAR [von Engeln, 2006] • 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

  5. 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

  6. 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 • Systematic ionospheric residual errors degrade accuracy • Errors due to calibration of excess phase (POD and single-differencing) affect precision and accuracy • Insufficient tracking depth (including loss of L2) degrades accuracy • Different tracking depths for a pair of occultations degrades precision

  7. 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.

  8. Statistical Comparison of Refractivity for FM3-FM4 Mean STD # matches < 0.2 % between 10 and 20 km

  9. Precision Parameters

  10. Statistical Comparison 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

  11. Statistical Comparisons for 1D-Var Retrievals Kinetic Temperature Water vapor pressure < 0.4K between 10 and 20 km < 0.7mb [K] [mb]

  12. Statistical Comparisons for 1D-Var Retrievals Pressure < 0.15 % at surface [fractional difference]

  13. Impact of Tangent Point Separation, 2006.111-277

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

  15. Impact of Ionospheric Scintillations for TPs < 10km , 2006.111-277 Middle troposphere Upper troposphere • >50 collocations per Lat/LT box

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

  17. Real-Time vs Post-Processed Results

  18. COSMIC POD Issues • Attitude errors • GPS phase center offsets and variations, • ~1 cm variation over FOV for POD antenna • ~0.01-0.03 mm/sec for Limb antenna • Local spacecraft multipath • Changing center of mass, ~1-3 cm variation • Data gaps and latency are improving

  19. Radio Occultation Inter-Agency Comparisons • Radio Occultation Sensor Evaluation (ROSE) • GFZ/JPL/UCAR inter-agency comparison • CHAMP data, 2002.213-243 • GFZ/UCAR [van Engeln, 2006] • CHAMP data, similar differences as ROSE • GRAS SAF ROPIC is starting • CHAMP data, COSMIC? Courtesy: Wickert et al., 2004

  20. Summary • Estimates of RO precision from COSMIC are close to theoretical estimates • Sufficient straight-line tracking depth (~ -150 km) important for lower troposphere retrievals • Continue assessment of COSMIC POD quality and investigate methods to minimize error sources • Continue inter-agency comparisons (ROSE, Gras SAF ROPIC) to understand observed differences • Continue to study impact of residual ionospheric errors as more RO data become available

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