AGU Fall MeetingDec 11-15, 2006San Francisco, CA 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

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Outline Processing overview RO retrieval errors - Previous results RO precision from COSMIC Summary

AGU Fall MeetingDec 11-15, 2006San Francisco, CA CDAAC Processing Overview 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 - GPS Orbits/EOP ’ s (Final/IGU) - IGS Weekly Station Coordinates - 30-sec Ground GPS Observations Estimate LEO Orbit And Clocks Single/Double Difference Occultation Processing Estimate Ground Station ZTD ’ s and Station Coordinates - 1-Hz Ground GPS Observations - 50-Hz LEO Occultation GPS Obs sec LEO GPS Observations - LEO Attitude (quaternian) data Retrieved Profiles Abel Retrieval 1D-Var Retrieval

AGU Fall MeetingDec 11-15, 2006San Francisco, CA 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

AGU Fall MeetingDec 11-15, 2006San Francisco, CA COSMIC Collocated Occultations Occultation map of atmPhs.C G nc Occultation map of atmPhs.C G nc

AGU Fall MeetingDec 11-15, 2006San Francisco, CA 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

AGU Fall MeetingDec 11-15, 2006San Francisco, CA 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.

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Statistical Comparison of Refractivity for FM3-FM4 Mean STD # matches < 0.2 % between 10 and 20 km

AGU Fall MeetingDec 11-15, 2006San Francisco, CA PPLT0 < ALT < 5 km PPMT10 < ALT < 20 km PPUT25 < ALT < 35 km Precision Parameters

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Statistical Comparison of Refractivity, ALL Collocated pairs Pairs with similar straight- line tracking depths Setting Occultations with Firmware > v4.2 Tangent Point separations < 10km Same QC for all retrievals One outlier removed Near real-time products used 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

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Statistical Comparisons for 1D-Var Retrievals Kinetic TemperatureWater vapor pressure < 0.4K between 10 and 20 km [K][mb] < 0.7mb

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Statistical Comparisons for 1D-Var Retrievals < 0.15 % at surface Pressure [fractional difference]

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Impact of Tangent Point Separation,

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Impact of Latitude for TPs < 10km,

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Impact of Ionospheric Scintillations for TPs < 10km, Middle troposphere Upper troposphere  >50 collocations per Lat/LT box

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Post-Processed External Overlaps: UCAR-NCTU RadialAlong- Track Cross- Track 3-D Mean [cm] (Vel: mm/s) 0.9 (0.01) -3.1 (-0.03) 0.2 (0.00) - STD [cm] (Vel: mm/s) 10.6 (0.13) 11.1 (0.14) 10.6 (0.18) 18.9 (0.26) UCAR - NCTU FM1-6 Radial Along-track Cross-track PositionVelocity

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Real-Time vs Post-Processed Results

AGU Fall MeetingDec 11-15, 2006San Francisco, CA COSMIC POD Issues Attitude errors GPS phase center offsets and variations, –~1 cm variation over FOV for POD antenna –~ mm/sec for Limb antenna Local spacecraft multipath Changing center of mass, ~1-3 cm variation Data gaps and latency are improving

AGU Fall MeetingDec 11-15, 2006San Francisco, CA Radio Occultation Inter-Agency Comparisons Radio Occultation Sensor Evaluation (ROSE) –GFZ/JPL/UCAR inter-agency comparison –CHAMP data, GFZ/UCAR [van Engeln, 2006] –CHAMP data, similar differences as ROSE GRAS SAF ROPIC is starting –CHAMP data, COSMIC? Courtesy: Wickert et al., 2004

AGU Fall MeetingDec 11-15, 2006San Francisco, CA 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