Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Outline POD overview Current POD results from COSMIC RO retrieval errors - Previous results RO precision from COSMIC Summary

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan LEO POD at CDAAC with Bernese v5.0 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 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 Excess Phase Data LEO POD

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Current POD Results - Near Real-Time Internal overlaps for –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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Post-Processed POD Results (cont) External Orbit Overlaps with initial orbits from Univ. of Texas-CSR for FM1-3 on (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 (courtesy Da Kuang). Some orbits show mean along track differences. Under investigation Internal Orbit Overlaps for (27-hr arcs) –Average: ~9 cm (~0.1 mm/s) 3D RMS for 12 overlaps

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan COSMIC Collocated Occultations Occultation map of atmPhs.C G nc Occultation map of atmPhs.C G nc

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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.

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Statistical Comparison of Refractivity for FM3-FM4 Mean STD # matches

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Statistical Comparisons 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

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Statistical Comparisons for 1DVar Retrievals Kinetic TemperatureWater vapor pressure < 0.4K between 10 and 20 km [K][mb] < 0.7mb

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Impact of Tangent Point Separation,

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Impact of Latitude for TPs < 10km,

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan Real-Time vs Post-Processed Results

Formosat-3/COSMIC WorkshopNov 28 - Dec 1, 2006Taipei, Taiwan 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