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

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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
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Outline

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

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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

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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) Solns
• 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

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

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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

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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

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

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Collocated Retrievals
Inversions of pairs of collocated COSMIC
occultations with horizontal separation of ray TP
lt 10 km. Upper panel tropical soundings, 2006,
DOY 154, 1523 UTC, 22.7S, 102.9W. Lower panel
polar soundings 2006, DOY 157, 1314 UTC, 72.6S,
83.5W.
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Statistical Comparison of Refractivity for FM3-FM4
Mean STD matches
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Statistical Comparisons of Refractivity,
2006.111-277

• Setting Occultations with Firmware gt v4.2
• Tangent Point separations lt 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
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Statistical Comparisons for 1DVar Retrievals
Kinetic Temperature
Water vapor pressure
lt 0.4K between 10 and 20 km
lt 0.7mb
K
mb
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Impact of Tangent Point Separation, 2006.111-277
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Impact of Latitude for TPs lt 10km , 2006.111-277
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Real-Time vs Post-Processed Results
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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