AGU Fall MeetingDec 11-15, 2006San Francisco, CA Estimates of the precision of GPS radio...
-
Upload
dorcas-turner -
Category
Documents
-
view
218 -
download
0
Transcript of AGU Fall MeetingDec 11-15, 2006San Francisco, CA Estimates of the precision of GPS radio...
AGU Fall MeetingDec 11-15, 2006 San 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, 2006 San Francisco, CA
Outline
• Processing overview• RO retrieval errors - Previous results• RO precision from COSMIC• Summary
AGU Fall MeetingDec 11-15, 2006 San 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-secGPS Clocks
- GPS Orbits/EOP’s (Final/IGU)- IGS Weekly Station Coordinates- 30-sec Ground GPS Observations
Estimate LEO OrbitAnd Clocks
Single/Double DifferenceOccultation Processing
Estimate Ground StationZTD’s and Station Coordinates
- 1-Hz Ground GPS Observations- 50-Hz LEO Occultation GPS Obs.
- 30-sec LEO GPS Observations- LEO Attitude (quaternian) data
Retrieved Profiles
Abel Retrieval
1D-Var Retrieval
AGU Fall MeetingDec 11-15, 2006 San 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, 2006 San Francisco, CA
COSMIC Collocated Occultations
Occultation map of atmPhs.C003.2006.157.04.30.G13.0001.0001.ncOccultation map of atmPhs.C002.2006.157.04.30.G13.0001.0001.nc
AGU Fall MeetingDec 11-15, 2006 San 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, 2006 San 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, 2006 San Francisco, CA
Statistical Comparison of Refractivity for FM3-FM4
MeanSTD# matches
< 0.2 %between 10 and 20 km
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
,n
PPMT
n
ii∑
=σ
PPLT 0 < ALT < 5 km
PPMT 10 < ALT < 20 km
PPUT 25 < ALT < 35 km
Precision Parameters
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
Statistical Comparison of Refractivity, 2006.111-277
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, 2006 San Francisco, CA
Statistical Comparisons for 1D-Var Retrievals
Kinetic Temperature Water vapor pressure
< 0.4Kbetween 10 and 20 km
[K] [mb]
< 0.7mb
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
Statistical Comparisons for 1D-Var Retrievals
< 0.15 %at surface
Pressure
[fractional difference]
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
Impact of Tangent Point Separation, 2006.111-277
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
Impact of Latitude for TPs < 10km , 2006.111-277
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
Impact of Ionospheric Scintillations for TPs < 10km , 2006.111-277
Middle troposphere Upper troposphere
>50 collocations per Lat/LT box
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
Post-Processed External Overlaps: UCAR-NCTU
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)
UCAR - NCTU 2006.216-218 FM1-6
RadialAlong-trackCross-track
Position Velocity
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
Real-Time vs Post-Processed Results
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
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
AGU Fall MeetingDec 11-15, 2006 San Francisco, CA
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
AGU Fall MeetingDec 11-15, 2006 San 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