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


Outline

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


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


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


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


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


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.


Statistical Comparison of Refractivity for FM3-FM4

MeanSTD# matches

< 0.2 %between 10 and 20 km


,n

PPMT

n

ii∑

=σ

PPLT 0 < ALT < 5 km

PPMT 10 < ALT < 20 km

PPUT 25 < ALT < 35 km

Precision Parameters


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


Statistical Comparisons for 1D-Var Retrievals

Kinetic Temperature Water vapor pressure

< 0.4Kbetween 10 and 20 km

[K] [mb]

< 0.7mb


Statistical Comparisons for 1D-Var Retrievals

< 0.15 %at surface

Pressure

[fractional difference]


Impact of Tangent Point Separation, 2006.111-277


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


Impact of Ionospheric Scintillations for TPs < 10km , 2006.111-277

Middle troposphere Upper troposphere

>50 collocations per Lat/LT box


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


Real-Time vs Post-Processed Results


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


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


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

agu fall meetingdec 11-15, 2006san francisco, ca estimates of the precision of gps radio...

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