C. Shum, C. Zhao, Y. Yi, and P. Luk The Ohio State University GFO Calibration/Validation Meeting NOAA Laboratory for Satellite Altimetry Silver Spring, Maryland June 12, 2001 QUALITY ASSESSMENT OF GFO SENSOR AND DATA PRODUCTS

Routine quality assessment of GFO data productRoutine quality assessment of GFO data product –Cycle by cycle GFO sensor and data verifications: –NGDR (GfoM and GfoO) data editing summary Restricted web: web: Summary of sensor and data assessmentSummary of sensor and data assessment –OODD and GSFC MOE orbit verifications (POE to follow) –Time tag, USO, SWH,  0, sea state bias evaluations –Radiometer, ionosphere and geophysical corrections –NGDR vs. IGDR comparisons

Restricted Web:

Summary of Data Editing Percentage for Recent GFO NGDR data CyclePoints flag as bad EM bias out of range (-900,0 mm) RMS (Range=0 or >500 mm)  0 or SWH Not available 614.4%4.4%7.5%2.5% 715.2%4.4%8.0%2.7% 816.0%4.6%8.6%2.8% 917.2%4.8%9.6%2.8% Restricted Web:

GFO VERIFICATION WEB-PAGE Restricted Web:

GFO VERIFICATION WEB-PAGE

GFO-1 ORBIT DETERMINATION AND ANALYSIS Averaged statistics for OSU orbit, RA time bias, range bias, sea state bias *RA bias is relative to T/P MSS and negative value means range is short Selected orbit (6-day arcs) comparison (different gravity models) Selected orbit (6-day arcs) comparison (different gravity models)

Precise Orbits: Nov. 30, 2000 – Feb. 5, 2001 data (Operational Data) RMS in cm

Evaluation of Doppler Orbits Using Crossovers Doppler Orbits: Nov. 30, 2000 – Feb. 5, 2001 data (Operational Data) Crossover range: cm rms

Evaluation of GSFC MOE/SLR Orbit Using Crossovers MOE/SLR Orbits: Nov. 30, 2000 – Feb. 5, 2001 data (Operational Data) Crossover range: mm rms

Evaluation of OODD (NGDR) Orbits Using Crossovers Doppler Orbits: April 15 – May 18, 2001 data (Operational Data) Crossover range: cm rms

Evaluation of MOE (NGDR) Orbit Using Crossovers MOE/SLR Orbits: April 15 – May 18, 2001 data (Operational Data) Crossover range: cm rms (Day 113 picked lower accuracy orbit than IGDR did)

Evaluation of MOE Orbit (IGDR) Using Crossovers MOE/SLR Orbits: April 15 – May 18, 2001 data (NOAA IGDR Data) Crossover range: cm rms

RADIAL ORBIT ERROR ASSESSMENT AFTER BIAS/TILT/1-CPR ERROR REMOVAL Estimated error for MOE/SLR orbits after adjustment: ~40,000 km arcs (Ocean-wide crossovers): ~40,000 km arcs (Ocean-wide crossovers): 8.6 cm rms (days ); 9.7 cm rms (days ) 3,000 km arcs (30° Zonal Band between 25S-5N): 3,000 km arcs (30° Zonal Band between 25S-5N): 3.9 cm rms (days ); 4.4 cm rms (days ) 3.9 cm rms (days ); 4.4 cm rms (days ) 1,000 km arcs (10° Zonal Band between 15S-5S): 1,000 km arcs (10° Zonal Band between 15S-5S): 1.3 cm rms (days ); 2.3 cm rms (days ) Atlantic: 1.1 cm (days ); 1.0 cm (days ) Pacific: 1.3 cm (days ); 1.2 cm (days ) All crossover data used (No editing) Estimated error for OODD (Doppler) orbits after adjust.: 1,000 km arcs (10° Zonal Band between 15S-5S): 1,000 km arcs (10° Zonal Band between 15S-5S): 1.8 cm rms (days ); 6.5 cm rms (days ) Atlantic: 1.2 cm (days ); 1.8 cm (days ) Pacific: 2.1 cm (days ); 1.3 cm (days )

Estimated GFO Radial Orbit Error SUMMARY: ORBIT ERROR ASSESSMENTS Precise Orbits (NASA GSFC orbits using improved models) SLR rms: 4 cm, crossover rms: 8-9 cm Estimated radial orbit accuracy: 5-6 cm rms Near-Real Time (12-24 hours) Orbits Opnet Doppler (OODD) orbits estimated accuracy: cm rms Opnet Doppler (OODD) orbits estimated accuracy: cm rms Estimated orbit accuracy for GSFC MOE (SLR+Doppler): cm Estimated orbit accuracy for GSFC MOE (SLR+Doppler): cm Estimated orbit accuracy after crossover adjustment (bias & tilt): Estimated orbit accuracy after crossover adjustment (bias & tilt): OODD: cm rms (1,000 km arc) OODD: cm rms (1,000 km arc) MOE/SLR: cm rms (1,000 km arc) MOE/SLR: cm rms (1,000 km arc)

SWH AND  0 CALIBRATION Geographical SWH comparison: 1 ± 14 dm Geographical SWH comparison: 1 ± 14 dm GFO and ERS-2 (June 1999) GFO and ERS-2 (June 1999) Recommended calibrations (D. Hancock, 2000): Recommended calibrations (D. Hancock, 2000): Add 0.37 dB to  0 (1-parameter model) Add 0.37 dB to  0 (1-parameter model) Add 0.24 m to SWH (1-parameter model) Add 0.24 m to SWH (1-parameter model) Study based on Tran et al. [2000], T/P comparisons Study based on Tran et al. [2000], T/P comparisons Buoy validations (D. Cotton, 2000): Buoy validations (D. Cotton, 2000): Preliminary results with limited data Preliminary results with limited data 2-parameter (bias and offset) model 2-parameter (bias and offset) model Ohio State University study (Y. Yi, 2000): Ohio State University study (Y. Yi, 2000): Preliminary results: comparison with TOPEX Preliminary results: comparison with TOPEX Applying Hancock biases improves buoy fit Applying Hancock biases improves buoy fit for wind-speed (  0 ), but not SWH [D. Cotton, pc] for wind-speed (  0 ), but not SWH [D. Cotton, pc] Hancock calibration was recommended and implemented in GFO data processing

SWH Buoy Calibration (D. Cotton) Buoy data fit: 12 cm rms (26 cm for TOPEX; 32 cm for ERS-2) Preliminary results (limited calibration data used)

Buoy Wind Speed (  0 ) Calibration (D. Cotton) Buoy data fit : 1.28 m/s (1.27 m/s for TOPEX; 1.23 m/s for ERS-2) Preliminary results (limited calibration data used)

COMPARISONS WITH TOPEX SWH/  0 10-day Averages within 66S-66N Preliminary results indicate GFO offsets with TOPEX SWH and  0 values, confirming D. Hancock’s calibration results

ASSESSMENT OF TIME BIAS AND SSB Time Tag Accuracy and Timing Stability: Time Tag Accuracy and Timing Stability: RA time tag accuracy assessed using crossover analysis over three Cal/Val periods and first 4 operational cycles data sets: Sea State Bias: One parameter model (dependence on SWH) estimates based on crossover analysis varies over different versions of Cal/Val data (possibly due to changes of  0 /SWH and imperfect modeling, such as orbits): Cal/Val I Data (1999): 4.3% SWH Cal/Val II Data (1999): 3.0% SWH Cal/Val III Data (2000): 3.5% SWH Cal/Val IV Data (2000): unavailable Operational Data (2001): 4.7% SWH Time Tag accuracy: 0-3 ms (cal/val), 1.5 ms (operational) USO Drift Range Correction: 15 cm since launch [Lillibridge et al., 2000; Hancock, personal communication]

Precise Orbits: Nov. 30, 2000 – Feb. 5, 2001 data (Operational Data)

GFO Timing Stability Comparisons GFO Geosat

USO Height Correction Comparisons GFO Geosat

COMPARISON OF GFO MWR AND NCEP WET DELAY GFO MWR – NCEP Mean (mm) RMS (mm) GFO MWR – NCEP Mean (mm) RMS (mm) Cycle 2 (Jan 3-19, 2001) Cycle 6 (Mar 12-18, 2001) Cycle 8 (April 15-May 01) Cycle 9 (May 02-18) GFO MWR - ERS2 MWR: -11  31 mm (Dec 6-22, 1999 data)

COMPARISON OF GFO MWR AND ERS-2 MWR (ATSR) WET TROPOSPHERE DELAY

VALIDATION OF IONOPSHERE CORRECTION Comparison of JPL and CODE GIM (Global Ionosphere Map) and IRI95, high solar activities (Sept. 00 – April 01)Comparison of JPL and CODE GIM (Global Ionosphere Map) and IRI95, high solar activities (Sept. 00 – April 01) CODE and JPL GIM, bias ~ 2.9 mm, rms: ~ 1.5 cm IRI95 and CODE GIM, bias ~ -10 mm, rms: ~ 5.4 cm IRI95 and JPL GIM, bias ~ -7.1 mm, rms: ~5.1 cm Sea level drift error could be 0-3 mm/yr directly due to ionosphere correction error during this periodSea level drift error could be 0-3 mm/yr directly due to ionosphere correction error during this period Assessment of IRI-95, GIM (CODE) versus TOPEX “truth” GIM is better than IRI-95 during high solar activities on both RMS and drift; and comparable to IRI95 during low solar activities Assessment of IRI-95, GIM (CODE) versus TOPEX “truth” GIM is better than IRI-95 during high solar activities on both RMS and drift; and comparable to IRI95 during low solar activities IRI95-T/P: 5.6 cm rms, GIM –T/P: 2.3 cm rms IRI95-T/P: 5.6 cm rms, GIM –T/P: 2.3 cm rms (TOPEX data used: Sept 00 - Feb 01) (TOPEX data used: Sept 00 - Feb 01) IRI95-T/P drift: 0.35 mm/yr (Data: ) IRI95-T/P drift: 0.35 mm/yr (Data: )

COMPARISON OF GFO IONOSPHERE DELAY FROM IRI95 AND JPL, CODE GIM (12/1999-2/2001)

Apparent sea level rise: 3 cm/60 days

GFO Range (SSH) Data Noise Estimation SSH (Sea Surface Height) NGDR data for July-August, 1999 (Cal/Val I) and for Aug-Oct (days , Cal/Val IV) Track segments ~400 sec. long (12 tracks used) Single repeating cycles used Method Estimate SSH signal by cubic splines SSH Noise = SSH - SSH Signal Selected data in deep oceans with least variability (1 Hz data) mm rms in Atlantic and mm rms in Pacific (1-3 m wave heights) Comparison with other altimeters ERS-2 SSH Noise: 19/27 mm rms (Sep, 1997) TOPEX SSH Noise: 11/15 mm rms (Sep, 1999)

Blue: 1 Hz Red: 3-sec. average GFO Noise Estimate: July - August, 1999 (Cal/Val I Data)

GFO Noise Estimate: (Cal/Val IV Data, 2000, September)

Precise Orbits: Nov. 30, 2000 – Feb. 5, 2001 data (Operational Data)

GFO RANGE BIAS Bias Rel. to T/P (Cal/Val I, II, III): -3 ± 5 cm Lake Michigan GPS- Buoy Campaign (March 24, 1999): 31 ± 42 cm Note: “Precise” GDR not Available for March 24, 1999 GFO Pass

COMPARISON OF NGDR AND IGDR NGDR and IGDR MOE agree well on time tags, SWH,  0, AGC, solid Earth tide and wet troposphere delay Ionosphere delay, ocean tide and dry troposphere delay have differences as different models were used NGDR MOE and OOD agree perfectly on time tag, SWH,  0, AGC and all other corrections except ionosphere delay NGDR MOE and IGDR MOE orbits are not always same Differences exist in Uncorrected Altimeter Range between NGDR MOE, IGDR MOE and NGDR OOD products Comparisons conducted using Cycle 8 and 9 data products Mismatch orbit (1 Hz vs 10 Hz processing) was suspected and confirmed by other studies (J. Lillibridge, S. Klingenberger, R. Vaughan et al.)

GFO Sensor and Data Assessment Estimated Doppler orbit accuracy: cm; GSFC MOE orbit accuracy: cm; GSFC precise orbit accuracy: 5-6 cm rms Orbit accuracy due primarily to availability of SLR tracking Time bias insignificant (0-1.5 ms); USO Drift is 15 cm/mission Noise of GFO SSH (1Hz): ~19 mm; TOPEX noise: ~13 mm rms, ERS-2 noise: ~23 mm rms, over same regions Sea state bias: ~4.7% of SWH. GFO absolute range bias: ~-3 cm  0  0Recommend to adopt David Hancock’s suggested calibrations for SWH and  0 : add 0.24 m to SWH and add 0.37 db to  0 Wet troposphere correction: 0.2 cm bias, 2.6 cm rms (w/NCEP Model); ~1 cm bias, 3 cm rms (w/ ERS-2) CODE GIM and IRI95 provide ionosphere corrections accurate to 1-5 cm rms during medium to high solar activities; GIM performs better during high solar activities

FUTURE POTENTIAL IMPROVEMENTS Estimation of 4- or 5-parameter sea state biasEstimation of 4- or 5-parameter sea state bias –Improved calibration of SWH and  0 needed –Crossover analysis using “best” processed data set Barotropic ocean tide modelsBarotropic ocean tide models –NAO99.2b [Matsumoto et al., 2001], GOT99.b [Ray, 2000 Ionosphere modelsIonosphere models –GIM (CODE or JPL?), IRI2001 [Bilitza, 2001] Dry troposphere and IBDry troposphere and IB –NCEP operational, FNOC (?), ECMWF (for GDR?) Need Wallops “internal calibration”Need Wallops “internal calibration” Concerns: USO drift “large”, inland lake/sea data quality, lack of waveform data, land-sea flag, improved center of gravity (SLR retroreflector and RA), further gravity field improvement?Concerns: USO drift “large”, inland lake/sea data quality, lack of waveform data, land-sea flag, improved center of gravity (SLR retroreflector and RA), further gravity field improvement?

GFO Great Lakes data are being edited due primarily to large SSB(?)

ASSESSMENT OF TIDE ERROR USING MODEL COMPARISONS

Assessment Of Tide Error Using Model Comparisons (RSS of 8 constituents) 6 recent models used: NAO99.2b, GOT99.b, 6 recent models used: NAO99.2b, GOT99.b, CSR4.0, DW98, Delft, YATM4D CSR4.0, DW98, Delft, YATM4D Shum et al. [2001] Shum et al. [2001]

TIDE MODEL EVALUATIONS USING ALTIMETER SEA LEVEL DATA [Shum et al. 2001] Global coastal ocean (depth<1000m) GEOSAT (Residual rms, cm) Tide Models GM ERM ERS-1 T/P CSR YATM4d CSR GOT NAO Smith Sea Level = Altimeter SSH - OSUMSS95 Latitude weights applied [Yi and Rapp, 1995] Edit criteria = 1000 cm