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STRATEGY for ENVISAT RADAR ALTIMETRY CROSS-CALIBRATION and VALIDATION

STRATEGY for ENVISAT RADAR ALTIMETRY CROSS-CALIBRATION and VALIDATION. J. Benveniste (1) and the ENVISAT RA-2/MWR Cross-Calibration and Validation Team (1) European Space Agency, Earth Observation Applications Department ESRIN, Via Galileo Galilei, I-00044, Frascati (RM), Italy,

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STRATEGY for ENVISAT RADAR ALTIMETRY CROSS-CALIBRATION and VALIDATION

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  1. STRATEGY for ENVISAT RADAR ALTIMETRY CROSS-CALIBRATION and VALIDATION J. Benveniste (1) and the ENVISAT RA-2/MWR Cross-Calibration andValidation Team (1) European Space Agency, Earth Observation Applications Department ESRIN, Via Galileo Galilei, I-00044, Frascati (RM), Italy, (e-mail: Jerome.Benveniste@esrin.esa.it) ENVISAT RA-2/MWR Cross-Cal & Val Team

  2. Presentation Overview • Objectives • Relative Calibration definition • Cross-Calibration and Validation Strategy • The Cross-Calibration and Validation Team • Requirements • Experience from ERS-2 Commissioning Working Group • Conclusion ENVISAT RA-2/MWR Cross-Cal & Val Team

  3. Objectives • Cross-calibrated data products:- • Determine the relative bias between ENVISAT RA-2 (resp. MWR) and ERS-2 RA (resp. MWR) at the mm level, or/and other altimeters (T-P, GFO, Jason). • Rationale: to ensure ERS-ENVISAT data products consistency • Validated data products:- • Geophysical validation of the processing models and the data products. • Rationale: to authorize distribution of validated data products to all users, within 6 months after launch. ENVISAT RA-2/MWR Cross-Cal & Val Team

  4. Relative Calibration • Relative calibration, so-called cross-calibration, consists of estimating the offset, plus its associated error, of each measured parameters by differentiating geophysical level data products from two sources: Envisat and a reference (e.g., ERS-2, T-P, GFO, Jason, buoys, models). These offsets, or relative biases, are the ensemble averaged deltas between the two measurement sources.Rationale: When the two measurement sources are of same nature (e.g., ERS-2 RA and ENVISAT RA-2), the two mission-long time series can be united with high precision. ENVISAT RA-2/MWR Cross-Cal & Val Team

  5. Cross-Calibration and Validation Strategy (1) • (i) Cross-calibration of the RA-2 main geophysical parameters against ERS-2 (and/or TP, GFO, Jason):- • Range, Significant Wave Height and Wind Speed (Sigma0) • Global comparison at cross-overs and along the collinear tracks of millions of globally distributed data (significant error reduction) • Global statistical comparison against ERS-2, Models, In-situ data • (ii) Calibration of MWR geophysical retrieval algorithms:- • against radiosondes (+spaceborne) measurements of water vapour. • cross-calibration against ERS (and/or other radiometers) of MWR brightness temperatures and water vapour. • (iii) Preparation of long-term drifts detection:- • Global tide gauge network for RA-2 range monitoring. ENVISAT RA-2/MWR Cross-Cal & Val Team

  6. Cross-Calibration and Validation Strategy (2) • (iv) Product validation • Verify with real data the consistency of the product package (document, media, format and actual dataset), global valid data coverage, outliers, flags. Tune data editing criteria. • Quantifying the inherent validity and accuracy of the range and altitude measurements, geophysical corrections, of the SWH and Wind speed measurements. • Determination of orbit error in the RA-2/MWR data products (via crossover analyses). Estimation of time tag errors by minimization of crossover height differences. • The validated data will be used for the cross-calibration activities: This will enhance the quality and thoroughness of the validation. ENVISAT RA-2/MWR Cross-Cal & Val Team

  7. RA2/MWR Cross-Cal and Val Team Composition • Experts in the Members States have proposed to contribute to the RA-2/MWR Cross-Calibration and validation via the AO proposals. • A selection of AOs have been retained to contribute to the ESA core cross-calibration and validation planned activity to reach the objectives. • Guest participants from non Member States have also been invited to collaborate in the CCVT. • Expert Support Laboratories, Ground Segment processor expert • ESA Staff to coordinate Team and subgroups and to support activities ENVISAT RA-2/MWR Cross-Cal & Val Team

  8. Special Tasks • Impact assessment of the novel automatic mode switching on geophysical applications. • Cross-calibration of low and medium resolution modes on ERS-2 ice mode. • Sea State Bias estimation in both Ku and S band. • Dual frequency ionospheric correction validation • Using the novel S Band data. • Dual frequency rain flag validation • Using the novel S Band data. ENVISAT RA-2/MWR Cross-Cal & Val Team

  9. Requirements (1) • Flying Formation Configuration • ENVISAT will fly a 35-day repeat orbit and the same ground track as ERS-2, 30 minutes ahead. This meets the cross-calibration requirements (shorter than the decorrelation time of fastest meteocean signals). • Minimal relative cross-track distance to reduce the influence of cross-track geoid gradient: 100m (better was achieved for ERS-1/2) • Operation in low/medium resolution over ocean: 3 days in same part of 3 cycles. • Data products • Near Real Time (FDGDR // URA/QLOPR) • Off-Line (IGDR, GDR // OPR ; SGDR // WAP) • Other altimeter/radiometer data • In situ data and models ENVISAT RA-2/MWR Cross-Cal & Val Team

  10. Requirements (2) • Validated Orbit • Best available validated orbit will be used, including validated DORIS tracking and laser ranging data. • Wet tropospheric path delay correction • Model correction will be used until the MWR has been calibrated and validated • The ERS-2 MWR took 50 days to stabilise. ENVISAT RA-2/MWR Cross-Cal & Val Team

  11. Results from ERS-2/ERS-1 Cross-Calibration (1) • URA, QLOPR and OPR products were validated • The calibration values for each geophysical parameter estimated by the different institutes converged to similar values within the specified error limits • This provides high degree of confidence in the results • This shows the robustness of the calibration strategy • Simultaneous flight of altimeters on ERS-1 and ERS-2 permitted to discover jumps (up to 4 cm) between RA switch-offs. • ERS RA cal. scheme (SPTR) allows to derive a correction file ENVISAT RA-2/MWR Cross-Cal & Val Team

  12. ENVISAT RA-2/MWR Cross-Cal & Val Team

  13. ENVISAT RA-2/MWR Cross-Cal & Val Team

  14. Results from ERS-2/ERS-1 Cross-Calibration (2) • Sigma0 in URA/QLOPR and OPR were adjusted by global statistical comparisons • There is therefore no bias between ERS-2 and ERS-1 • Wind speed was validated against ECMWF model ENVISAT RA-2/MWR Cross-Cal & Val Team

  15. ENVISAT RA-2/MWR Cross-Cal & Val Team

  16. Results from ERS-2/ERS-1 Cross-Calibration (3) • Controversy on SWH  Buoys Vs. WAM • Calibration coefficients eventually agreed! • But it was recommended not to apply them is the products • More work needs to be done in the Wave community: • Calibration seems to depend on sea age. ENVISAT RA-2/MWR Cross-Cal & Val Team

  17. ENVISAT RA-2/MWR Cross-Cal & Val Team

  18. Results from ERS-2/ERS-1 Cross-Calibration (3) • Microwave Radiometer • Gains were stable 50 days after switch-on • Water vapor was calibrated with radio soundings • Path delay well cross-calibrated with ERS-1 MWR (within 2 mm). • Verified with TMR ENVISAT RA-2/MWR Cross-Cal & Val Team

  19. Validation and Cross-Calibration output • URA, QLOPR and OPR were verified and used by the RA CWG. This was an enhancement to the validation work • An OPR validation report was sent to all RA & MWR product users, to others on request • RA Cross-Calibration • The range bias is not significantly different from zero • USO and SPTR correction files were created and made available to users (on www) • Sigma0 (wind) calibration value is applied in URA/QLOPR, OPR • Fine tuning of Waveheight calibration values • Consensus: LUT unchanged, calibration to be applied by users ENVISAT RA-2/MWR Cross-Cal & Val Team

  20. Conclusion • Solid experience of relative calibration and validation has been acquired with the novel ERS-2/ERS-1 RA & MWR cross-calibration activities. • The ENVISAT RA-2/MWR cross-calibration and validation activities are based on proven methodology. Experience to apply the Cal/Val methods exist in Europe in various institutes, each of which have their own algorithms and methodology, experience, and areas of interest. • The RA-2/MWR Cross-Calibration and Validation Team has been constituted with AO PIs, ESLs, ESA staff, and is at work. • The RA-2/MWR Cross-Calibration and Validation Plan is drafted and is being peer-reviewed and refined by the Team. ENVISAT RA-2/MWR Cross-Cal & Val Team

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