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ENVISAT/MWR Calibration Review. E. Obligis, L. Eymard, A. Pilon. 1. History of the characterisation parameters changes/corrections 2. Description of the products/algorithms used for the calibration 3. Methodology for the MWR calibration/validation 4. Intercalibration with ERS2

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ENVISAT/MWR Calibration Review


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    1. ENVISAT/MWR Calibration Review E. Obligis, L. Eymard, A. Pilon

    2. 1. History of the characterisation parameters changes/corrections 2. Description of the products/algorithms used for the calibration 3. Methodology for the MWR calibration/validation 4. Intercalibration with ERS2 TBs over Ocean TBs over Antarctic L2 products over Ocean 5. Next steps Calibration over stable continental areas L2 products validation New side lobe algorithm NN geophysical algorithms 6. ERS2/MWR Status « Reprocessing » 7. Conclusions

    3. 1. History of the characterisation parameters changes/corrections • Transmission coefficients of the reflector : ref • first fixed to the ERS2/MWR values • then adjusted => acts like a linear correction over TBs • Sky horn feed transmission coefficients : a_cc • adjusted to produce gain values close to the on ground ones • 9.68 at 23.8 GHz and 10.16 at 36.5 GHz • Main antenna transmission coefficients : c1=1/(ar.afeed) • adjusted to correct last biases

    4. Calibration corrections

    5. 2. Description of the products/algorithms used for the calibration

    6. 3. Methodology of calibration • Algorithms formulated using the 2000/01 version of the ECMWF model • Important change in the model in January 2002 • Algorithms and calibration have to be consistent • Not suitable to calibrate ENVISAT over 2002 analyses • Calibration made over ERS2 TBs adjusted on the GI01 database

    7. Comparison measurements/simulations ERS2 - 2000/2001 database

    8. 4. Comparison ERS2_adjusted/ENVISAT TBs over Ocean

    9. Comparison ERS2_adjusted/ENVISAT TBs over Antarctic

    10. Comparison L2 ERS2/ENVISAT products

    11. 5. Next steps • TBs calibration : • statistics over stable hot and cold continental areas • Sahara desert and Amazonian forest • Greenland Glacier and Antartic Plateau • need to reprocess all TBs with the new characterization parameters • L2 products validation : • using radiosounding measurements (not enough at that time) • using GPS measurements (if any usable ?)

    12. 5. Next steps -2 • Smoother correction of the gain in case of temperature variations • use of a polynomial law for g=f(T) instead of a discrete table • New side lobe contribution • this contribution has been estimated using ERS2 TBs over land and ocean • use of 4 seasonal maps for each frequency Tslg=href x Tsl + (1-href) x Tref 1. Tsl constant : Tsl_24=8.21K Tsl_36=0.413K 2. Tsl=hsun x Tsun + hsky x Tsky +hearth x Tearth + hsat x Tsat with Tearth estimated over each mesh of 1°x1° over the globe, for each season

    13. 5. Next steps -3 • NN instead of parametric algorithms • dh = c0 + c1 ln(280.-TB23.8) + c2 ln(280.-TB36.5) + c3 1/s0Ku2

    14. 6. ERS2 status • Analysis of ERS2/MWR data over continental targets : • Antarctic plateau (cold target) • Sahara desert and Amazonian forest (hot target) • Greenland plateau (rather cold) for comparison with Topex/TMR Low-pass filtering of the time series, and estimation of the trend

    15. Comparison ERS2 / Topex over Greenland ERS2 Topex GR23 GR37 Good comparison of time series at corresponding channels (initial and filtered) Gradient ratio : GR37=(T36.5-T37)/(T36.5+T37) and G23=(T23.8-T21)/(T23.8+T321) over the same 6 years, trend +0.5K at 36.5/37GHz, -0.7K at 23.8/21GHz

    16. ERS2 reprocessing • ENVISAT calibrated on ERS2 TBs adjusted to 2001 model • Retrieval algorithms formulated over this model • Same algorithms can be applied over Envisat and ERS2 TBs • For ERS2, a preliminar adjustment to 2000/2001 model is needed • tb_ers_23.8=tb_ers_23.8+4.90 • tb_ers_36.5=tb_ers_36.5+2.16

    17. 7. Conclusions • Good behaviour of the radiometer • Necessary adjustment in the level1 processing => new href,acc,c1 • These new characterization parameters have to be checked • using the sigma0 in Ku band to compute exact level2 products • looking at atmospheric attenuations • using the new version of the processing chain (side lobe,g=f(T)…) • by comparison with other in-flight radiometers (TOPEX, JASON, SSM/I, TMI) • final validation after at least one year to get enough radiosounding measurements • With these new characterization parameters the wet tropo correction will be usable by altimeter community