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Desert calibration results

Desert calibration results. Direct comparison of TOA reflectances (865 nm). Meris. Meris. Polder. Ground measurement campaign (93). Algeria 4. Libya 1. Meris. Seawifs. Libya 1. AATSR visible and near infrared calibration over desert sites . François Cabot, Olivier Hagolle CNES.

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Desert calibration results

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  1. Desert calibration results • Direct comparison of TOA reflectances (865 nm) Meris Meris Polder Ground measurement campaign (93) Algeria 4 Libya 1 Meris Seawifs Libya 1

  2. AATSR visible and near infrared calibration over desert sites François Cabot, Olivier Hagolle CNES

  3. Definitions AATSR on board calibration is the nominal calibration method AATSR is calibrated against reflectance k: Spectral band DN : Digital Number (AATSR Level 0, corrected for instrumental defects) r : Reflectance Ak : Sensitivity of instrument for spectral band k Calibration results are expressed this way Calibration of AATSR using natural targets

  4. Calibration of AATSR over desert sites • Calibration over desert sites • 20 desert sites (Nth Africa, Arabia) selected for their homogeneity and stability over time • Since 1996, a desert reflectance data base is built with • POLDER, VEGETATION (1 and 2), ATSR2, • MODIS, MISR, SeaWiFS, AVHRR • MERIS, AATSR • Comparison of acquisitions with similar geometries : • qs(AASTR)-2° <qs(REF)< qs(AASTR)+ 2° • qv(AATSR)-2° <qv(REF)< qv(AASTR)+ 2° • f(AATSR)-5° < f (REF) < f(AASTR)+ 5° • Reciprocal geometries are accepted

  5. Calibration of AATSR over desert sites AATSR POLDER Available geometries for Libya 1

  6. Sensor 2 Reference Sensor Atmosph correction to surface reflectance (6S), tau=0.2 Comparison Spectral resampling 865 nm Atmosph. simulationto ToA reflectance (6S), tau=0.2 Surface reflectancefor sensor 2 Calibration of AATSR over desert sites • Desert site calibration method • Multi-instrument accuracy (3s) : 2% • Multi-temporal accuracy (3s) : 1% Comparison of SeaWiFS Multi-date calibration obtained with deserts (blue) or Moon (green)

  7. AATSR Desert calibration results • cross cal. with POLDER as a function of time (all sites) AATSR/POLDER 555 1.08% 670 1.08% 865 1.14% Variations with time for 555 ? 555 670 AATSR/MERIS (2 points) 555 1.01% 670 1.02% 865 1.03% 870

  8. Desert calibration results • Calibration bias increasing in the near infrared • between POLDER and MERIS • POLDER, VGT and SeaWiFS are quite consistent

  9. Calibration of MERIS using natural targets • Conclusions • Very preliminary Results • Deserts • Two sensor families : • POLDER, ATSR2,SeaWiFS, VEGETATION • AATSR, MERIS have a calibration discrepancy of about 10 % (near infrared) • No explanation for this discrepancy • deserts processing has been deeply verified • consistent with D. Smith results • very simple processing (comparison of reflectance) • Much more data are needed

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