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

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

FDefinitions ïAATSR on board calibration is the nominal calibration method ïAATSR is calibrated against reflectance 7k: Spectral band 7DN : Digital Number (AATSR Level 0, corrected for instrumental defects)   : Reflectance 7A k : Sensitivity of instrument for spectral band k ïCalibration results are expressed this way Calibration of AATSR using natural targets

Calibration of AATSR over desert sites FCalibration over desert sites ï20 desert sites (Nth Africa, Arabia) selected for their homogeneity and stability over time ïSince 1996, a desert reflectance data base is built with 7POLDER, VEGETATION (1 and 2), ATSR2, 7MODIS, MISR, SeaWiFS, AVHRR 7MERIS, AATSR FComparison of acquisitions with similar geometries :   s(AASTR)-2° <  s(REF)<  s(AASTR)+ 2°   v(AATSR)-2° <  v(REF)<  v(AASTR)+ 2°   (AATSR)-5° <  (REF) <  (AASTR)+ 5° 7Reciprocal geometries are accepted

Calibration of AATSR over desert sites AATSRPOLDER Available geometries for Libya 1

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

AATSR Desert calibration results Fcross cal. with POLDER as a function of time (all sites) AATSR/POLDER % % % Variations with time for 555 ? AATSR/MERIS (2 points) % % %

Desert calibration results ïCalibration bias increasing in the near infrared 7between POLDER and MERIS ïPOLDER, VGT and SeaWiFS are quite consistent

ïGood consistency between Rayleigh and Glitter ïA bias between 0 to 3 % is measured Rayleigh calibration results Reference spectral band Error bars represent standard deviation of results

Calibration of MERIS using natural targets FConclusions ïVery preliminary Results ïDeserts 7Two sensor families : [POLDER, ATSR2,SeaWiFS, VEGETATION [AATSR, MERIS have a calibration discrepancy of about 10 % (near infrared) ïNo explanation for this discrepancy 7deserts processing has been deeply verified 7consistent with D. Smith results 7very simple processing (comparison of reflectance) ïMuch more data are needed