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DEVELOPMENT OF IN- FLIGHT CALIBRATION METHODS USING S PECULAR REFLECTION FOR THE MULTI -VIEWING, MULTI -CHANNEL ,MULTI -P OLARIZATION I MAGER (3MI) MOHAMED S. DJELLALI 1 ,J ÉRÔME RIEDI 1 ,SÉBASTIEN MARCQ 2 ,BERTRAND FOUGNIE 3 , SOUICHIRO HIOKI 1 ,J EAN-MARC NICOLAS 1 1 LABORATOIRE D’OPTIQUE ATMOSPHÉRIQUE (LOA), UNIVERSITÉ DE LILLE,LILLE,FRANCE 2 CENTRE NATIONAL D’ETUDES SPATIALES (CNES), TOULOUSE,FRANCE 3 EUROPEAN ORGANISATION FOR THE EXPLOITATION OF METEOROLOGICAL SATELLITES,DARMSTADT,GERMANY 1. MOTIVATION 3MI lacks on-board calibration systems. Therefore, it will rely on vicari- ous calibration techniques. What we propose: • Use the sunglint as a basis for an absolute calibration technique. Why ? • Strong intensity throughout the solar spectrum. • Well-known spectral and geometrical behavior. Challenges: • An absolute calibration objective accuracy of 2 %. • Surface wind speed uncertainties. • Atmospheric composition. 2. METHODOLOGY The method’s underlying idea relies on the statistical nature of the phenomena. We define an optimal zone for absolute calibration using differences between two simulations of the sunglint reflectance via the Cox & Munk model. The sensitivities to surface wind speed as well as the atmosphere are minimized. 3. RESULTS ON POLDER The selected case is from the 6th of March 2009, near the Australian coast. The main conditions for selecting the case are a clear sky over the deep ocean with low aerosol loads (less than 0.1 for the AOT at 550 nm). We can observe a much lower sensitivity to the surface wind speed as well as the aerosol quantity within the defined optimal zone compared to the close-to-specular zone, for all the tested spectral bands. The objective accuracy of 2 % using our proposed calibration method has been achieved during the first approach using POLDER data. 4. OSIRIS: AN AIRBORNE SIMULATOR FOR 3MI OSIRIS (Observing System Including PolaRisation in the Solar Infrared Spectrum) and 3MI have similar measurement principles as well as spec- tral ranges. We observe less variations in the relative differences between the data and the simulations within the theoretically defined zone. This particular approach could be used to monitor an airborne instru- ment’s calibration. 5. CONCLUSIONS • 3MI is going to be important for the remote sensing of aerosols and clouds as well as for the air quality monitoring and NWP. • The instrument is strongly inspired from the POLDER concept and will also strongly rely on natural targets for its in-flight calibration. • Sensitivity to surface wind speed has been minimized using the sunglint reflectance’s geometrical behavior. • Tests using POLDER data show promising results with accuracies better than or about 2 % for all the tested spectral bands. • The proposed method could be used on 3MI as well as the OSIRIS airborne radiometer. 6. OUTLOOK &PERSPECTIVES • Validate more cases using the same approach. • Evaluate the technique’s applicability for the commissioning phase. • Improve the method by obtaining the surface wind speed parameters as well as the absolute calibration coefficients using an optimal estimation technique.
