Bertrand Fougnie, Sophie Lachérade (CNES) GSICS Annual Meeting 29th Feb.- 4th March 2016, Tsukuba, Japan Discussion Cross-calibration (over the Moon) Bertrand Fougnie, Sophie Lachérade (CNES) Jack Xiong (NASA)

Context Digit = A. L Digit = A. Es. R Radiance (or irradiance) is measured by sensors physical quantity close to the measurement depends on the observed target but also depends on the illumination of the target (in general the sun) Reflectance (or normalized radiance) is a property of the target physical quantity close to the target depend only the target properties but doesn’t depend on the illumination Some sensors are calibrated in reflectance R, other in radiance L calibration = ratio measurement / reference a calibration “in reflectance” adjust the product calibration x solar irradiance (A.Es) Es is required to convert radiance into reflectance, or reversely Many solar irradiances are used – in general, no common reference What do you/we calibrate ? What does GSICS want to calibrate ? Digit = A. L Digit = A. Es. R

Discussion Proposition (to be discussed): A cross-calibration approach (result) must address the question a cross-calibration result could simply show a difference between solar irradiances considered by both systems cross-calibration between Pleiades and MODIS/VIIRS : results were impacted by the difference between solar irradiances considered by Pleiades and MODIS Proposition (to be discussed): If the reference sensor is calibrated “in reflectance”, cross-calibration should be done on reflectance If the reference sensor is calibrated “in radiance”, cross-calibration should be done on radiance or irradiance otherwise the result will be impacted by the difference between solar irradiances of both sensors. Note : The GSICS reference MODIS is calibrated in reflectance After the cross-calibration, every GPRC can use its own solar irradiance Es to convert radiance <-> reflectance

Example Cross-calibration between Pleiades and MODIS : results were impacted by the difference between solar irradiances considered by Pleiades and MODIS For the cross-calibration VIIRS-Pleiades the following cross-calibration was applied : Reflectance to calibrate corresp. ROLO reflectance PHR1-B vs VIIRS Reference reflectance corresp. ROLO reflectance Normalization factor for sun/moon/sensor distances