In-flight calibration of the POLDER polarized channels using the sun's glitter

The spaceborne sensor Polarization and Directionality of the Earth Reflecta nces (POLDER), launched on the Japanese platform Advanced Earth Observation Satellite (ADEOS) on August 17, 1996, is a new instrument devoted to multi spectral observations of the directionality and polarization of the solar r adiation reflected by the earth-atmosphere system, Polarization measurement s are performed in three channels, centered at 443, 670, and 865 nm, As POL DER has no onboard calibration system, in-flight calibration methods have b een developed. We address in this paper the calibration of the polarization measurements. The method uses the sunlight reflected within the sun's glit ter. While the radiance of the sun's glitter depends strongly on the sea su rface roughness, its intrinsic degree of polarization depends only on the o bservation geometry, which is specially convenient for calibration purposes . However, the degree of polarization measured at the satellite level is af fected by the atmosphere, The proposed calibration scheme allows us to take into account the influence of the atmosphere on the degree of polarization measured in some viewing direction within the glitter pattern hy using the radiance measured in the same viewing direction and in another direction f ar from the glitter. The expected accuracy is about 0.5% in the near-infrar ed channel 865 nm and about 2% in the visible channels, in terms of percent polarization. The method has been applied successfully to measurements ach ieved over ocean areas with the airborne version of the POLDER instrument.