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

The spaceborne sensor Polarization and Directionality of the Earth Reflectances (POLDER), launched on the Japanese platform Advanced Earth Observation Satellite (ADEOS) on August 17, 1996, is a new instrument devoted to multispectral observations of the directionality and polarization of the solar radiation reflected by the Earth-atmosphere system. Polarization measurements are performed in three channels, centered at 443, 670, and 865 nm. As POLDER has no onboard calibration system, in-flight calibration methods have been developed. The authors address in this paper the calibration of the polarization measurements. The method uses the sunlight reflected within the Sun's glitter. While the radiance of the Sun's glitter depends strongly on the sea surface roughness, its intrinsic degree of polarization depends only on the observation geometry, which is specially convenient for calibration purposes. However, the degree of polarization measured at the satellite level is affected by the atmosphere. The proposed calibration scheme allows the authors to take into account the influence of the atmosphere on the degree of polarization measured in some viewing direction within the glitter pattern by using the radiance measured in the same viewing direction and in another direction far from the glitter. The expected accuracy is about 0.5% in the near-infrared channel 865 nm and about 2% in the visible channels, in terms of percent polarization. The method has been applied successfully to measurements achieved over ocean areas with the airborne version of the POLDER instrument.