Concept for Absolute Radiometric Calibration of MOMS-2P

Abstract

The evaluation of the pre-mission radiometric calibration data of MOMS-2P showed that the derived relative calibration factors depended strongly on the intensity. For this reason the existing laboratory measurements could not be used for absolute calibration. Due to lack of time this problem could not be further investigated before launch. Therefore alternative concepts for radiometric calibration during the mission were considered. For monitoring of the long term radiometric properties of MOMS-2P the inner side of the optics protection cover is coated with spectralon. This spectralon plate acts as diffuse reflector of known spectral reflectance characteristics. By tilting the spectralon plate by 20 degrees with respect to the entrance optics the direct sunlight is reflected into the optics. As the spectrum of the sun is very well known these measurements can be used for absolute calibration. The principles of these measurements will be described and first data will be presented. In addition to the sun calibration an in-field calibration method is in preparation. For this method large ground targets of approx. 100 m in diameter with known spectral reflectance characteristics must be available. During the MOMS-2P overflight the sun irradiance on the ground and the atmospheric transmittance have to be measured. The basic concept of this in-flight calibration method will be presented.