CLARREO Reflected Solar Spectrometer: Restrictions for Instrument Sensitivity to Polarization

The foundation for future space mission Climate Absolute Radiance and Refractivity Observatory (CLARREO) is the ability to produce climate change benchmark records and provide on-orbit calibration standard through the highly accurate and Système Internationale-traceable observations. The accuracy of CLARREO measurements is set to 0.3% (k=2) for spectrally resolved reflectance. The instrument sensitivity to polarization and polarization of reflected light at the top of atmosphere are the sources for systematic uncertainty. In this paper, we estimate radiometric errors due to polarization effects for CLARREO benchmark and reference intercalibration observations. Data from the Polarization and Anisotropy of Reflectance for Atmospheric Sciences coupled with Observations from Lidar (PARASOL) instrument, a spaceborne polarimeter, have been used in combination with the orbital modeling of Earth's sampling. For the CLARREO benchmark data, we used simulated annual nadir sampling for the polar orbit with 90° inclination, and for the intercalibration with cross-track sensors on the JPSS, such as CERES and VIIRS, we simulated on-orbit matched data sampling. Selected PARASOL data over one full solar year provided polarization parameters in visible (VIS) spectral range. For estimating polarization in near infrared (NIR) spectral range, we used a radiative transfer model. Our results show that to limit error contribution due to polarization to half of the allowed total, the sensitivity to polarization of CLARREO reflected solar instrument should not exceed 0.5% (k=2) in spectral range from VIS to NIR.