Simulation of CLARREO Reflected Solar Instrument Errors due to Polarization

The goal of CLARREO Reflected Solar (RS) imaging spectrometer is to enable rigorous climate decadal change observations by providing highly accurate SI-traceable measurements of reflected spectral radiances. The allowed CLARREO RS error budget is set for 0.3% (2 sigma) for the spectrally integrated broadband radiance. One of the major contribution into error budget of the spectrometer comes fromuncertainty of its sensitivity to polarization and knowledge of polarization state of incomming light.

In this study we estimate CLARREO RS radiometric errors due to polarization effects for both, benchmark and reference inter-calibration operating modes. The PARASOL Level-1 polarization and Level-2 Clouds data have been used in combiation with orbital simulation of sampling. For benchmark mode we used simulated annual sampling for 90 polar orbit, and for reference inter-calibration of CLARREO RS with instruments on NPOESS (VIIRS, CERES) simulated cross-track data collecting mode. Selected PARASOL data over year 2006 provided realistic global sampling of polarization levels in visible spectral range and scene types. For error estimate in near-infrared spectral range we used a radiative transfer model.

The results of this study lead to the formulation of requirement for sensitivity to polarizaion for CLARREO RS imaging spectrometer: sensitivity to polarization of the instrument should not exceed 0.25% (1 sigma) in visible (below 1000 nm wavelength), and 0.75% (1 sigma) in near-infrared spectral ranges (beyond 1000 nm wavelength).