Assessment and Correction of View Angle Dependent Radiometric Modulation due to Polarization for the Cross-Track Infrared Sounder (CrIS)

Abstract

The Cross-track Infrared Sounder (CrIS) is an infrared Fourier-transform spectrometer that measures the Earth’s infrared radiance at high spectral resolution and high accuracy. The potential for polarization errors contributing significantly to the radiometric uncertainty of infrared remote sounders has been well recognized and documented, particularly due to polarization-dependent scene select mirrors operated in conjunction with grating-based instruments. The issue is equally applicable to FTS-based sensors. While the CrIS sensor utilizes an unprotected gold scene select mirror which has extremely low polarization in the infrared and the angle of incidence at the mirror is maintained for all calibration and Earth scene views, the radiometric bias due to polarization effects was determined to be non-negligible for cold scenes. A model for the polarization-induced calibration bias and the associated correction is presented for the CrIS instrument, along with details of the model parameter determination, and the impact of the correction on the calibrated radiances for a range of scene temperatures and types.

Simulation of CrIS Radiances Accounting for Realistic Properties of the Instrument Responsivity That Result in Spectral Ringing Features

“…2023, 15, 334 2 of 20 geolocation, radiometric calibration, spectral calibration, and a range of quality metrics and monitoring activities. Radiometric calibration accounts for various effects including detector nonlinearity, the basic two-point linear calibration, and scene mirror induced polarization [4,5]. Spectral calibration includes accounting for the well-known effects of self-apodization, and processing to produce calibrated spectra on a standard wavenumber scale with a standard Spectral Response Function (SRF).…”

“…Non-flat instrument SRFs inside measured spectral bands; (4). Artifacts of the spectral resampling and selfapodization correction steps of the calibration algorithm; and (5). Lack of perfect circular sampling of the onboard numerical filter.…”