Polarization scramblers in Earth observing spectrometers: lessons learned from Sentinel-4 and 5 phases A/B1

Sensitivity to polarization is a major design driver for Earth observing dispersive spectrometers. While the measured Earth radiance observed from space in the UV, visible and near IR bands has a strong and highly variable linearly polarized component, most essential components in spectrometers are inherently sensitive to polarization: scan mirrors, gratings, dichroics. Minimisation of the resulting radiometric errors is a challenge and cannot be only achieved with careful optical designs. Depolarization by passive optical components such as birefringent polarization scramblers has been demonstrated with the last generation of atmosphere monitoring instruments (MERIS, OMI). In order to achieve the demanding performances targeted by future instruments (Sentinel-4, Sentinel-5, CarbonSat) the available degrees of freedom left for optimisation shall be explored, and new polarization scrambler designs must be found. This paper summarizes design rules and performance aspects identified by ESA during phases A/B1 of the Sentinel-4 and Sentinel-5 missions. The following aspects have been investigated and will be discussed: minimization of polarization dependent spectral oscillations, use of a polarization scrambler in converging beam or parallel beam at large angles of incidence, polarization dependent pointing error.