Articles | Volume 11, issue 1
Research article
15 Jan 2018
Research article |  | 15 Jan 2018

In-flight calibration of SCIAMACHY's polarization sensitivity

Patricia Liebing, Matthijs Krijger, Ralph Snel, Klaus Bramstedt, Stefan Noël, Heinrich Bovensmann, and John P. Burrows

Abstract. This paper describes the in-flight calibration of the polarization response of the SCIAMACHY polarization measurement devices (PMDs) and a selected region of its science channels. With the lack of polarized calibration sources it is not possible to obtain such a calibration from dedicated calibration measurements. Instead, the earthshine itself, together with a simplified radiative transfer model (RTM), is used to derive time-dependent and measurement-configuration-dependent polarization sensitivities. The results are compared to an instrument model that describes the degradation of the instrument as a result of a slow buildup of contaminant layers on its elevation and azimuth scan mirrors. This comparison reveals significant differences between the model prediction and the data, suggesting an unforeseen change between on-ground and in-flight calibration in at least one of the polarization-sensitive components of the optical bench. The possibility of mechanisms other than scan mirror contamination contributing to the degradation of the instrument will be discussed. The data are consistent with a polarization phase shift occurring in the beam split prism used to divert the light coming from the telescope to the different channels and polarization measurement devices. The extension of the instrument degradation model with a linear retarder enables the determination of the relevant parameters to describe this phase shift and ultimately results in a significant improvement of the polarization measurements as well as the polarization response correction of measured radiances.

Short summary
This article describes a method to determine the polarization sensitivity of SCIAMACHY, a spectrometer on Envisat, from in-orbit data. Polarization is a preference of a direction in which light oscillates, and many optical instruments suffer from a dependence of their measured signals on this. To measure and correct for this effect, a statistical analysis of in-flight data combined with a model of the atmosphere and the instrument was performed, showing that the instrument changed after launch.