An Assessment of the In-flight Polarization Response of SCIAMACHY

Abstract:

SCIAMACHY, onboard ENVISAT, had been measuring spectrally resolved radiances in the UV-VIS-NIR-SWIR range for almost ten years until 2012. While the spectra from observations in nadir and limb modes have been used successfully to retrieve trace gas columns and profiles, one of the biggest remaining issues is their accurate absolute radiometric calibration, thereby accounting for the polarization sensitivity of the instrument.

To measure the linear polarization, the instrument is equipped with broadband sensors, so called Polarization Measurement Devices (PMDs). The polarization response of the spectral channels and the PMDs had been calibrated on ground, but comparison of in-flight data with model and other instruments revealed that those measurements may have been compromised or that the response changed from on ground to inflight conditions. In order to understand and monitor the instrument behavior, several statistical methods have been explored and evaluated. They make use of the in flight limb and nadir data and reference data from radiative transfer models and instruments such as GOME-2 and PARASOL. Triggered by the observed inconsistencies between on-ground and in-flight data, the on-ground measurements have been reanalysed at SRON. The resulting on-ground polarization sensitivities now show much better agreement with the data derived from in-flight. The results for the effective polarization response from various methods will be presented and compared with the old and new on-ground data sets.