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Polarization Correction

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6.2.2.7 Polarization Correction

The polarization correction is necessary because SCIAMACHY does not employ a polarization scrambler and the detected signal is thus dependent on the polarization of the incoming light. The polarization response was determined on-ground using the Polarization Measurement Devices (PMDs). The key data contain the ratio of the PMD signal to the science channel signal for several instrument configurations and polarization directions. The polarization is also measured in-flight and from the calculated polarization and the key data a linear polarization correction factor is calculated for all wavelengths. All data necessary to calculate the polarization correction factors are stored in the Level 1b product: The polarization sensitivities from on-ground measurement are stored in the POLARIZATION SENSITIVITY ADS for nadir, limb and occultation. The polarization fractions and the integrated PMD values for each observation are directly stored in the MDS.

6.2.2.8 Radiometric Correction


Finally, the data have to be radiometrically calibrated. This is done by using on-ground key data. The radiometric sensitivity for each observation is stored directly in the MDS of the level 1b product.

6.2.2.9 Sun Mean Reference

The sun mean reference (SMR) is used to calculate the reflectance. It is derived from daily diffuser measurements of the Elevation Scan Mechanism (ESM) diffuser and the Azimuth Scan Mechanism (ASM) diffuser. For both diffusers a fully calibrated SMR and a SMR with all calibrations applied except the radiometric calibration are available. The latter is provided, because tests showed that some retrieval have better quality when the radiometric calibration was switched off. All SMRs are stored in the SUN MEAN REFERENCE ADS. This ADS also contains sun occultation measurements and ESM diffuser measurements that are not used to calculate the reflectance. The identifiers are as follows:


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E0 : Not radiometrically calibrated ESM sun spectrum, Neutral Density Filter (NDF) in

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E1 : Not radiometrically calibrated ESM sun spectrum, NDF out

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A0 : Not radiometrically calibrated ASM sun spectrum

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A1 : Fixed ASM spectrum (manually provided at begin of mission)

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D0 : Fully calibrated ESM sun spectrum, NDF in

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D1 : Fully calibrated ESM sun spectrum, NDF out

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D2 : Scaled ASM sun spectrum, using ESM calibration data, i.e. these data are only scaled and not calibrated

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Ox : Occultation data (calibrated and scaled using limb radiometric response)

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S : Subsolar data (calibrated and scaled using nadir radiometric response)

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U : Occultation data (calibrated excluding limb radiometric response)

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V : Subsolar data (calibrated excluding nadir radiometric response)




Note that only the spectra in bold type are useful for sun normalization of the Earthshine data! The ESM diffuser data without the NDF in are made for monitoring purposes and partly saturate. The scaled diffuser ASM spectrum (A1) does not give the absolute radiometric response but only corrects roughly instrument characteristics in the spectrum. The occultation and subsolar measurements have the general problem that they are measured with the small aperture that was not thoroughly characterized on-ground. Thus they are mainly useful for monitoring purposes. All sun spectra are averaged over the number of available and suitable measurements in a state (see the level 0-1c ATBD for details).

6.2.2.10 Degradation Correction


The degradation correction with the monitoring factors (m-factors) is implemented in the Level 0-1b processing, but at the moment not operationally used. The radiometric degradation correction is applied in the level 1b-2 processing (see section xxx). However, the user has the possibility to apply the radiometric degradation correction with the SciaL1c tool and thus produce corrected Level1c data.


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