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Dark correction

Table of Contents Dark correction

Dark Signal values are measured in every orbit on the night side and once a month around the whole orbit. Operationally the dark signal is divided into the following parts:


The analogue offset or fixed pattern noise. This is a signal added by the electronics to each pixel to avoid negative or near zero signals. This value is independent from external changes to the observation condition and also independent from exposure time. However, for each readout, the analogue offset is added meaning that a measurement with coadding factor n contains n x analogue offset. (???)


The leakage current. This is the time dependent part of the dark signal mainly caused by thermally created electron-hole pairs. It is conceptually divided in two parts:


One that is independent form orbit phase. This part is derived from the orbital measurements on the dark side of the Earth


One that depends on orbit phase because of the different illumination of the instrument and resulting small temperature changes. This part is derived as a function of twelve orbit segments from the monthly dark measurements that are done for a complete orbit. The variable leakage is only relevant for the SWIR channels.


The stray light. Near sun rise, the limb port of the instrument is directly illuminated by the sun leading to a large amount of spatial stray light. This stray light is characterised during the monthly dark orbits. It is only relevant for orbit phases around sun rise, i.e. over the Northern pole.

All orbit phase independent dark parameters are part of the LEAKAGE CONSTANT ADS. The LEAKAGE VARIABLE ADS contains the orbit phase dependent components of the dark signal. Both ADS’s are used for the dark correction. The LEAKGE NEW ADS contains the newly calculated parameters that were obtained on that orbit for each block of dark measurements. For the UV/VIS channels 1-5 the dark signal is calculated from the fixed pattern noise, the orbit independent leakage current and the stray light component. The SWIR channels are treated differently, because more variations can be expected due to the thermal background of the instrument and the ice layers on the detectors of channel 7 and 8. For these channels the dark parameters are calculated from the measurements of the same orbit, i.e. from the LEAKAGE NEW ADS. Additionally the variable leakage signal from the LEAKAGE VARIABLE ADS is used to take into account the thermal background. The individual dark measurements are no longer part of the level 1b product, but state wise averages are stored in the DARK AVERAGE ADS. Stray Light Correction

The stray light correction uses the key data to obtain a correction value for each pixel in each measurement. The correction values are stored directly in each MDS of the level 1 product in one byte (i.e. 255 values). Wavelength Calibration

The wavelength calibration uses on-ground and in-flight data for channels 1-6. From weekly and monthly in-flight measurements of the spectral line source, the difference to the on-ground calibration is determined. The line positions are determined using the Falk algorithm. Subsequently a wavelength for each pixel is stored in the product. Since the spectral line source does not have enough lines in the infrared and bad pixels interfere with the line position determination, for channels 7 and 8 the on-ground wavelength calibration is taken without any changes for these channels. PPG/Etalon Correction

The PPG correction aims to correct relative gain changes with time in the pixels of the SWIR channels. It is determined in-flight using the White Light Source (WLS) measurements. However, observations show that the PPG does not change significantly in-flight and the correction is currently not applied.
In the measured spectra interference structures can be seen as a result of multiple reflections on the boundaries of the protective layer of SiO on the light detecting silicon. This so-called etalon effect can change if material is deposited on top of the protective layer. The correction for this change is derived from the weekly WLS measurements and is only relevant for channels 1-5. Both corrections are stored in the PPG ETALON ADS.

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