SCIAMACHY monitoring factors: observation and end-to-end correction of instrument performance degradation

SCIAMACHY (SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY) is a grating spectrometer in the UV-Vis-NIR spectral range. SCIAMACHY is part of the ENVISAT payload and since April 2002 in a sun-synchronous orbit, observing Earth’s atmosphere in nadir, limb and occultation geometry. To ensure the SCIAMACHY data quality over the life-time of the instrument, its performance is continuously monitored. The throughput of the instrument is monitored end-to-end by regular observations of the sun. From these measurements, the so-called monitoring-factors (m-factors) are calculated. Each light path of the instrument has its own set of m-factors. By applying the m-factors, a stable radiometric calibration of the radiances and irradiances from SCIAMACHY over mission lifetime can be ensured.

We describe the measurements, the calculations and the operational implementation of the m-factors. Based on the m-factors, the performance of the SCIAMACHY over seven years in orbit is assessed. Furthermore, first examples will be shown how the retrieval of atmospheric parameters benefits from m-factors.