In-orbit Optical Path Degradation: Gome Experience and Sciamachy Prediction

It is well known that degradation of optical components is to be expected after prolonged exposure to the conditions experienced in orbit. In particular it is expected that the instrument response to ultra-violet light will change. GOME and SCIAMACHY are prone to degradation. In order to minimise the impact on the scientific data products, several means of monitoring possible degradation were incorporated in the instrument design and operations. GOME degradation is monitored on a regular basis, using light sources both internal and external to the instrument. This paper presents some results of the measured GOME instrument monitoring using the Sun and the Moon as sources of stable radiation. The unexpected behaviour of the degradation observed can be qualitatively explained by changes in the reflective properties of the scan mirror, resulting in scan-angle and polarisation dependent degradation. With the degradation model, based on the Fresnel equations for reflection coefficients of media with known refractive index, predictions are made for GOME degradation behaviour for other angles and polarisation states than the Solar and Lunar viewing angles. In addition, predictions are made for SCIAMACHY degradation, under the assumption that a similar degradation mechanism is present and dominating in the SCIAMACHY instrument.