Let's Get Clean Again: Contaminated Mirrors in Space and Throughput Recovery
Krijger, Matthijs¹; Snel, Ralph¹; Sciamachy, Q.W.G.^2
1SRON Netherlands Institute for Space Research, NETHERLANDS; ²IUP University of Bremen, GERMANY

Accurate long-term climate and atmosphere constituent trends are becoming more important with current climate changes. The study of these global trends requires that space borne instrument remain radiometrically stable over long periods. However, degradation of optical components in space is a problem affecting many remote sensing instruments, due to the prolonged exposure to the conditions in orbit. Degradation has been observed for GOME-1 on-board ERS-2 , SCIAMACHY on-board ENVISAT , and GOME-2 on-board METOP-A, but also the various other ESA missions. Current retrieval techniques require such high (relative) radiometric accuracy that the originally planned degradation correction schemes are no longer adequate, as degradation introduces changes in various instrument sensitivities. Not only the spectral throughput is affected but also the polarization response and their scan angle dependence.
A special case is SCIAMACHY which, unlike other instruments where throughput only decreases, showed a large throughput recovery in it's last years.
We will explain this temporal evolution by modelling the mirror (and diffuser) surfaces and their (multi-layer) contaminations in the scanner unit of the instruments, employing Fresnel equations. By combining with routine inflight monitoring measurements, the contaminant thicknesses are determined. All this information combined allows for spectral-, polarisation- and scan-angle-dependent degradation correction. This approach is currently being implemented for SCIAMACHY degradation correction. We will also explain the temporal evolution of the unexpected throughput recovery of SCIAMACHY in the last years and how this general approach can be applied to other missions together with lessons learned for future missions.