Abstract: The Global Ozone Monitoring Experiment is a nadir-viewing spectrometer covering a broad wavelength range, from the UV to the near IR. It has been operating since 1995 on board the ESA ERS-2 satellite, monitoring a large range of atmospheric trace constituents, with particular emphasis on ozone. The performance of the instrument is monitored in- flight by means of routine on-board calibration measurements, observing the sun and, occasionally, the moon. In this way, degradation of optical components in space can be monitored. For example, the performance of the broad-band detectors which monitor the polarization state of the incoming light is analyzed by means of solar measurements. The measurements of the polarization detector which samples UV light show a degradation of 6 percent per year. The optical components affected have been identified by monitoring the fractional polarization, which is a characteristic of the light back-scattered by the Earth's atmosphere. The influence of the observed degradation of Earth radiation measurements is estimated to be in the order of 1. 5 percent per year in the UV wavelength range. !20
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