The Global Ozone Monitoring Experiment (GOME) is a nadir-viewing spectrometer covering a broad wavelength range, from the UV to the near infrared. 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.