Anomalous solar array performance on GPS

The general issue of degradation of optical surfaces on spacecraft is reviewed in order to understand the observed behavior of the Navstar solar cell arrays. The solar arrays on GPS Navstars 1-6 have shown anomalous degradation during the 5-year mission life and beyond. The departure from predicted performance consists of an extra 2.5% per year degradation in excess of the radiation model estimates. Examination of optical solar reflector (OSR) data from a variety of spacecraft reveals variations in OSR degradation rates which correlate with the vehicle design. These data support the idea that contaminants outgassing from the vehicle are photodeposited on the optical surfaces, leading to degradation of their reflectivity. Contamination data taken from an OSR flown on Navstar 5 are used to predict the solar cell array degradation. The predicted effect of contamination on the array output is consistent with the observed behavior of the five Block I vehicles.