After about three years on orbit (altitude « 20,000 km), it became apparent that the solar array output from the first block of Navstar Global Positioning System (GPS) satellites was degrading faster than could be accounted for on the basis of radiation damage alone. There are at least two possible explanations. Either the Van Alien radiation environment was causing unexpected damage, or contamination, such as would result from outgassing by the spacecraft itself, was partially obscuring the solar panels. An extensive analysis is presented of the outgassing properties of the materials used on the GPS Block I vehicles, their masses, temperatures, locations, and possible outgassing paths. It is shown that if a small fraction of the matter impinging upon the solar panels is subject to a photochemical reaction initiated by the solar uv and sticks, a sufficient amount of matter will remain on the panels to account for the unexplained degradation. The calculation is repeated for the GPS Block II vehicles now being launched, and estimates for the expected lifetimes of these vehicles are presented.
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