The Multispectral Scanner (MSS) was launched on the Earth Resources Technology Satellite (ERTS-1) 23 July 1972. The MSS has two calibration systems, one internal and one external. Both calibration systems have shown strong, spectrally dependent performance degradation since launch. This paper presents details on the optical system of the MSS and data on the performance degradation as a function of both spectral interval and time in orbit. The history of the MSS during tests is traced, and it is shown that hydrocarbons from an external source may have been deposited on optical surfaces in the instrument. It is postulated that these contaminant coatings may have polymerized as a result of the exposure to uv light from the sun, increasing their blue absorbtion and accounting for the observed performance degradation. Arguments supporting this postulate are presented, and other possible sources of the performance degradation are discussed.
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