SSBUV sensitivity drift determined using solar spectral irradiance measurements

The Shuttle Solar Backscatter Ultraviolet (SSBUV) experiment flew eight Space Shuttle missions between 1989 and 1996, in order to maintain and validate the calibrations of a number of satellite-borne ozone-monitoring instruments. The SSBUV sensitivity drifted during each mission. Reversible drifts resulted from the outgassing of the optics, while non-reversible drifts were the consequence of changes induced by solar UV radiation. These changes varied from mission to mission, but were typically less than 1% near 400 nm, increasing to 5% to 10% near 200 nm. In addition to using traditional calibration data, the SSBUV solar irradiance data were used to assess radiometric sensitivity changes. In the near- and mid-UV, short-term solar irradiance variations can be accurately modelled using the Mg II index. Therefore, during the course of each relatively brief Shuttle flight, the Mg II index-corrected solar irradiance provides a "constant candle" and any changes in the observed solar irradiance result from sensitivity drift. Using the Sun as a source, SSBUV sensitivity drifts were modelled via an iterative approach as the product of linear functions of outgassing time and accumulated solar exposure time. The resulting time-dependent corrections were validated using the conventional calibration data and used in conjunction with the absolute radiometric sensitivity determined in the laboratory to process the solar irradiance data. It is shown that the application of this technique reduces the uncertainty in the SSBUV solar measurements associated with radiometric sensitivity drifts to approximately 1% at 200 nm.