A number of other, 'secondary', experiments and sensors were also located in and around Columbia's payload bay and middeck. One of these was the Shuttle Solar Backscatter Ultraviolet (SSBUV) instrument, designed to help to calibrate atmospheric ozone measurements being conducted by the Earth-orbiting Total Ozone Mapping Spectrometer (TOMS) and Upper Atmosphere Research Satellite (UARS). It had flown on several previous Shuttle missions as a calibration tool and during STS-62 was used to detect tropospheric sulphur dioxide emissions from several Central American volcanoes, including Colima in Mexico. It also observed sulphur dioxide from industrial byproducts in the troposphere above China and Japan.
The instrument's value lay in its ability to provide very precisely calibrated ozone measurements; it was verified to laboratory standards before launch and checked during and after each mission. This provided a highly precise 'standard' against which data from other Earth-resources satellites - some of which had spent years aloft and whose accuracy may have degraded - could be compared. Specifically, it enabled researchers to update the calibration of another ultraviolet instrument on the National Oceanic and Atmospheric Administration's NOAA-11 satellite, which had been in orbit since 1988. This also allowed calibrations with even earlier TOMS data going back to 1978.
In combination, this provided a highly prized 16-year 'data set' of differing ozone levels in the Earth's atmosphere, which in turn could better inform scientists' theories of climatic changes. Among other disturbing findings, SSBUV data has helped to confirm a 10% ozone depletion in the northern hemisphere at mid-latitudes, resulting from the combined effects of residual aerosols distributed in the upper atmosphere following the eruption of Mount Pinatubo in the Philippines in 1991 and cold stratospheric temperatures during the winter of 1992-1993. Simultaneous observations of the physics and chemistry of the atmosphere by UARS helped to build a more complete picture.
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