he Solar Stellar Irradiance Comparison Experiment (SOLSTICE) is one of ten instruments on the Upper Atmosphere Research Satellite (UARS) One of the main UARS science goals is to understand better the ozone chemistry and trends in the upper atmosphere. Ozone abundance is known to change naturally by solar UV variability and dynamics in the polar regions and by man's influence such as CFCs. UARS was launched in September 1991 from the Space Shuttle Discovery during the STS-48 mission.

        The primary scientific objective for the SOLSTICE program is to make precise and accurate measurements of the solar ultraviolet (UV) spectral irradiance over the spectral range 119 to 420 nm. Moreover, it has the goal of measuring solar variability over arbitrarily long periods, for example, over the duration of the UARS mission that may exceed ten years. The requirement for absolute accuracy is on the order of ± 10% (2 value), but the requirement for absolute accuracy between any two measurements spaced throughout the UARS mission is ± 2% (2 value). To achieve these goals the instrument response is determined from both preflight calibrations and from in-flight calibration and validation programs. SOLSTICE has been designed with the unique capability of monitoring a number of bright blue stars (those with O and B spectral type) using the same optical elements and detectors employed for the solar observations. These stars, which vary by only small fractions of a percent over long time periods, provide a stable reference for deriving the SOLSTICE instrumental degradation rates.

        The primary science requirement for SOLSTICE is to provide one full solar spectrum per calendar day, and to achieve this, the data processing algorithm combines typically 15 individual observations to form the single daily spectrum, adjusted to 1 Astronomical Unit (AU: mean Sun-Earth distance). This daily SOLSTICE spectrum, called the Level 3BS product is reported for each 1.0 nm interval (centered on the half nm) between 119 to 420 nm and is available from the NASA Goddard data center.

        A second instrument, the Solar Ultraviolet Irradiance Monitor (SUSIM), is also aboard UARS measuring the solar UV irradiance with basically the same spectral coverage and resolution as SOLSTICE. However, SOLSTICE and SUSIM have quite difference optical designs and, moreover, employ dramatically different in-flight calibration techniques. The UARS solar instruments are mounted on the Solar Stellar Pointing Platform (SSPP) that provides tracking of the Sun and the stars from the UARS satellite.