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The NASA Earth Observing Systems’ (EOS) SOlar Radiation and Climate Experiment (SORCE) mission consists of four instruments aboard a small satellite to measure the total solar irradiance (TSI) and solar spectral irradiance from 1 to 2000 nm. Solar irradiance, being the dominant energy source in the Earth's atmosphere, establishes much of the atmosphere's chemistry and dynamics. The SORCE measurements will therefore provide the requisite understanding of one of the primary climate system variables for the NASA EOS program. The SORCE primary science data product will be the TSI and solar spectral irradiance on a 6 hour cadence for a period of 5 years or more. The SORCE science team will study how much the solar irradiance varies, how the solar variability affects the Earth’s atmosphere, and how the magnetic structures on the Sun change its irradiance. The SORCE instruments are the Total Irradiance Monitor (TIM), the Spectral Irradiance Monitor (SIM), the SOLar STellar Irradiance Comparison Experiment (SOLSTICE), and the XUV Photometer System (XPS). The TIM is an active cavity radiometer similar in design to previous cavity radiometers, such as the VIRGO, ACRIM, and ERBS instruments, but with significant improvements in sensor and electrical design. TIM will provide a measurement of TSI directly traceable to SI units with an absolute accuracy of 0.01% and relative accuracy of 0.001% per year. The SIM is a Fery prism spectrometer with an Electrical Substitution Radiometer (ESR) as the reference detector and Si and InGaAs photodiodes as the working detectors. SIM will measure the solar spectral irradiance from 200 nm to 2000 nm with a spectral resolution varying from 0.5 nm to 34 nm, an absolute accuracy of 0.03%, and a relative accuracy of 0.006% per year. The SOLSTICE is an improved version of the UARS SOLSTICE instrument, both being ultraviolet (UV) grating spectrometers with photomultiplier tube detectors. SOLSTICE will measure the solar spectral irradiance from 115 nm to 320 nm with a spectral resolution of 0.1-0.2 nm, an absolute accuracy of 5%, and a relative accuracy of 0.5% per year. The XPS is a set of soft x-ray (XUV) photometers, consisting of Si photodiodes with thin-film filters to select moderate spectral bands. XPS will measure the solar spectral irradiance in the XUV (1-31 nm) and at Lyman-? (121.6 nm) with bandwidths of about 5 nm, an absolute accuracy of 20%, and a relative accuracy of 4% per year. Orbital Sciences Corporation is providing the SORCE satellite, a version of their GALEX spacecraft bus tailored for the SORCE mission. The SORCE satellite is a 3-axis stabilized satellite for pointing the instruments towards the Sun for the primary solar measurements as well as for pointing towards stars for the SOLSTICE in-flight calibrations. The SORCE spacecraft is scheduled for a launch on a Pegasus XL in July 2002 into an orbit with a 645 km altitude and 40° inclination.
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