The XUV Photometer System (XPS) is one of four solar irradiance measurement experiments that was launched in January 2003 as part of the Solar Radiation and Climate Experiment (SORCE). Studies of the solar XUV radiation began in the 1960s with space-based rocket experiments, but the knowledge of the solar XUV irradiance, both in absolute magnitude and variability, has been questionable due largely to the very limited number of observations. With the launch of Solar & Heliospheric Observatory (SOHO) in 1995, Student Nitric Oxide Explorer (SNOE) in 1998, and Thermosphere-Ionosphere-Mesosphere-Energetics-Dynamics (TIMED) spacecraft in 2001, there is now a continuous data set of the solar XUV irradiance, and advances in the understanding of the solar XUV irradiance have begun. The SORCE XPS, which evolved from earlier versions flown on SNOE and TIMED, continue on these solar XUV irradiance measurements with improvements to accuracy, spectral image, and temporal change.
Woods et al.  describe the SORCE XPS instrument design, calibration, and irradiance algorithms. Woods and Rottman  describe the early results from XPS.