The first Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) instrument flew aboard NASA's Upper Atmosphere Research Satellite (UARS) and was an ultraviolet spectrometer that measured the Sun’s ultraviolet (UV) energy in three channels. Scientists calibrated the instrument by looking at stars during the nighttime portion of the UARS orbit. During the daylight portion of the orbit, SOLSTICE measured the solar spectral irradiance of the total solar disk in UV wavelengths from 115 to 430 nanometers.

A follow-on SOLSTICE instrument was launched as part of the Solar Radiation and Climate Experiment (SORCE) in 2003. SORCE SOLSTICE makes daily solar ultraviolet (115-320 nm) irradiance measurements and compares them to the irradiance from an ensemble of 18 stable early-type stars. This approach provides an accurate monitor of instrument in-flight performance and provides a basis for solar-stellar irradiance comparison for future generations.