The first Solar-Stellar Irradiance Comparison Experiment (SOLSTICE) flew aboard NASA's Upper Atmosphere Research Satellite (UARS). This spectrometer 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, 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 in 2003 as part of the Solar Radiation and Climate Experiment (SORCE). SORCE SOLSTICE made daily solar ultraviolet (115-320 nm) irradiance measurements and compared them to the irradiance from an ensemble of 18 stable early-type stars. This approach provided an accurate monitor of instrument in-flight performance and a basis for solar-stellar irradiance comparison for future generations.