Only a portion of the Sun's radiation penetrates the Earth's atmosphere to its surface; and at some wavelengths, the radiation is absorbed entirely. It is therefore not possible to use ground-based observations of the Sun to estimate the top-of-the-atmosphere (TOA) radiation. Although there was an ambitious ground observing program during most of the past century, it provided only ambiguous estimates of irradiance, and little or no information on whether the Sun varied. Therefore, space-based observations are required.
The SOlar Radiation and Climate Experiment (SORCE) continues the precise measurements of Total Solar Irradiance (TSI) that began with the ERB instrument in 1979 and that has continued to the present with a number of NASA, NOAA, and European programs – most recently ACRIMSAT, launched in 1999. SORCE is also providing measurements of the Solar Spectral Irradiance (SSI) from 1 nm to 2000 nm (excluding 31-120 nm, which is not covered by the SORCE instruments) on a uniform wavelength scale, which will vary from 1-33 nm over the entire spectral interval, accounting for 95% of the spectral contribution to TSI. SORCE carries four types of instruments, including the Total Irradiance Monitor (TIM), the Spectral Irradiance Monitors (SIM), two Solar Stellar Irradiance Comparison Experiments (SOLSTICE), and the XUV Photometer System (XPS).