One of two on-board external solar calibrators on Landsat 7 ETM+. The FASC consists of a nearly white panel, which is extended in front of the full aperture of the ETM+ as it ascends over the North Pole from the night side so the Sun's irradiance can be reflected into the aperture. FASC measurements are typically made once a month. The gain or responsivity of the detectors can be calculated from factors that include, known solar spectral irradiance, measured solar zenith and azimuth angles relative to the solar panel, known distance from the Sun, and pre-launch measurements of the nearly diffuse bidirectional reflectance factor of the solar panel. Also see PASC and IC.