The National Oceanic and Atmospheric Administration (NOAA) has been observing the earth and its atmosphere in the visible spectrum for well over three decades, using instruments on Geostationary Operational Environmental Satellite (GOES) and Polar-orbiting Operational Environmental Satellite (POES). These instruments have no onboard calibration for visible channels. These channels must be calibrated vicariously.

Calibration is the process of quantifying the instrument response to know signals. For onboard calibration, these signals are provided by an onboard device such as a solar diffuser. For vicarious calibration, these signals are substituted with those from certain targets on earth. Knowledge of calibration signals directly affects the accuracy of calibration. This knowledge is far from being complete, leading to bias of up to 10%. The homogeneity and spectral characteristics of targets must also be considered to minimize calibration uncertainty.