The empirical Calibration of Satellite sensors by measurement of upwelling Radiation from targets that are viewed on the Ground or from aircraft at the SAME time as from space. The at-satellite radiance is calculated by using radiative transfer models of the atmosphere along with measurements of the atmospheric Transmittance or Optical Thickness for the spectral band-pass being calibrated. Also see ground truth/ground observations, radiometric reflective band calibration, and reflectance-based calibration. (Source: Dr. John Barker)