The field Calibration of Satellite sensors by measurement of reflected solar Radiation from targets of known or measured Reflectance that are viewed on the Ground 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, radiance-based calibration, and radiometric reflective band calibration. (Source: Dr. John Barker)