In-orbit radiometric Calibration of a Satellite sensor by a method independent of that used to perform the initial laboratory calibration. Two methods are commonly used when making vicarious measurements of Ground truth: the radiance-based method in which Data from the sensor are compared with radiance measured by a sensor mounted on an aircraft, and the reflectance-based method in which a radiative transfer model is used to estimate the top-of-atmosphere radiance from a ground target of known reflectance. Also see ground truth/ground observations, radiance-based calibration and reflectance-based calibration. (Source: Dr. John Barker)