The radiometrical comparison of one sensor to another sensor on different satellites. Imagers are often considered to be cross-calibrated IF they are calibrated to a common Source such as an integrating sphere before launch. However, testing for changes with time requires on-orbit cross-calibration being performed by looking at the SAME target at as close to the same time as possible to avoid possible changes in the target scene, especially atmospheric changes. The best target for this is the Moon because it has no atmosphere and is spectrally flat. The common assumption in cross-calibrations is that the spectral band-passes of the two sensors are identical or that there are no spectral features in the target, including the atmosphere. Even with sensors designed to have nearly identical band-passes, differences in relative spectral response (RSR) curves can result in uncorrectable radiometric differences of 5-15% due to real spectral differences in the target viewed by the two sensors. Also see relative spectral response and spectral striping. (Source: Dr. John Barker)