The Ground Calibration Unit (GCU) is described. The GCU uses a solar-illuminated diffuser as calibration source. The absolute radiance of the diffuser is calculated from its absolute reflectivity and the incoming solar irradiance. The GCU was used to calibrate the on-board lamp sources of the flight model 1 of the Scanner for Radiation Budget (ScaRaB) instrument in February 1993.
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