The Ground Calibration Unit (GCU) is described. The GCU uses a solar-i
lluminated diffuser as calibration source. The absolute radiance of th
e diffuser is calculated from its absolute reflectivity and the incomi
ng solar irradiance. The GCU was used to calibrate the on-board lamp s
ources of the flight model 1 of the Scanner for Radiation Budget (ScaR
aB) instrument in February 1993.