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An in-flight visible calibration system, VISCAL, is used to calibrate the visible/near infra-red channels of ATSR-2. The in-flight monitoring of the VISCAL is described and results presented. Data shows that the visible channels ate affected by condensation effects as well as some long term. degradation. The accuracy of the measured ground reflectances is dependent, on the long term stability of the calibration. An area within the Libyan Desert has been used to determine calibration drift rates of 0.5% (1.6 mu m), 1.8% (0.87 mu m), 0.8% (0.66 mu m) and 1.7% (0.56 mu m) per year.