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The only indication of sensor degradation affecting the shortwave channels of the AVHRR is the changing response of targets which are assumed to be invariant in time. Using daily data of the whole lifetime of NOAA 11 three different long term trends in the signal response could be identified and quantitatively separated: real sensor degradation, atmospheric influence caused by the changing local time of observation, and surface anisotropy by increasing solar zenith distance. Together they cause the decrease of the measured signal and should be called effective signal degradation. If atmospheric corrections are applied to get surface reflectances it is important to avoid duplicate corrections that would lead to long term trends caused by changing atmospheric attenuation. The degradation coefficients were calculated by postulating a constant NDVI of the desert area. Corrections for surface ansisotropy to the time series of the reflectances were applied. Thus a precise quantitative inter-calibration of the NOAA 14 to the NOAA 11 time series could be performed. The resulting degradation coefficients can be updated and applied in real time. (C) 1997 COSPAR. Published by Elsevier Science Ltd.