Preflight calibration anomaly in the NOAA 14 AVHRR Channels 1 and 2

Preflight calibration data for the visible and near-infrared channels of the NOAA 14 AVHRR suggests a kink in the response function at a reflectance of ∼ 6% in Channel 1 and ∼ 12% in Channel 2. Analysis of the ratio of the instrumentʹs response in the two channels shows that this is an artifact of the preflight calibration performed in 1993, and not a genuine response anomaly. This effect may be explained by the entry of stray light from the integrating sphere into the instrument during the period when the space clamp target is viewed. The preflight operational calibration equations were based on this flawed data set. Fortunately, there is sufficient information to permit a recalibration. The revision leads to a reduction in reflectance for a given digital count value, by comparison with the published preflight calibration. The revision had significant consequences for low radiance applications such as the NOAA/NESDIS aerosol optical depth product, which showed anomalously high values during the first months of operation of NOAA 14.