Radiometric calibration and monitoring of NOAA AVHRR visible data
Results and corrections to calibration for the NOAA-7, -8, -9, -10, and -11 AVHRR Channel 1 data, covering the period from July 1983 through December 1989, are presented. Results from the NOAA-7 and NOAA-9 analysis are used as examples to illustrate key points and present detailed results. In the AVHRR monitor procedure, the spatial variability index is calculated first to classify the data as CLEAR or CLOUD. This classification is then used to sort the data to produce three 1D radiance histograms for different surface types. The surface reflectances are retrieved from the radiances, and a set of reflectance filters are applied to sort the data into global surface reflectance maps. An examination of the NOAA-7 results shows that the NOAA-7 radiometer sensitivity actually decreased at a rate of about 0.5-1.0 percent per year. Trends inferred from the point measurements representing independent calibrations at different times, using models, known sites, and coincident aircraft measurements, agree well with the satellite methods.