Lowest order correction for solar zenith angle to Global Vegetation Index (GVI) data

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Abstract
NOAA produces vegetation indices as part of a project to develop the uses of meteorological satellite data for global agricultural monitoring (Henderson-Sellers et al. 1986). However, no consideration is given to the variability of vegetation indices with the solar zenith angle. This paper focuses on this particular issue. A brief summary of an inversion technique is presented in which raw values of the normalized difference vegetation indices (NDVIs) for a variety of surface-cover types are simulated as a function of solar zenith angle. A relationship between a change in NDVI and solar zenith angle is presented. This relation is used to correct global vegetation index (GVI) data. The results show that for NOAA-7 and NOAA-9 data there is little correction in the neighbourhood of the equator (± 10") but the amount of correction increases with increasing latitude. Such corrections are also shown to be important in data comparison and integration. For example, in comparing the NOAA-6- and NOAA-8-derived NDVI with that derived from NOAA-7 and NOAA-9 for a given date and location the solar zenith angle correction is important.