EFFECTS OF ORBITAL DRIFT ON ADVANCED VERY HIGH-RESOLUTION RADIOMETER PRODUCTS - NORMALIZED DIFFERENCE VEGETATION INDEX AND SEA-SURFACE TEMPERATURE

Although orbits of the NOAA TIROS-N satellites are designed to be sun- synchronous, epheremis data shows that the afternoon, ascending node s atellites currently cross the equator hours later than they did upon l aunch. This delay results in different illumination conditions for mea surements made by the Advanced Very High Resolution Radiometer (AVHRR) . The effects of illumination on two standard AVHRR products-normalize d difference vegetation index (NDVI) and sea surface temperature (SST) -are modeled here. Combining orbital data with model results, the effe cts of the NOAA-II orbital drift on NDVI are quantitatively assessed f or three earth targets: an equatorial Africa site (0 degrees N), the F irst ISLSCP Field Experiment (FIFE) site (39 degrees N), and the Borea l Ecosystem-Atmosphere Study (BOREAS) site (55 degrees N). Top-of-atmo sphere NDVI corrections for solar zenith angle are developed for a den se, deciduous forest. Orbital drift effects on SST are given for an eq uatorial site. Although results vary with season, latitude, atmosphere and time since launch, NDVI differences of up to 0.23 and SST differe nces of up to 0.5 K may occur due strictly to orbital drift.