Effects of orbital drift on advanced very high resolution radiometer products: Normalized difference vegetation index and sea surface temperature

Abstract Although orbits of the NOAA TIROS-N satellites are designed to be sun-synchronous, epheremis data shows that the afternoon, ascending node satellites currently cross the equator hours later than they did upon launch. This delay results in different illumination conditions for measurements made by the Advanced Very High Resolution Radiometer (AVHRR). The effects of illumination on two standard AVHRR products—normalized difference vegetation index (NDVI) and sea surface temperature (SST)—are modeled here. Combining orbital data with model results, the effects of the NOAA-11 orbital drift on NDVI are quantitatively assessed for three earth targets: an equatorial Africa site (0° N), the First ISLSCP Field Experiment (FIFE) site (39° N), and the Boreal Ecosystem-Atmosphere Study (BOREAS) site (55° N). Top-of-atmosphere NDVI corrections for solar zenith angle are developed for a dense, deciduous forest. Orbital drift effects on SST are given for an equatorial site. Although results vary with season, latitude, atmosphere and time since launch, NDVI differences of up to 0.23 and SST differences of up to 0.5 K may occur due strictly to orbital drift.