Solar zenith angle correction of global NDVI time-series from AVHRR

Previous studies found that global NDVI time-series products from many different producers have the same kind of systematic errors, including the increase of NDVI values with the aging of a NOAA POES satellite and a drop of NDVI values when a new satellite replaced the old one. By examining the NOAA/NASA pathfinder AVHRR Land product, this study found that the increase of solar zenith angles due to the orbital drifting of the NOAA satellites is the main cause of the systematic errors. It is also found that statistically NDVI values increase linearly with the increase of solar-zenith angles, and this relationship is basically the same for NOAA-7, NOAA-9, and NOAA-11. By using the regression method, we obtained the rate of increase of NDVI values with the increase of solar-zenith angle. The rate is 0.1571 NDVI units per radian. The rate was applied back to the global annual maximum NDVI composites from 1982 to 1994 derived from NOAA/NASA Pathfinder AVHRR Land product to compensate the increase of solar-zenith angles. Results showed that most of the increase in NDVI values related to the aging of NOAA satellites was removed