Normalization of the directional effects in NOAA–AVHRR reflectance measurements for an improved monitoring of vegetation cycles

This paper describes a correction of directional effects in AVHRR reflectance time series. The method relies on a priori directional signatures, in the red and near infrared, for various surface covers. The reflectance normalization is applied to the historical Pathfinder AVHRR Land (PAL) data set acquired between 1981 and 1999. The high frequency variability in the reflectance time series, which is interpreted as a noise due to varying observation geometry and directional effects, is reduced by a factor greater than 1.7 for most vegetated surfaces. For the analysis of the vegetation annual cycle and its inter-annual variations, we recommend the use of the Difference Vegetation Index (DVI), following a prior correction of the directional effects, instead of the commonly used NDVI. Indeed, DVI appears more robust than NDVI to noise resulting from atmospheric perturbations, which makes it a good candidate for long-term ecological surveys and climate change studies.