Sun-sensor geometry effects on vegetation index anomalies in the Amazon rainforest

Abstract

The capacity of orbital sensors to monitor vegetation functioning and phenology in the Amazon rainforest using vegetation indices (VIs) has been broadly challenged in recent years. In particular, recent studies indicate that artifacts associated with sun-sensor geometry are likely to have major influence on the variability of some VIs. Nevertheless, the magnitude of this influence and the impacts on different VIs are still poorly understood. This study evaluates the scaling and magnitude of the phase angle variation effects on six different VIs, as well as in the individual spectral bands used for computing these indices. Our results show a significant and consistent relationship between phase angle and VI anomalies. The scaling (i.e. the change rate of VI anomaly with phase angle) of this relationship varies according to spectral bands and VIs. Median anomalies in individual Moderate Resolution Imaging Spectroradiometer (MODIS) bands showed a variation of up to -0.045 degree-1, while median VI anomaly variations reached -0.047 degree-1. The scaling of all relationships was shown to be constant throughout the year. Average phase angle values in the Amazon basin ranged from approximately 25 degrees in October to 41 degrees in June. Such variation can, on average, cause changes of up to 0.75 in some VIs, therefore having major impacts on the interpretation of the relationships between VIs and forest phenology.