Impact of Orbital Drift on Long-Term AVHRR NDVI Time-Series Data

The Normalized Difference Vegetation Index (NDVI) time-series data derived from Advanced Very High Resolution Radiometer (AVHRR) have been extensively used for studying inter-annual dynamics of global and regional vegetation. However there are significant variations in the data as a result of the orbital drift of the satellites through their active life. The research presented in this poster investigates the contribution of surface BRDF (Bidirectional Reflectance Distribution Function) and atmosphere to the observed impacts of orbital drift on AVHRR-NDVI timeseries data and to assess the effectiveness of a new MODIS based semi-empirical BRDF correction technique developed for MODIS reflectance products in removing the impact of orbital drift in AVHRR data. It was found that, for dense tropical forest sites, atmosphere is the dominant source of the inter-annual NDVI trends that have generally been attributed to orbital drift. However, the contribution of surface BRDF to the overall signal increases considerably at tree cover closer to 50%. At both 10% and 95% tree cover, the surface BRDF contributions are again negligible. Investigation into effectiveness of MODIS based BRDF parameters for remediation of the impact of orbital drift on AVHRR NDVI data indicated it to be a viable method for operational implementation.