The importance of early morning local overpass times for BRDF retrieval, modeling of spectral reflectance and fAPAR estimation

Blueprints for near-future moderate resolution satellite-borne sensor systems such as the VIIRS (Visible Infrared Imaging Radiometer Suite) sensor on the National Polar-orbiting Operational Environmental Satellite System (NPOESS) do not include plans to ensure that land surface observations are made from an early morning (AM) orbit (07:30-08:30 local time) as well as in the afternoon (PM), and current spaceborne sensors designed to sample the bidirectional reflectance distribution function (BRDF) achieve this by off-nadir viewing with a small range of solar zenith angles (SZA). This may limit the utility of current and future systems in important Earth observation applications such as estimating the fraction of absorbed photosynthetically-active radiation (fAPAR). The lack of observations at 50° < SZA < 70° may only be partly offset by the 09:30 orbit foreseen for EUMETSAT´s METOP satellite, to carry a third generation Advanced Very High Resolution Radiometer (AVHRR). Here existing AVHRR data are used to examine the likely impact of inverting models with observations at a small range of Sun angles.