The Importance of Early Morning Local Overpass Times of BRDF Retrieval, Modeling of Spectral Reflectance and Fapar Estimation

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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 many 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.

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