Normalization of directional effects in 10-day global syntheses derived from VEGETATION/SPOT: I. - investigation of concepts based on simulation

摘要

Since April 1998, the VEGETATION/SPOT-4 sensor has acquired global observations at kilometer scale in four optical spectral bands on a daily basis. Its large field of view results in a strong dependency of surface reflectances on the Sun-target-sensor geometry. Our objective is to define a method to remove this anisotropy during the processing of 10-day syntheses derived from data acquired at every VEGETATION orbit pass. This article investigates the concepts that were developed for AVHRR/NOAA, POLDER/ADEOS, and MODIS/TERRA. The investigation is based on the statistical analysis of 18,000 simulated time series of red and near infrared reflectances, close to the ones acquired on various land classes by the VEGETATION sensor under different geometric, cloudiness, and atmospheric conditions. Conclusions are reached on the successive key stages that are required for the removal of surface reflectance anisotropy during the processing of syntheses. Firstly, we suggest to separate the time window devoted to the retrieval of Bidirectional Reflectance Distribution Functions (BRDFs) from a second one (shorter) that is used for combining the most recent observations. Secondly, we verity the robustness of the Roujean's BRDF model through various angular sampling, land classes, cloudiness, and atmospheric noise. Thirdly, we improve the stability of the formula that normalises reflectances at nadir views. Fourthly, we show that a simple averaging is relevant to combine the observations after normalisation. The comparison with the common Maximum Value Compositing (MVC) method shows that the normalisation of directional effects greatly improves both consistency and accuracy in time series of surface reflectances. Despite the use of two time windows that allows to increase the efficiency of BROF retrieval, anisotropy removal is still not possible in many cloudy regions of the world. The associated Part II article exposes a method that is fully operational on a 10-day basis under high cloudiness conditions.