"Land Surface" Level 2 algorithms
The methodology of the first "Land Surfaces" processing of the POLDER data has been described in Leroy et al (1997). The advanced algorithm prepared for POLDER 2 is presented below (see also the flowchart of the Processing Line).
In addition to correction of the Rayleigh diffusion, two main improvements have then been brought to the atmospheric scattering corrections module (complete description in PDF format):
1.We no longer use the lambertian hypothesis for the final inversion of surface reflectances:
instead we take into account the surface anisotropy by choosing between three a priori BRDF form prototypes (one standing for developped vegetation, one quasi-isotropic standing for desert and snow and one last for intermediate vegetation).
2. Our second aim is to perform some aerosol correction, but without taking the risk of over correcting the surface reflectances. As a consequence, the correction will be significant only for large aerosol load. For this purpose, we use the POLDER tropospheric aerosols characterization, based on a fixed aerosol model that displays a large polarized phase function. Because the inversion is based on the polarized reflectance, this model tends to generate an optical thickness estimate that is in the low range of possible values.
Another major interest of the new processing line is the addition, in the product, of the surface reflectance at 565 nm (green channel).
Level 2 Product
The level 2 (orbital swath) product contains the directional surface reflectances at 443, 565 nm, 670, 765 and 865 nm, the polarized reflectance at 865 nm and the viewing geometries (for each of the 14 viewing directions). It also contains non-directional parameters (sun geometry). See list of level 2 parameters to get a complete description.
Development of these algorithms results from a joint effort of Laboratoire des Sciences du Climat et de l'Environnement (LSCE), POSTEL at Medias France and Laboratoire d'Optique Atmosphérique (LOA). It has been supported by CNES (Centre National d'Etudes Spatiales).
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