"LAND SURFACE" LEVEL3 ALGORITHM:
The filtering module
The bi-directional spectral reflectances are the inputs of the "Land Surface" Level 3 processing line. Their quality controls the relevance of biophysical parameters. In order to complete the Level 2 cloud masking and to eliminate the disturbed data, a multi-temporal filtering module has been implemented in the ADEOS-2/POLDER-2 advanced algorithm. It determines the type of surface (ground, snow or mixed), identifies the temporal inconstancies of the measured reflectances over the synthesis period of 30 days, and filters the observations contaminated by residual clouds and/or aerosols. This latter point is based on the fitting of the directional model of Walthall (1985) on the reflectances acquired at 443nm under angular configurations close to the perpendicular plane.
The filtering module has been tested on ADEOS-1/POLDER-1 data. The inconsistent spatial variability of biophysical parameters over cloudy areas is clearly reduced. Figure 1 shows this improvement on the Directional Hemispherical Reflectance over the Central Africa. The patchy features over the equatorial forest on no-filtered POLDER-1 product (map at the top) is clearly smoothed and the area appears more homogeneous on the filtered POLDER-1 product (map at the bottom). The decrease of the Directional Hemispherical Reflectance at 670nm over the Southwest of the area is due to the filtering of the bi-directional reflectances contaminated by aerosols of biomass burning.
The same improvement is obtained on the NDVI. Figure 2 displays maps over the Amazonian forest. The spots of low values on the no-filtered POLDER-1 NDVI (map at the top) disappeared with an increase of the NDVI (map at the bottom). The physical characteristics of the surface, such as rivers, are enhanced. The mapping is more accurate and the patterns can reasonably be linked to true surface structures.
Walthall, C. L., J. M. Norman, J. M. Welles, G. Campbell and B. L. Blad, Simple equation to approximate the bidirectional reflectance from vegetative canopies and bare soil surfaces, Applied Optics, 24, 383-387, 1985.
© CNES Copyrights 2002