October 18, 2013



Validation Plan and Results

The "Land Surface" Level 3 advanced algorithm processes the bi-directional ADEOS-2/POLDER-2 reflectances over a synthesis period of 30 days with a sliding window to get a temporal resolution of 10 days. It generates 2 products:

    the "Directional Parameters" product that contains the 3 directional coefficients resulting from the inversion of a BRDF model.
    the "Albedo and Vegetation" product that contains the spectral Directional Hemispherical Reflectances (DHR), the NDVI corrected from the directional effects, the LAI and the Fraction of Vegetation Cover (FVC).

    These products represent the properties of the continental ecosystems. They are essential for weather forecast and climate modeling, water and carbon cycles studies, whose results are partly controlled by the relevance of the input biophysical parameters. Then, they must be validated, first to estimate their accuracy for the user community, and also to provide feedback so that retrieval algorithm can be improved.

    The validation procedure has been adapted to each product. Indeed, some of them are structural vegetation properties (LAI, FVC), others depend on the spectral characteristics of the sensor (DHR, NDVI). In the best case, the validation procedure has relied on the following steps:

      Verification of the realism of the products:
        Analyze of the spatial and temporal distribution of pixels for which parameter values are out of physical range.
        Analyze of the spatial variability, i.e. the representation of the gradients at the continental scale, and temporal evolution, basically over an annual cycle.
        Comparison with ADEOS-1/POLDER-1 products, when existing, to estimate the improvements brought by the new modules implemented in the advanced ADEOS-2/POLDER-2 algorithms.
        Comparison with available equivalent products derived from other sensors, such as SPOT-5/VEGETATION-2, ENVISAT/MERIS, NOAA/AVHRR, TERRA/MODIS, by similar or different approaches. Note that the 6 km resolution of POLDER limits the relevance of comparisons with kilometric or sub-kilometric products.
        Comparison with in-situ measurements:
          Data collected during international campaigns or regional studies, and available in literature or various databases. Such measurements are usually local in time and space, what limits seriously the significance of the comparisons. However, the large number of points allows a first estimation of the accuracy of the satellite product. A catalogue compiled by CNRM (Centre National de Recherches Météorologiques) gathers some hundreds of such LAI ground measurements.
          Spatially-distributed reference data sets. At the present time, only LAI maps exits and are available, produced in the frame of the VALERI (VAlidation of Land European Remote sensing instrument) project. They are elaborated from ground-based measurements and high resolution satellite images. The validation activities of the MODLand group have also been implemented to generate such reference products to validate biophysical parameters derived from TERRA/MODIS data.

          The validation of ADEOS-2/POLDER-2 "Land Surface" level 3 products has been achieved by POSTEL at Medias-France. The results are detailed in the "Level 3 User Manual - Algorithm Description & Product Validation" .

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