"Land Surface" Level 3 algorithms
Download a PDF "Level 3 User Manual - Algorithm Description & Product Validation".
Monitoring of terrestrial vegetation from satellites at global and regional scales requires accurate and frequent measurements of surface reflectance. In this context, the POLDER instrument leads a key improvement providing, at high temporal resolution, measurements of the Bi-directional Reflectance Distribution Function (BRDF) corrected for atmospheric effects.
The first algorithms of the "Land Surface" processing line have been applied to the 8 months of ADEOS-1/POLDER-1 data. The methodology, which takes advantage of the POLDER directionality, is presented in Leroy et al. (1997). The Level 3 products are generated over a synthesis period of 30 days with a sliding window to get a temporal resolution of 10 days. Advanced algorithms have been developed, and tested on ADEOS-1/POLDER-1 acquisitions, to be applied to ADEOS-2/POLDER-2 data. In addition to the Leaf Area Index (LAI) retrieval, the major improvements are:
The inversion of a new 3-coefficients semi-empirical BRDF model (Maignan et al., 2003) to normalize the bi-directional measurements
The application of temporal weighting favoring the data collected in the middle of the synthesis period. Thus, the smoothing of the temporal variations of the biophysical parameters due to the monthly synthesis decreases.
An error is associated to each parameter. This value depends on the noise on the input data (i.e. the measured bi-directional spectral reflectances) and on the retrieval algorithm.
The "Land Surface" Level 3 processing line will generate 2 products from ADEOS-2/POLDER-2 data:
The "Albedo and Vegetation" product contains the spectral Directional-Hemispherical Reflectances, DHR, and the Normalized Difference Vegetation Index (NDVI) corrected for directional effects, already available in ADEOS-1/POLDER-1 products. Two new biophysical parameters are added: the Leaf Area Index (LAI) and the Fraction of Vegetation Cover (FVC).
The "Land Surface" Level 3 algorithm relies on 3 major steps (see diagram):
the linear inversion of a BRDF model
the inversion of a vegetation model by a neural network to retrieve the LAI.
A more complete description of the ADEOS-2/POLDER-2 "Land Surface" Level 3 advanced algorithms will be soon available for downloading.
Development of the "Land Surface" Level 3 algorithms results from a collaboration between Noveltis, the Laboratoire des Sciences du Climat et de l'Environnement (LSCE) and the Centre d'Etudes Spatiales de la BIOsphère (CESBIO). It has been supported by the Centre National d'Etudes Spatiales (CNES). The validation of the ADEOS-2/POLDER-2 products has been carried out by POSTEL à Medias France.
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