干旱区生态环境知识资源中心

Land surface bidirectional reflectance distribution function (BRDF) measurements were acquired from November 1996 to June 1997 at global scale and 6 km spatial resolution with the POLDER instrument onboard the ADEOS-1 satellite. We selected 395 BRDF data sets on areas distributed on the 17 biomes of the IGBP 1-km land cover classification (DISCover data set) at 443, 670, and 865 nm, at several periods (November and December 1996, May and June 1997). The selected BRDF data are characterized by a low noise level, a sufficient number of clear days during the month, and a roughly even sampling of directional space. The data show large differences of the directional and spectral signatures of the various land cover classes, both in shape and in magnitude. Except for the desert and ice classes, all signatures present a peak in the backscattering direction, with sometimes an additional strong peak in the specular direction for wetlands. The data permit an assessment of the BRDF temporal evolution due to changes of surface state or Sun elevation, as well as a quantification of the BRDF variability within a land cover class. The maximum error level of the BRDF database is estimated to be of the order of 0.01 and 0.03 (in units of reflectance) in the red and near infrared, respectively. The BRDF database is available to the science community through the Internet. It should be helpful for the prototyping of various science applications, including the test of radiative transfer models and algorithmic schemes of corrections of angular effects on remote sensing data. As an example of application of the database, various semiempirical BRDF models published in the literature are tested and intercompared. Whereas all tested models catch reasonably well the overall shape of the BRDF, some differences appear between the red and the near infrared, between classes, and between models, which the use of the database permits to quantify.