The “radiation” biogeophysical products of Postel are spatialized variables derived from optical or micro-wave sensors measurements acquired over many years at regional to global scales.
Land Surface Albedo
The albedo is the fraction of the incoming solar radiation reflected by the land surface, integrated over the whole viewing directions. The albedo can be directional (calculated for a given sun zenith angle, also called “black-sky albedo”) or hemispheric (integrated over all illumination directions, also called “white-sky albedo”), spectral (for each narrow band of the sensor) or broadband (integrated over the solar spectrum). The surface albedos are derived from many sensors (Vegetation, Polder, Meteosat) in the frame of different projects, namely Geoland and Amma.
Bidirectional Reflectance Distribution Function (FDRB)
The Bidirectional Reflectance Distribution Function (FDRB) describes how terrestrial surfaces reflect the sun radiation. Its potential has been demonstrated for several applications in land surface studies (see Bicheron and Leroy, 2000).
The BRDF has been measured in the field (e.g. Kimes, 1983 ; Deering et al., 1992) or from airborne instruments (Irons et al., 1991 ; Leroy and Bréon, 1996), with most often an adequate sampling of directional space but with a poor spatial coverage. Directional effects on land surfaces have been seen from space with AVHRR (e.g. Gutman, 1987) or with ATSR (Godsalve, 1995). Then, the spatial coverage is potentially adequate, but the sampling of the BRDF is limited in the angular plane of acquisition.
The space-borne POLDER-1/ADEOS-1 instrument (November 1996 – June 1997) has provided the first opportunity to sample the BRDF of every point on Earth for viewing angles up to 60°-70°, and for the full azimuth range, at a spatial resolution of about 6km, when the atmospheric conditions are favorable (Hautecoeur et Leroy, 1998). From April to October 2003, the land surface BRDF was sampled by the POLDER-2/ADEOS-2 sensor. From March 2005, the POLDER-3 sensor onboard the PARASOL microsatellite measures the bi-driectional reflectance of the continental ecosystems.
These successive observations allowed building :
- a BRDF database from the 8 months of POLDER-1 mesurements
- a BRDF database from the 7 months of POLDER-2 measurements
- 4 BRDF databases from one year of POLDER-3 measurements
Downwelling Longwave Radiation (DLR)
The Downwelling Longwave Radiation (W.m-2) (DLR) is defined as the thermal irradiance reaching the surface in the thermal infrared spectrum (4 – 100 µm). It is determined by the radiation that originates from a shallow layer close to the surface, about one third being emitted by the lowest 10 meters and 80% by the 500-meter layer.The DLR is derived from several sensors (Meteosat, MSG) using various approaches, in the framework of the Geoland project.
Bicheron, P, and M. Leroy, Bidirectional reflectance distribution function signatures of major biomes observed from space, Journal of Geophysical Research, 105 , 26,669-26,681, 2000.
Deering, D.W., T.F. Eck, and T. Grier, Shinnery oak bidirectional reflectances properties and canopy model inversion, IEEE Transaction on Geoscience and Remote Sensing, 30, 339-348, 1992.
Goldsave, C., Bidirectional reflectance sampling by ATSR-2 : a combined orbit and scan model, International Journal of Remote Sensing, 16, 269-300, 1995.
Gutman, G.G., The derivation of vegetation indices from AVHRR data, International Journal of Remote Sensing, 10, 107-132, 1989.
Hautecoeur, O., and M. Leroy, Surface bidirectional reflectance distribution function observed at blobal scale by POLDER/ADEOS, Geophysical Research Letter, 25, n°22 , 4197-4200, 1998.
Irons, J.R., K.J. Ranson, D.L. Williams, R.R. Irish, and F.G. Huegel, An off-nadir pointing imaging spectroradiometer for terrestrial ecosystem studies, IEEE Transaction on Geoscience and Remote Sensing, 29, 66-74, 1991.
Kimes, D.S., Dynamics of directional reflectance factor distributions for vegetation canopies, Applied Optics, 22, 1364-1373, 1983.
Leroy, M., and F.M. Bréon, Surface reflectance angular signatures from airborne POLDER data, Remote Sensing of Environment, 57, 97-107, 1996.