Full text: Mesures physiques et signatures en télédétection

A more direct and general use of the two-angles data will be in estimation of the surface albedo. 
Albedo estimates from satellite data are required to monitor environmental change, but currently suffer from 
limitations in the accuracy of data, when calculated on the basis of the nadir reflectances only. ATSR-2 data 
will give the opportunity to refine the weighting factors applied to spectral bands and viewing angles, and 
therefore to improve albedo estimates. 
The soil background is a major surface component controlling the spectral behaviour of vegetation 
canopies, and on which the retrieval of biophysical characteristics of the canopy is depending. Sensitivity of 
different vegetation indices to soil colour effect has been investigated. Although vegetation indices considerably 
reduce these soils effects, satisfactory estimation from the indices of the vegetation characteristics at relatively 
low cover would be possible if at least the general type of soil is known (peat or mineral soil). The Simplified 
Transformed Soil Adjusted Vegetation Index (Simplified TSAVI), presented here, enhanced by a soil map of 
the studied area, appears as the best candidate for such a purpose. This index offers a high degree of precision 
in cover estimates and is attractive because of its simple formulation, requiring minimal knowledge of soil type. 
However, even if the 26 soil samples entail a very large range of spectral values, a question still remains about 
their representativity. The 26 soils were derived from a basic set of five types, with additional variations 
induced by moisture and roughness. Therefore the results of these simulations need to be tested on real data, 
and on a larger range of organic soils in order to generalize the distinction between organic and non-organic 
(mineral) soils. 
The main peculiarity of the ATSR-2 sensor is the two-look angle geometry. Directional effects of the 
soil-insensitive vegetation index can be quite well related to the canopy architecture, as long as the LAI 
saturation is not reached. Differences between the nadir view and the 55 degrees view angle should be large 
enough to distinct a change in a canopy as due to a stress or disease. Experimental data are again needed for 
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Reviews, 7: 65-82. 
Campbell G.S. (1986). Extinction coefficients for radiation in plant canopies calculated using an ellipsoidal 
inclination angle distribution. Agricultural and Forest Meteorology, 36: 317-321. 
devers J.G.P.W., Verhoef W. (1993). LAI estimation by means of the WDVI: a sensitivity analysis with a 
combined PROSPECT-SAIL model. Remote Sens. Environ. 7:43:64. 
Huete A.R. (1988). A Soil-Adjusted Vegetation Index (SAVI). Remote Sens. Environ., 25: 295-309. 
Jacquemoud S., Baret F., Hanocq J.F. (1992). Modelling spectral and bidirectional soil reflectance. Remote 
Sens. Environ., 41: 123-132. 
KuuskA. (1991). The hot-spot effect in plant canopy reflectance, in Photon-vegetation interactions, Application 
in optical remote sensing and plant ecology. Myneni and Ross Eds, Springer, pp 139-159. 
Rondeaux G., Herman M. (1991). Polarization of light reflected by crop canopies. Remote Sens. Environ. 
38(1): 63-75. 
Sellers P.J., Berry J.A., Collatz G.J., Field C.B., Hall F.G. (1992). Canopy Reflectance, photosynthesis and 
transpiration III. A reanalysis using improved leaf models and a new canopy integration scheme. Remote Sens. 
Environ. 42: 187-216. 
Verhoef W. (1984). Light scattering by leaf layers with application to canopy reflectance modelling: the SAIL 
model. Remote Sens. Environ., 16: 125-141.

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