745 
where c and d are positive constants, s is the angular distance from the sun to a point in the sky hemisphere 
(0,<|>), and 0y > s the angle from zenith. Zq is the solar zenith angle. The parameters AO, Al, and A2 are functions 
of the atmospheric conditions and control the isotropic, or mean, shape of the diffuse sky distribution, the 
horizon or limh brightening, and the shape of the solar aureole, respectively. Figure 5 illustrates the output from 
this model for a similar solar zenith and azimuth angle. The main differences between figures 4 and 5 are due to 
the scaling of the maximum values within the particular graphics package used. Further work is required to 
assess the wavelength dependency of the three parameters and to relate them to a physical measure of 
atmospheric loading such as optical depth. 
lOO' 
Figure 5. Sky radiance anisotropy modelled using three parameters to control the functional form of the 
distribution for a similar solar zenith and azimuth angle to figure 4. 
3.2 Surface Bidirectional Reflectance Distribution Function (BRDF) 
The BRDF of terrestrial surfaces is important in the derivation of surface albedo from remotely sensed data since 
surface reflectance from nadir viewing sensors generally provide a poor measure of the albedo. The directional 
reflectance of soils and vegetation canopies can be highly asymmetric in the forward and backscattering 
directions and it is essential to measure and model these directional effects before interpreting remotely sensed 
data from nadir and mult-look direction sensors. Figure 6 illustrates the asymmetry in directional reflectance 
from an area of lateritie soils characteristic of the Tiger-Bush sub-site. This three dimensional visualisation is 
one of 252 such images for this one target alone which contain data on the spectral variation of the directional 
reflectances. A number of visualisation tools are still to be developed before the full information content of the 
data can be extracted. 
The BRDFs of all the characteristic cover-types will be used in aggregating the directional reflectances over 
varying spatial scales from airborne and satellite remotely sensed data to develop a model for the surface 
radiation budget at sub-site, super-site and GCM scales.