621
Figure 1 : Polarised reflectance measured at the
surface in the principal plane over bare soil
(Fig. la), and over vegetation (Fig. 2b). The
lines indicate the results of the models
defined by eq. (2) and (3).
Pp(e,. 6 V , cp) =
My) < 2 >
4(cos(0j) + cos(0 v ))
where y is the incidence angle (equal to half
the phase angle), and Fp is the polarised
fraction of the specular reflectance as given by
Fresnel laws. The model is valid for covering
canopies (i.e. large Leaf Area Indices).
We developed a different model for bare
ground surfaces since those have a structure
different than vegetation: There is no
attenuation on the incident and outgoing path
as there is for the vegetation. The model
assumes that the ground is composed of
isotropically distributed facets (rough surface).
One representation for such a distribution is
obtained considering the surface to be entirely
covered with hemispheres of varying radii.
The model writes (Br6on et al. 1994):
Fi gure 2 : Same as Fig. 1 but for the
perpendicular plane.
p P (0 J ,0 v ,(p v )
My)
4 cos Q s cos 0 V
(3)
This formulation is clearly not satisfying for
limb viewing or illumination as it diverges for
those angles. This results from our
approximation of neglecting mutual
shadowing of the facets. For smaller zenith
angles, however, it agrees with the
measurements as shown below.
4 RESULTS AND DISCUSSION
4.a Polarised reflectance at surface level
Fig. 1 show typical angular signatures of the
polarised reflectance in the principal plane for
bare soil (Fig. la) and vegetation (Fig. lb). We
recall that the reflectances are positive (resp.
negative) when the polarisation is
perpendicular (resp. parallel) to the plane of
scattering. On each diagram, the solid line
shows the model results and the diamonds
indicate the measurements. We use one of the
