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Title
Mesures physiques et signatures en télédétection

539
Fig. 3 shows the brightness temperature
(measured at e = 45*) of a layer of
wheat ears as a function of the azimuth
angle The ears are placed close
together in order to form an almost
uniform layer. When the main axis of
ears is in the incidence plane (*=0*), the
V component of the emission is much
higher than the H one, while the reverse
is obtained for * = 90*. A similar
behavior is shown by the layer of stems
(Fig. 4) and leaves (Fig. 5).
All the diagrams confirm the anisotropic
emission of these elements, the
emissivity being much higher when the
main axis of plant constituents is aligned
with the electric field of the antenna. It
should be noted that there is not a full
symmetry between V and H components
because the incidence angle is 45*.
Figure 6 - The brightness temperature (f = 37 GHz, 0
= 45’) at V and H pol. of three layers of wheat leaves
(PWC = 0.5 Kg/m 2 ) as a function of azimuth angle #.
When medium becomes optically thick as
in three layers of leaves, the anisotropy tend to disappear and the normalized temperature is
almost independent of azimuth angle, with a vertical polarized component higher than the
horizontal one according to the Brewster effect (Fig. 6). As a consequence, and due to the almost
random distribution of leaves in the standing crop, the anisotropic effect is significantly smoothed
when plants ore observed in field
conditions.
The measurements on alfalfa indicated
different emission characteristics of this
crop. Indeed the alfalfa plant shape is
characterized by small, quasi circular
leaves and a ramified structure. The
anisotropic effects are much less
conspicuous in this case. As an example,
Fig. 7 shows the brightness temperature
of layered stems with leaves of an alfalfa
crop, with a total plant water content
equal to 0.70 Kg/m 2 . We see that the
difference between V and H
polarizations is quite small, even in the
case of one single layer; moreover the
increase of emission with increasing
biomass is very fast and attains
saturation from the second layer.
Figure 7 - The brightness temperature (f «= 37 GHz, 0
= 45*) of layered stems with leaves of alfalfa plants. (V
= vertical pol., H « horizontal pol.)