538 
The decrease of normalized temperature 
when the ears are cut can be interpreted 
in terms of the bistatic scattering cross 
section. Indeed, assuming the energy 
conservation, the emissivity of a half 
space of absorbing and scattering 
elements can be written as 1 minus the 
fraction of power incident on the 
surface from a given direction (at a 
specific polarization and frequency) that 
is rescattered in the upper half space 
(Peake 1959, Ferrazzoli et al. 1989). The 
polar pattern of the bistatic scattering 
cross-sections (measured at 0 = 45*) of 
an ensemble of vertical cylinders 
simulating ears, indicates that the 
incoming radiation is either absorbed or 
scattered downward for both vertical (V) 
and horizontal (H) polarizations, 
(Ferrazzoli et al. 1994). It follows that 
the total upward scattered power is very 
low and the emissivity is close to 1 for 
both polarizations. 
Figure 4 - The brightness temperature (f= 37 Ghz, 6 
= 45*) at V and H pol. of a layer of wheat stems (PWC 
= 0.4 Kg/m J ) as a function of azimuth angle ♦. 
On the other hand, the scattering pattern of thin discs simulating leaves is almost symmetrical with 
respect to the horizontal plane since each scatterer gives a significant contribution to the upward 
scattered power, and the emissivity is 
lowered. Thus, emissivity seems to be 
significantly influenced by ears for full 
crop (1) and by leaves when ears are cut 
(2). For an ensemble of ears and thin 
stems (3), the downward scattered power 
is further attenuated by the lower plant 
constituents; the contribution to the 
upward scattered radiation is even lower 
than for full crop and the emissivity is 
higher. 
Finally, when vegetation is cut (5) the 
radiation is scarcely affected by the 
Brewster angle due to surface roughness 
and the residue of cut stems, and the 
vertical polarized component is only 
slightly higher than the horizontal one. 
When soil is covered with a layer of long 
and thin (with respect to wavelength) 
vertical stems (4), the screening effect 
due to scattering from thin cylinders is 
stronger for V polarization and 
emissivity can result higher at H 
polarization. 
Figure 5 - The brightness temperature (f = 37 GHz, 6 
=45*) at V and H pol. of a layer of wheat leaves (PWC 
= 0.17 Kg/m l ) as a function of azimuth angle #. 
In order to further investigate the effects of plant geometry on the canopy emission, we tried to 
characterize the emissivity of plant constituents by measuring the emission of single elements 
placed on a reflecting plate.