535 
MICROWAVE EMISSIVITY OF PLANT CONSTITUENTS AT 37 GHZ 
G. MACELLONI, S. PALOSCIA, P. PAMPALONI, R. RUISI and C. SUSINI 
Istituto di Ricerca sulle Onde Elettromagnetiche - CNR 
via Panciatichi 64, 50127, Firenze (Italy) 
ABSTRACT 
Experimental investigations and theoretical analysis, carried out over the past years, have 
indicated that microwave emission is sensitive to vegetation biomass and depends on plant 
geometry. In general a significant information is obtained by using an appropriate combination 
of polarimetrie, multi-frequency data collected in a frequency range between 1.5 and 19 GHz. 
On the other hand vegetation monitoring on a global scale from satellite would benefit from 
observations at higher frequencies, which allow a better ground resolution. For this reason it 
appeared advisable to further investigate microwave features of crops at 37 GHz. Field measurents 
have been carried out on standing vegetation and on plant constituents with a dual polarized 
sensor. The results of the experiments, which show a significant anisotropy of microwave emission 
from single plant constituents of wheat, are discussed. 
KEY WORDS: Microwave radiometry, Vegetation. 
1 - INTRODUCTION 
Microwave radiometry has proven to be a relevant technique for global scale monitoring of 
geophysical processes of the atmosphere and the ocean. Microwave radiometers also appear to be 
a very promising tool over land in the detection of soil moisture and estimation of snow water 
equivalent and vegetation biomass. In fact, experimental investigations and theoretical analysis, 
carried out over the past years by our group at CNR in Florence and by other scientists 
worldwide, pointed out that microwave emission is sensitive to vegetation biomass (e.g.: 
Kirdiashev et al. 1979, Ulaby et al. 1983, Pampaloni and Paloscia 1985, Huppi et al. 1985). The 
electromagnetic features of many crop types have been studied on a frequency range between 1 
and 37 GHz and several relations between brightness temperature and vegetation parameters have 
been found (Paloscia and Pampaloni 1992). 
A major result of our previous investigations is that microwave emission from vegetation canopies 
is not only influenced by the total biomass, but by the plant geometry as well (Pampaloni and 
Paloscia 1986, Pampaloni 1992). The effect of biomass, associated with a monotonic variation of 
the emissivity as vegetation grows, is dominant when the observation wavelength is much smaller 
or much larger than the dimensions of plant constituents, while the type of crop mainly affects 
emission at a wavelength in the order of leaf dimension (Ferrazzoli et al. 1992a). Therefore, also 
for this reason, a universal relation (valid for all vegetation types) between a microwave parameter 
and vegetation biomass has not been found. 
In general, we have seen that significant information is obtained by using an appropriate 
combination of polarimetric data, collected at an incidence angle close to 50 degrees, and in a 
frequency range between 1.5 and 20 GHz (Pampaloni and Paloscia 1985, Paloscia and Pampaloni 
1988, Ferrazzoli et al. 1992b). On the other hand, since the most convenient method for 
vegetation monitoring on a global scale is the use of satellite sensors, the question of ground 
resolution becomes important. Consequently, observations at a higher frequency, which should 
allow a more prcatical antenna dimension, would be advisable, although investigations with 
ground based and airborne sensors (Paloscia and Pampaloni 1992) had indicated that on