186 
In a first order approximation (9) can be written as: 
2 2 
Y = F.(4ak cos 0) 
for a « X/4ttcos0 
We see that for low frequencies and large incidence 
angles only the product of moisture content and rough 
ness, contained in F.a can be determined. 
We selected three bare or barely vegetated fields 
and will compare the gamma versus incidence plots 
with equality (9). The first results of the applica 
tion of the described model are presented in fig. 8. 
Corresponding field parameters of interest are given 
in table 1. 
FIELD: 1 
Incidence angle <cteg. > —> 
field: 6 
Incidence ANGLE <degr. > > 
FIELD: 15 
Incidence angle <d=g- > —> 
Figure 8. Comparison between the calculated backscat- 
tering coefficient for field 1, 6 and 15 and relation 
(9) for large incidence angles. 
Table 1. Characteristic field parameters. 
Field 
a 
F (dB) 
Mg(g/g) 
1 
40 
-10 
0.34 
6 
11 
- 5 
0.36 
15 
11 
- 3 
0.42 
In this table we see the rms roughness parameter a 
as measured during the ground data collection and 
the model parameter F found for field 1, 6 and 15 
along with the measured gravimetric water content, 
M , in terms of gram water per gram dry soil. 
As expected, F and M do exhibit a similar behavior. 
It must be stated though that the relationship between 
the intensity factor and the gravimetric water content 
needs more investigation. Comparison of the results 
for the same fields on two succeeding days might shed 
some more light on this relationship. In this however 
we are restricted by fact that only large moisture 
contents were present during the SIR-B experiment. 
The large values of gamma observed in fig. 8 for 
small incidence angles are caused by a coherent con 
tribution which has to be added with the right side 
of equation (9). This coherent term is best under 
stood if we consider a perfectly conducting flat 
surface that appears to the radar as a mirror from 
which the main contribution to the received power 
comes from nadir. At the intersection angle of the 
incoherent term of eq. (9) and the coherent term the 
backscattering coefficient exhibits the least depen 
dence on roughness (Ulaby 1979) . Because of this 
fact and eq. (10) also the behavior of y for small 
incidence angles will have to be taken into consider 
ation in future investigations in spite of the bad 
resolution here. 
6 CONCLUSIONS 
It is possible to use the data gathered in the SIR-B 
project with DUTSCAT working at 1.2 GHz as an extra 
source in the semi-empirical solution of the inter 
action problem of microwaves with various types of 
agricultural fields and the question how to extract 
information from this interaction. We must note 
however that the data set is rather limited in soil 
moisture variation and inherent to the airborne si 
tuation not very accurate for small incidence angles 
At this moment a variety of aspects is studied more 
closely. 
REFERENCES 
Attema, E.P.W. 1979. Radar backscattering from bare 
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Attema, E.P.W. & P.J. van Kats & L. Krul 1982. A 
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