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 —>

field: 6

Incidence ANGLE >

FIELD: 15

Incidence angle —>

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

soil, model prediction and observed responses.

Proc. Earsel-workshop: Microwave Remote Sensing on

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Attema, E.P.W. & P.J. van Kats & L. Krul 1982. A

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Attema, E.P.W. & P. Snoeij 1984. Dutscat, a 6-fre

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Krul, L. 1979. The modelling problem, an introduc

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