Agriculture Reference
In-Depth Information
y
wave direct.
z
x
y
wave direct.
z
x
Fig. 3.6
Polarization of radar waves in a horizontal- or in a vertical direction (From Fink et al.
2010
, altered)
Because the transmit and receive situations in the fi rst two cases are the same,
they are denoted as “
like-polarized
”. The last two cases are “
cross-polarized
” .
These detailed sensing alternatives can - when used in combination with suitable
wavelengths and incidence angles
of the radiation (Fig.
3.5
) - substantially improve
the results obtained (Heinzel
2007
; McNairn et al.
2009
; Shimoni et al.
2007
).
Yet whatever results are obtained when sensing with visible, infrared or radar
waves, it should always be kept in mind that the signals indicate just phenomena
that are of interest because of their known relation to soil- or crop properties. The
signals never directly explain the reasons for the sensed phenomena. So still a clever
mind is needed for the analysis of the causes. However, a farmer has to make this
analysis too when he inspects his fi elds visually. The difference is in the amount and
kind of data available.
And the success achieved with the large pool of signals often depends very
much on an intelligent processing of these. It might be necessary to create special
mathematical indices
based on the respective electromagnetic spectrum. These
indices are calculated from special selected wavelength bands by algebraic or
differential operations. Suitable indices for soil- and crop properties have been
and still are the object of intensive research. Some of them will be dealt with in
later chapters.
However, there are still fundamental differences in the sensing potential of
visible and infrared radiation on the one hand or radar waves on the other hand.
This potential is listed in Table
3.2
. Summing up, it can be seen that sensing
with visible and infrared radiation is mainly focused at
constituents
of soils and
crops, whereas the applications for radar waves are more pointed towards
θ
