Agriculture Reference
In-Depth Information
Formerly, the dielectric properties have been defi ned by a so called dielectric
constant. This property, however, is not constant at all, it varies widely. Consequently,
now it is denoted as “
permittivity
”. This physical term generally is expressed as
relative permittivity because it is related to the permittivity of a vacuum and hence
is without dimensions. It defi nes the potential to store electric energy. Air has
approximately the permittivity of a vacuum, which is 1 (one). This is the minimum.
Compared with other matters or materials, water has a very high permittivity of
around 80. The respective data for dry natural materials including soils and plant
matter are much lower, they are in the range of 3-8 (Paul and Speckmann
2004
;
Lillesand and Kiefer
1979
).
These large differences in the permittivities or in the dielectric properties of
water on the one hand and dry soils on the other hand are the base of
moisture sens-
ing
by radar waves in precision farming. However, a prerequisite for sensing the
moisture is that effects of differences in the surface roughness do not show up.
Rather long radar wavelengths can help in this respect, at least with sensing of soils.
This is because long waves react less on the roughness of the soil surface. Another
advantage of rather long waves is their ability to sense the moisture not exclusively
on the top surface of the soil, but instead also for some vertical distance down from
the surface. The moisture solely on the surface of soils is hardly important for crops,
since their water is supplied by a soil layer of some thickness.
The potential of sensing by radar waves can be enhanced by
polarizing the
radiation
. The normal case is that the radiation vibrates or fl uctuates in all direc-
tions perpendicular to the propagation at random, even if the wavelength is uniform.
Polarizing the radiation aims at controlling the direction in which the photons
vibrate. So a polarizer is a device that allows only radiation with a specifi c angle or
a specifi c direction of vibration to pass through. The signal is fi ltered by a polarizer
in such a way that the wave vibrations are restricted to a single plane that is
e.g.
perpendicular or horizontal to the direction of wave propagation (Fig.
3.6
). There
can be additional alternatives in polarizing directions.
It should be mentioned that this polarizing in a vertical- or horizontal direction
does not alter the fact that every radiation has an electrical- as well as a magnetic
fi eld. These fi elds incidentally also move in perpendicular planes. Yet the polariza-
tions shown in Fig.
3.6
only refer to electric fi elds.
When a polarized radar radiation is transmitted to crops or soils, it generates
refl ectance with a variety of polarizations. So - in a simplifi ed way - there is again
a somewhat random situation. But this random radiation too can be polarized
again when it is received by the radar sensor. Today, many radar sensors are
designed to transmit and receive waves that are either horizontally (H) or verti-
cally (V) polarized. With these, there can be four combinations of transmit- and
receive polarizations:
HH - for horizontal transmit to the target and also horizontal receive
VV - for vertical transmit to the target and also vertical receive
VH - for vertical transmit to the target but horizontal receive
HV - for horizontal transmit to the target but vertical receive.
