Agriculture Reference
In-Depth Information
from the crop and to some extent are influenced by the soil below, it is very difficult
to interpret them.
Polarization
of microwaves has become an important feature when regarding
the ability to sense crop properties. Whereas for non polarized microwaves the pho-
tons vibrate around the axis of propagation in all directions at random - though the
wavelength is uniform - with polarized microwaves the vibrations are restricted to
oriented. Yet the waves that are sent to the canopy and those that are returned to the
receiver might not be in the same plane. Only those microwaves, which get back to
the recording receiver after a
single reflection
on the surface of the canopy, main-
tain their original polarization or their plane of vibration. These waves are recorded
by the receiver as “
like-polarized
” and represent
surface scattering
. However, this
type of scattering occurs mainly with short waves.
When very short waves are excluded, the majority of the waves are thrown
back and forth within the crop before eventually being scattered towards the receiver.
The result is that these waves - at least partly - get back to the receiver in a depolarized
state. This means, the original polarization is lost. The depolarization does not alter
the wavelength. And depolarization hardly occurs with waves that hit soil instead of
a crop.
Hence the amount of depolarization can help to discriminate between soil and
vegetation. A radar receiver can detect the amount of depolarization and thus record
volume scattering
.
For this, the emitter of the satellite might just send polarized
radiation within a vertical plane and receive in a horizontal plane, or vice versa. This
would be a “
cross-polarized
” operational mode, which would indicate about volume
scattering.
The main orientation of the plants can affect the backscatter. For many grain
crops, the stems and its leaves are mostly oriented in a vertical direction, though
this might depend on the growth stage If the incident radiation has a plane of polar-
ization that is parallel instead of cross to the main orientation of the plants, the
backscatter is lower. Hence using horizontally as well as vertically oriented polar-
izations can allow discriminating between crops according to their canopy struc-
ture or their habitus.
Another factor that affects the radar backscatter of crop canopies as well as of
soils is the incidence angle. This is the angle with which the radiation coming from
the satellite hits the earth.
Which crop properties can be sensed by means of radar waves when the present
possibilities of varying frequencies or wavelengths, of different polarizations as
well as incidence angles are exploited? Details to this question have been dealt with
by Brisco and Brown
1998
; Gherboudj et al.
2011
; Jiao et al.
2010
; Mattia et al.
2003
; McNairn et al.
2009
; Shimoni et al.
2009
; Steingiesser and Kühbauch
1998
.
Summing up, the situation is:
•
Chlorophyll content cannot be recorded.
•
Indication of the crop water content is unstable. The results for water sensing via
infrared radiation are better (Sect.
6.5
).
