Sensing of Crop Properties - Precision in Crop Farming - page 132

100

75

50

25

incidence angle = 20°
HH polarization
(= horiz. transmit to target and horizontal receive)

0 0

50 100 150 200

biomass in dt / ha

Fig. 6.19 Transmittance of radar waves though a grass canopy (From Ulaby et al. 1996 , altered)

Radarwaves might penetrate crop canopies deeper than visible radiation. A rule
of thumb is that for radar waves the penetration depth in crops is about half the
wavelength (Heinzel 2007 ), whereas with soils it is one fifth of it (Sect. 5.2.3.2 ) .
Hence the penetration in crops is approximately 2-3 times deeper than in soils. But
this is rather a rough estimation. It is not the wavelength alone that determines the
penetration. Among others, the biomass of a crop (Fig. 6.19 ) and its water content
are important.
For microwaves, a plant canopy acts similar to a three-dimensional water-bear-
ing structure. Within this structure, especially the leaves are the carriers of water. It
is mainly the effect of the water on the reflection that allows to sense crop proper-
ties. And regarding this water effect, the basic functioning of microwaves within
canopies is very similar to that within soils. Instead of the term “reflection”, for
microwaves generally the notation “ backscatter ” is used. It is the backscatter that
can supply signals about crop properties via remote or proximal sensing. For details
to the measuring of the backscatter see Fig. 5.18 .
When a canopy is closed and the radar wavelength is approximately the same
size as the leaves, not much of the radiation energy may reach the soil underneath.
Yet depending on the crop and its canopy, long waves may get to the soil. So if the
objective is to sense only the crop without any interference by the soil below, limita-
tions in the length of the waves might be necessary. If the signals received are partly