Sensing of Crop Properties - Precision in Crop Farming - page 131

errors. So any real-time control system for sprinklers will have to provide separate
paths for the radiation that is used for sensing on the one hand and for the water
drops on the other hand.
Guiding the water from the sprinkler boom down into the canopy via vertical
flexible hoses would do away with this problem. This technique also reduces evapo-
ration losses. However, it results in line watering instead of treating the whole area
and is hardly used in large scale farming.
In short, site-specific irrigation in real-time based directly on the water supply of
crops or soils for large scale farming is not yet state of the art (Evans and King 2010 ,
2012 ) though it is an urgent matter because of the increasing lack of water.
Principally, site-specific irrigation could also rely on maps of topography and soil
texture . Both factors can influence the water supply of crops. However, the direct
control via the water stress of crops should be preferred.

6.6 Sensing Properties of Crops by Microwaves

Microwaves operate with the longest waves that are used for sensing - hence the
name might be misleading (Fig. 3.1 ) . The historical explanation for the name is
attributed to the fact that microwaves are shorter than radiowaves. Compared to
sensing by visible and infrared reflectance, observations in the microwave region
are complementary and also more complex. And applications are still more limited,
but they are growing.
A fundamental advantage is that microwaves can penetrate the atmosphere
including clouds during day and night. Only heavy rain or snow can prevent the use.
As far as the transmission through the atmosphere is concerned, microwaves are
predestined for remote sensing. Yet the crop properties that can be detected are quite
different from those sensed by visible and infrared radiation. Whereas the latter
waves can sense chlorophyll, water and leaves, the main objectives of recording via
microwaves up to now are canopy structure, vegetation type and biomass.
Satellites operate on artificially created microwaves, which are commonly called
radar-waves . Some radar-wave configurations that are used on modern satellites
are listed in Table 6.3 .

Table 6.3 Frequencies and
wavelengths of radar bands
within the 0.4-15 GHz range a

Letter designation Frequency in GHz Wavelength in cm
P-band 0.44 68
L-band 1.28 23
S-band 3.0 10
C-band 5.3 5.7
X-band 9.6 3.1
a Extracted from Ulaby et al. 1996 and altered. S-band wave-
length corrected. Data refer to synthetic aperture radar (SAR).
Additional bands can be available