Agriculture Reference
In-Depth Information
With sensing from aerial platforms, it might be possible to avoid the transmission
of signals through clouds by a low height above the surface of the earth. However,
up to now unmanned observations from aerial platforms hardly occur. This limits
sensing from aerial platforms. The development, permission and use of unmanned
quadrocopters might alter the situation.
3.5
Microwaves or Radar Instead of Visible
or Infrared Waves
Practically all sensing limitations that arise from atmospheric barriers (Fig.
3.3
)
including clouds are removed when
microwaves
are used. The name “microwaves”
can be misleading, since their spectral region has the longest waves used in remote
sensing. Hence the microwaves also have the lowest energy per photon. The limita-
tions that arise from this for sensing from satellites are overcome by using
active
sensors
with special antennas that provide a high sensitivity. The active sensors
both emit microwave energy and detect its return from the ground. They are gener-
ally known as
radar
sensors. Radar stands for
r
adio
d
etection
a
nd
r
anging.
Modern spaceborne radar sensors work in the wavelength range of 0.1-100 cm
and emit pulses of radiation in a “fl ashlight” manner. The signals that are refl ected
back to the satellite depend to a large extent on the roughness of the surface that was
hit. The rougher the surface, the better the return signal is. Because from a rough
surface, the radar echoes are scattered back in several directions. Hence the refl ec-
tion is at least partly thrown back for recording, whereas specular refl ection from a
smooth target might not get back to the satellite at all. With cultivated soils, clods in
the seedbed provide for a diffuse refl ection (Fig.
3.5
).
Yet the refl ection back to the satellite depends on additional factors, especially
on the wavelength and the dielectric properties of the soils or the plants. The longer
the waves are, the more radiation is refl ected back to the satellite and
vice versa
.
Hence with long waves, a rather fl at soil surface can appear as being rough, while
with shorter waves it can show up as being smooth (CRISP
2010
).
diffuse reflection
θ
Fig. 3.5
Refl ection of radar signals from a smooth- or from a rough target surface
