Agriculture Reference
In-Depth Information
35
time domain reflectometry
raw data
mean of 10 data respectively
25
15
average spacing of measurements: ~ 0.16 m
radar antenna located 1.3 m above ground
5
0
10
20
30
40
50
60
70
position along the recording path in m
Fig. 5.20
Soil water content sensed from a land-based vehicle by means of ground penetrating
radar signals that were obtained via surface reflectance. The data were processed by applying
Topp's equation to the signals (From Redman et al.
2003
, altered)
The two electrodes - a brass cone and a metallic ring - make up the tip of a
cultivator tine. They are separated by an insulator (Fig.
5.21
). The soil that sur-
rounds the tine is part of the capacitor. The sensing is done by measuring the
“impedance” that exists for the current flow. This electro-physical criterion defines
- in a rather simplified description - the resistance to current flow within a capaci-
tor. The design of the sensor allows simultaneous recording of soil moisture via
impedance as well as of penetration resistance, which depends heavily on soil water.
Of course, this does not prevent the system to be used solely for moisture sensing.
An important factor is the
electrical frequency
. The present implements operate
with frequencies between 40 and 175 MHz. Low frequencies make it difficult to
sense precisely. This is because with low electrical frequencies, the signals that are
obtained depend not only on the real permittivity or the dielectric constant, but on
electrical conductivity as well (see Table
5.1
and Sect.
5.2.2.2
last part). The effect
is that texture and perhaps also the ion concentration in the soil water influence the
sensor output and thus the moisture that is indicated. On the other hand, higher
frequencies increase the expenses for the electronic devices (Kizito et al.
2008
).
A way out of this situation can be using the results from conductivity sensing for
a
site-specific correction
of the signals from capacitance sensing in order to arrive
at precise water mapping. Kelleners et al. (
2009
) indicate that such a procedure
could improve the accuracy of water sensing. The instrumental solution for this
could be sensing of capacitance and conductivity in one operation by employing
several frequencies
simultaneously with combined processing of the signals for the
