STUDY OF MAIZE PLANTS EFFECTS IN THE RETRIEVAL OF SOIL MOISTURE USING THE
INTERFERENCE PATTERN GNSS-R TECHNIQUE
N. Rodriguez-Alvarez§, X. Bosch-Lluis§, R. Acevo§, A. Aguasca§, A. Camps§?, M. Vall-llossera§?, I. Ramos-Perez§,
§Remote Sensing Lab, Dept.Teoria del Senyal i Comunicacions, Building D3, Universitat Politècnica de
Catalunya and IEEC CRAE/UPC, 08034 Barcelona, Spain.
?SMOS Barcelona Expert Centre. Pg. Marítim de la Barceloneta 37-49, 08003 Barcelona, Spain
Tel. +34+934017362, E-mail: email@example.com
The use of Global Navigation Satellite Signals Reflections
(GNSS-R) techniques to retrieve geophysical parameters
from surfaces has been increased in the recent years. These
techniques have resulted in suitable tools to obtain
information about the sea state of oceans, which is very
useful to improve the ocean salinity retrieval [1-3], and also,
information about the soil moisture [4-6] of lands.
The present work focuses on the use of the Interference
Pattern Technique (IPT) [7-10], a particular type of GNSS-
R technique, to study vegetation-covered soils. The IPT
consists mainly of the measurement of the interference
pattern between the GPS direct and reflected signals (the
interference power), after they impinge over the ensemble
soil surface and vegetation layer. The measured interference
signal provides information on the soil moisture of the
surface and also, on the vegetation height.
Index Terms— GNSS-R, Interferometric-pattern, Soil
moisture, Vegetation, Growth, Retrieval
A previous study , showed that this kind of retrievals
can be performed over wheat and barley fields, where plants
reach 60 cm height and their dominant structure is vertical.
This work extends the previous studies to maize-covered
fields whose height reaches up to 250 cm and their structure
is mostly vertical, but also due to the long leaves, they have
a significant horizontal component, they are density packed,
and the vegetation water content is much higher than in the
case of wheat. The paper is divided in three main parts. First
the theoretical aspects of the IPT measurements over a
maize scenario are presented focussing on the simplification
of the model implemented to emulate the vegetation layer.
Secondly, it is presented the experimental measurements
carried out to test the theoretical aspects and the main results
achieved. Finally, the conclusions about the suitability of the
simplified model and its application to maize planted fields
2. MAIZE EFFECTS THEORETICAL
In a previous work , a complex model based in L-
systems  was used to model trunk and leaves scattering.
In the present work a simplification of this model has been
performed considering the vegetation layer as a constant
layer with an equivalent maize dielectric constant value,
using the Matzler model , and the resultant dielectric
constant value for the mixture plant air, using the Kerr
model . Figure 1 shows the main interaction of the GPS
signals and the instrument, which is called Soil Moisture
(SMIGOL) Reflectometer [9, 10].
Observations at L-band
Figure 1. Geometrical configuration of SMIGOL Reflectometer and the
GPS signals reflecting over a surface composed of several layers
characterized by their dielectric constant (?i), thickness (ti) and the
roughness between layers.
Most significant improvements in the theoretical
relationships between geophysical parameters and their
effects over the interference power are related to the plant
height. The algorithms related to soil moisture and
topography have also been
modifications are not as relevant as the height retrieval ones,
so they are not explained in more detail than it was in .
As previously observed in , vegetation height is
directly linked to the number of notches and their positions
improved, but their
3813 978-1-4244-9566-5/10/$26.00 ©2010 IEEE IGARSS 2010
in the interference power measured. A notch is defined as a
minimum amplitude oscillation in the interference power.
An example of notches present in the interference power can
be seen at Fig. 2. After simulating different heights using the
simple layer model the result obtained for the theoretical
evolution of notches is the one that can be observed in Fig.
Figure 2. Measured interference power (black line) and retrieved theoretical
power (grey-dashed line) for one satellite measurement on June 8th.
Figure 3. Vegetation height as a function of the incidence angle where
notches take place.
Compared with the previous result , where the
evolution of notches for wheat using a complex
electromagnetic model to emulate the scattering was
presented, the present study reaches higher vegetation
heights (150 cm), and a more complete analysis will be
presented at the conference (up to 250 cm). The important
result is that for the first 60 cm both  and the present
simplified model are equivalent, therefore the simplified
model can be used for most crops.
3. MAIZE FIELD EXPERIMENT
The SMIGOL-Reflectometer [9, 10] has been deployed in a
field campaign over a maize field, Fig. 4, at Palau
d’Anglesola, Lleida, Spain, since March 2010 and will last
up to October 2010, when maize will be harvested. In that
way it is covering different growth stages of the maize, from
no vegetation up to 230 - 250 cm vegetation height,
including the dry up process of the maize.
Figure 4. Maize field experiment at Palau d’Anglesola, Lleida, Spain.
Photographs correspond to May 30th (40 cm maize height) and June, 19th
(135 cm maize height)
Figure 5 shows the main ground-truth of the field
Figure 5. Palau d’Anglesola field experiment over maize. The soil moisture
probes ECH2O-EC5 are located around the SMIGOL-Reflectometer. The
main inclination directions of the field are shown using red arrows.
The soil moisture of the field has been monitorized
using ECH2O-EC5 soil moisture probes , which have
been located around the SMIGOL-Reflectometer to clearly
see the tendency of the water in the field. Probes A, C, E
and G are located at 5 cm depth while probes B, D, F and H
are located at 20 cm depth. Topography of the field has been
defined by the farmer to have two main inclinations, shown
in Fig.5 with red arrows, and the maximum topography
difference is 40 cm.
Topography, soil moisture and vegetation height
retrievals have been applied to the measurements. Figure 2
shows the satellite 30 interference power measured by the
SMIGOL-Reflectometer on June 8th, 2010, DoY = 159, and
the simulated interference power, which are in excellent
agreement. As it can be seen in Fig. 2, after applying the
three retrieval algorithms, the measured and the retrieved
powers are very close. Furthermore the height retrieved for
each one of the notches found is specified.
89.6 cm 88.5 cm
85.2 cm 87.3 cm
After processing the measurements achieved during the
field experiment some results have been achieved for the
topography, soil moisture and vegetation height.
In Fig. 6 the topography retrieval is shown.
Figure 6. Topography retrieval achieved by processing the SMIGOL-
This result has been achieved averaging 6 days of data,
approximately 20 satellites information. The maximum
difference is near to 20 cm which is coherent with the
maximum difference of the field informed by the farmer.
In Fig. 7 the main result for the vegetation height
retrieval is shown.
Figure 7. Maize plant height retrieval achieved by processing the
The maize growing presented (solid green line) does
not correspond to the whole growing process (on-going), but
extended results will be presented at the conference, with
more updated data from the field experiment. At the present
moment maize plant is 120 cm high. It was planted (brown
arrow) on DoY 100 and was irrigated (blue dashed arrow)
on DoY 160. Furthermore heavy rain events (blue bars)
occurred on DoY 160 and 161. Ground truth about rain
events has been achieved from the Servei Meteorologic de
Catalunya web page  thanks to the closeness (720 m) of
the Poal meteorological station to the field experiment. As it
can be seen, in Fig. 7, the retrieved values using the
SMIGOL-Reflectometer measurements are highly correlated
with the maize growing. The measurements mean value
agrees with the ground truth, obtained randomly selecting
maize plants. Table I summarizes the vegetation height
Table I. Vegetation height results
DoY mean value
150 47.50 cm
158 77.50 cm
159 88.33 cm
160 94.65 cm
161 101.05 cm
162 107.33 cm
164 119.80 cm
Figures 8 and 9 show the soil moisture retrievals.
Figure 8. Soil moisture retrieval achieved by processing the SMIGOL-
Reflectometer measurements for day June 8th, DoY = 159, prior to heavy
rain and irrigation.
Figure 9. Soil moisture retrieval achieved by processing the SMIGOL-
Reflectometer measurements for day June 10th, DoY = 161, after heavy rain
Figures 8 and 9 show the results achieved after Download full-text
processing the satellite data available for days June 8th and
10th, respectively. In table II the ground-truth measurements
and the SMIGOL-Reflectometer
Table II. Soil moisture retrieval results.
\ DoY (5 cm depth) (20 cm depth)
159 20 %
161 35 %
In one hand, for DoY 159, the error is [0.2 – 0.6] %
respect to the 20 cm depth probe and [3.4 – 4.2] % respect
to the 5 cm depth probe. In the other hand, for DoY 161, the
error is [0.8 – 0.9] % respect to the 5 cm depth probe and
[6.2 – 7.9] % respect to the 20 cm depth probe. When soils
are wet the retrieved value is closer to the surface ground
truth and when soils are dryer the retrieved values are near
to the 20 cm depth values.
The simplification of the previous developed model for
vegetation scenarios performed in  into a simple layered
model including only information about plant dielectric
constant, has been tested and good result has been achieved
for the three retrievals performed, topography, vegetation
height and soil moisture. The field experiment performed at
Palau d’Anglesola, Lleida, Spain over a maize field has
allowed testing not only the simplification of the model but
also the fact that the Interference Pattern Technique also
works over maize plants, which have clear vertical and
horizontal components in the growing structure.
EC5-Probe G EC5-Probe H SMIGOL-Reflect.
23.4 %, 24.2 %
34.2 %, 35.9 %
This work, conducted as part of the award “Passive
Advanced Unit (PAU): A Hybrid L-band Radiometer,
GNSS-Reflectometer and IR-Radiometer for Passive
Remote Sensing of the Ocean” made under the European
Heads of Research Councils and European Science
Foundation EURYI (European Young Investigator) Awards
scheme in 2004, was supported by funds from the
Participating Organizations of EURYI and the EC Sixth
Framework Program. Also by funds from the Plan Nacional
del Espacio of the Spanish Ministry in the frame of the
project with reference ESP2007-65567-C04-02. And also by
funds from the project with reference AYA2008-05906-
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 Soil Moisture Sensor.
Servei Meteorològic de catalunya.
L-band ground reflectivity