Hydrology and Vegetation Remote Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 242

The IPT has been successfully used to implement three kinds of retrievals:
surface topography (Rodriguez-Alvarez et al. 2009a ), vegetation height (Rodriguez-
Alvarez et al. 2009b ), and bare or vegetation-covered soil moisture (Rodriguez-
Alvarez et al. 2010a ). However, coarse resolution digital elevation model estimation
was used for surface's topography correction. Vegetation height was a function of
notch number and notch position and could be retrieved independently from the soil
moisture and surface roughness (Rodriguez-Alvarez et al. 2010b ).
Two field experiments were also done. The first one is Palau Field Experiment
(wheat) operated from February 2008 to October 2008. The second one is GRAJO
(the GPS and Radiometric Joint Observations) field experiment (Rodriguez-Alvarez
et al. 2011 ; Monerris et al. 2009 ). Three plots were include: bare soil, perennial
grass and barley. For Palau Field Experiment, soil moisture retrieval was about
2.5 4.7 %, but for GRAJO field experiment, the soil moisture retrieval error
was about 2.0 3.2 %. For barley and wheat, vegetation height retrieval was
quite satisfactory: RMSE D 3 5 cm. Maize height results were quite satisfactory:
RMSE D 6.3 cm. Therefore, the IPT was demonstrated to a useful tool for retrievals
of land geophysical parameters (Rodriguez-Alvarez et al. 2011 ).

10.3.3 Hydrology Sensing from GNSS

In order to receive weak GNSS reflected signals, Special receiver should be
designed. There are two commonly used receiver antenna: one is RHCP pointing
to the zenith to receive direct signal, and the other one is LHCP pointing to nadir
to receive reflected signals. Larson et al. ( 2008a , b ) showed the conventional GPS
receivers used in geophysics and geodesy network can be used to monitor global
soil moisture. Although the purpose of this kind of receivers is to maximize the
direct signals and suppress the reflected ones, it is well known that they cannot
be completely rejected. The kind of receiver is generally at lower height to about
2 m, so that the delay time between direct signal and reflected signal is short and
coherence will be produced at the antenna.
Soil moisture can also be measured by using existing networks of standard
GPS receivers. Its fundamental is to use the multipath reflection amplitudes at
GPS sites to estimate soil moisture. The GPS antenna receives not only the direct
signal but also the reflected signal, which is not along a direct path. Masters et al.
( 2000 , 2004 ) obtained soil moisture at a USDA/SCAN (United States Department
of Agriculture/Soil Climate Analysis Network) site, located on the Central Plains
Experimental Range of Colombia, with the peak power of the GPS reflections
(Fig. 10.1 ). Katzberg et al. ( 2006 ) has also successfully estimated soil reflectivity
and dielectric constant with GPS reflected signals.
Multipath also impacts the SNR (signal-to-noise ratio), and provides an alterna-
tive method for quantifying multipath effects. GPS signals are mostly influenced by
the near-surface (0 5 cm) soil moisture and its spatial resolution is much higher,
about 50-m radius, very similar to SMOS and SMAP datasets. The GPS multipath