Hydrology and Vegetation Remote Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 245

Wu and Li ( 2012 ) investigated the reflected signals' polarization properties
in microwave radiative transfer model. Assume that the transmitted signals are
RHCP, polarizations of RR, RL, RH, RV and R ˙ 45 ı were simulated. From their
simulations, it can be seen that the as for RL polarization, scattering values only exist
at larger incident angles. Dynamic ranges of linear polarization (both horizontal and
vertical) were larger and their scattering respond existed at the whole incident angles
(from 10 ı to 70 ı ). V polarization dynamic ranges were larger than H polarization.
Linear polarization is suitable for receivers which were able to track signals from
the whole scattering angles range. But LHCP polarization response only exists at the
larger incident angles. In addition to the scattering zenith angles, scattering azimuth
angles are also influenced the polarization scattering properties.

10.4 GNSS-R Forest Biomass Monitoring

As for GNSS-R scattering, there are two scattering zones: one is the first Fresnel
Zone and the other is glistening area. As for the former, it is usually defined as the
locus of points, surrounding the specular reflection point, and compared with the
one reflected by the specular point itself, the phase shifts of reflected waves are less
than or equal to ; as for the latter, it is defined as the area for which the scattered
power has dropped down by a factor 1/e( 4.3 dB) (Ferrazzoli et al. 2010 ). The
coherent scattering comes from the first Fresnel zone, and the incoherent scattering
comes from the glistening zone.
In the case of ocean surfaces, the coherent scattering is much larger than the
incoherent one and coherent component is consequently neglected. However, as
for the land conditions, the land surfaces roughness scales are always less than
the GNSS wavelength, unless topographic effects are considered. Therefore, land
surface does not own a glistening area and diffuse scattering is usually much lower
than the coherent component.
Ferrazzoli et al. ( 2010 ) presented some theoretical simulations of the specular
scattering coefficients of a forested area to demonstrate the potentiality of GNSS-
R in forest biomass monitoring. The model developed at Tor Vergata University
was used in the study. It is a discrete model. Intrinsically, the Tor Vergata model
is bistatic. However, it is well known that the GNSS signal is significantly high in
the specular direction only due to the low power flux density at the earth surface.
Therefore, only specular direction simulations were presented at their paper and the
forest specular scattering coefficient was modeled like the following:

pq . s ; s D 0/ D 0coh

pq

. i ; i ;0/ C 0inch D i ;' s ' i . i ; i ;0/ (10.2)

pq

where i and ' i are the incident zenith and azimuth angles, s and ' s are the
scattering zenith and azimuth angles, p and q stand for polarizations, 0coh
pq

is the

coherent scattering component and 0inc
pq

is the incoherent part. The simplified