Hydrology and Vegetation Remote Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 243

Fig. 10.1 Soil moisture
dielectric constant at the
USDA/SCAN (United States
Department of
Agriculture/Soil Climate
Analysis Network) site on the
Central Plains Experimental
Range of Colombia from
peak power of the GPS
reflections (Modified from
Masters et al. 2000 )

amplitudes were compared with the Noach land surface model. Although, GPS
multipath amplitudes and the land surface model were not calibrated, the multipath
amplitudes showed good correlation relationship with nearby precipitation records.
Experimental data conducted at Marshall, CO, showed that good correlation of 0.91
was shown with the shallow water content reflectometry over a period of 83 days.
Subsequently, Marshall Experiment was extended to span 210 days (Larson et al.
2010 ). Reflector depth and its variation through time were assessed. The penetration
depth of GPS signals should be used appropriately and a retrieval algorithm should
be developed to convert GNSS SNR data to soil moisture in the future.

10.3.4 GNSS-R Scattering Properties

The microwave scattering models have been developed to better understand the
interaction of microwave signals with targets (such as soil or vegetation). They
are either based on the distorted born approximation (DBA) or radiative transfer
(RT) theory. As for the latter, two processes (extinction and emission) are utilized
to describe the change of microwave intensity propagating through the medium.
Wu et al. ( 2012 ) used the Bi-mimics model for GNSS-R soil or vegetation
study, only the specular scattering coefficients at the first Fresnel zone should be
considered. So the model was modified to the specular direction. As it is well
known, the existing models are developed for linear radar cross section or scattering
coefficient. However, in order to overcome ionospheric effects, the transmitted
signals by GNSS constellations are RHCP. Therefore circular polarization should be
used for GNSS-R remote sensing, including co-polarization and cross-polarization.
Wu et al. ( 2012 ) used the modified model to do initial research on GNSS-R vege-
tation biomass, including crop (wheat) and forest (Aspen in Mimics's Handbook).
Specular scattering simulations were carried out at RV and RL polarization, where
the incident angles were between 10 ı to 70 ı . Figure 10.2 showed the theoretical
simulations of vegetation diameters versus specular scattering coefficients.