Hydrology and Vegetation Remote Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 240

similar with other microwave remote sensing technique. However, it has unique
advantages below (Zavorotny and Voronovich 2000a ):

1. GNSS can provide longtime, stable and free signals and GNSS-R receiver is
passive, small size, light weight and low power consumption;
2. GNSS working at L-band (1.57542 GHz and 1.22760 GHz) is suitable for soil
moisture retrieval;
3. Compared with microwave radiometry, thermal variation of background does not
contaminate GNSS reflected signals;
4. GNSS-R receiver can simultaneously receiving multiple satellite signals in the
field of view, and greatly improve the spatial and temporal resolution;
5. GNSS-R receiver has self-positioning and self-timing capabilities with easily
accessing geographic information network, and therefore it is easy to set up a
wide range of soil moisture observation network.

Therefore, GNSS-R remote sensing is able to create a low-cost global soil
moisture network monitoring system, validate and calibrate the space borne remote
sensing data sets.
In addition, vegetation cover is one of the difficult problems that should be
considered in soil moisture estimation. Meanwhile, GNSS-R with low-cost, small
power consumption, high spatial and temporal resolution and some other advantages
make it possible for vegetation parameters monitoring, such as vegetation biomass
and vegetation growth condition monitoring.

10.3 Hydrology Sensing from GNSS-R

GNSS-R for hydrology sensing is a new and promising tool. For example, Zavorotny
and Voronovich ( 2000a ) established the quantitative relationship between soil mois-
ture and the received waveforms. Rodriguez-Alvarez et al. ( 2011 ) have developed
a ground-based SMIGOL reflectometry. Interference pattern technique (IPT) was
used to retrieve land geophysical parameters, such as surface topography, soil
moisture and vegetation height, et al. In addition, the GPS receivers commonly used
in geodesy and geophysics is also used for soil and vegetation studies from GPS
multipath information (Larson et al. 2010 ). Meanwhile, the microwave scattering
models are used to do theoretical analysis of vegetation biomass or study GNSS-
R reflected signals' polarization properties. In the following, the commonly used
methods for land geophysical parameters are introduced.

10.3.1 Waveform Correlation

Scattering model, Z-V model, was previously used for calculations of GPS ocean
reflections in order to obtain the correlation function and the correlation power of