Hydrology and Vegetation Remote Sensing - GNSS Remote Sensing: Theory, Methods and Applications - page 241

the sea surface waveforms (Zavorotny and Voronovich 2000b ). It is the integral form
of bistatic radar equation as.

“

G 2 ƒ 2

2

j S j

ı ı d

P D (10.1)

4R ts R rs

A

where A is the integral area, ƒ is the correlation function used to define Fresnel
Zone, S is the Doppler filter function and Doppler zone is inferred, and G is the
antenna's effective coverage zone. As for ocean part, ı ı is calculated from ocean
surface scattering using GO model (Zavorotny and Voronovich 2000b ). In 2000,
Zavorotny and Voronovich ( 2000a ) extended the method toward remote sensing
of soil moisture and pointed out that GPS scattering signals would provide good
opportunities for soil moisture remote sensing from aircrafts and satellites. The peak
of reflection signals is correlated with soil moisture and its tail is affected by the
surface roughness. Zavorotny and Voronovich ( 2000a ) assume that direct signals
are RHCP and the down-looking antenna is omnidirectional LHCP. Elevation
angle of 40 ı is considered. Theoretical simulations show good sensitivity to soil
moisture, but directly retrieving soil moisture from waveforms would be difficult
due to the surface roughness effect. The polarimetric ratios (HR/VR and LR/RR)
were introduced to overcome surface roughness effect. Theoretical simulations
HR/VR had demonstrated better performance than LR/RR ratio especially at low
to moderate values of the moisture content (Zavorotny and Voronovich 2000a ).

10.3.2 Interference Pattern Technique (IPT)

Spanish researchers of UPC have been doing land geophysical parameters retrieval
using the interference pattern technique (IPT) (Rodriguez-Alvarez et al. 2011 ). IPT
measures the interference of the direct and the reflected signals, which has been
successfully retrieved the surface topography, vegetation height and soil moisture
in vegetation-covered soils. Ground-based instrument Soil Moisture Interference-
pattern GNSS Observations at L-band (SMIGOL) reflectometry was developed
during 2008 and had been used in the experimental campaigns to test the theoretical
results. Different from the previous works using LHCP antennas, when a LHCP
was used, soil moisture information could not be fully detected and the vertical
polarization antenna pointing to the horizon was used. The direct and reflected
signals performed interference automatically at the antenna (Rodriguez-Alvarez
et al. 2011 ). The received signals were a function of elevation and azimuth angles of
GPS satellite positions. These angles vary in time as GPS satellites move. Surface's
topography was the first time retrieved as a function of elevation and azimuth angles
of satellite. To revisit the same surface scenarios, 3 h were used. SMIGOL can be
located at different heights over the surface. But the maximum height should make
sure that the direct and reflected signals must be added coherently at the antenna,
which means that the direct and reflected signals must arrive within the same GPS
chip interval.