## 4 Future Perspectives
### GNSS-Reflectometry (GNSS-R)
#### 4.1.1 GNSS-R for Soil Moisture Recovery  
There are mainly three ways to sense the soil moisture. The first one consists of looking at the reflected signal only, which is the simplest solution but also the one that provides the least information. Such experiments were made in 2002, using a slightly modified commercial GPS receiver  
Figure 1: Example of a local-scale static GNSS-R experiment.  
airborne [91,97,98]. The reflected signals were obtained from a left-handed circularly polarized (LHCP) antenna at the bottom of the plane. A right-handed circularly polarized (RHCP) antenna was on the top of the plane to receive the direct signals, however the measures were corrupted by multipath and thus not used for normalization purpose. The signal-to-noise ratio (SNR) of the reflected signals were measured and compared to dielectric constant and moisture of the soil. It was shown that the SNR measurements were moderately correlated to the _in situ_ measurements. Still, it was also shown that it was possible to detect field boundaries and that the GPS reflectivity was more correlated to the top 1 cm soil moisture than the soil moisture between 1 and 5 cm. Such an experiment has been done also using Delay-Doppler Maps (DDM), where the power of the reflected signals is computed for different code delay and Doppler frequency [99]. However, it has been shown that only the peak of the DDM was useful [100].  
The second option is to receive separately the direct and the reflected signal, and looking at the difference of power [100, 101, 102]. This method has the advantage of providing instantaneous measures, however currently it lacks of validation campaign regarding the expected accuracy.  
The third possibility is to receive the direct and reflected signal simultaneously through the same antenna. The variation of phase between the two signals causes constructive and destructive interferences that can be exploited to measure the soil moisture. This concept, called Interference Pattern Technique (IPT), has been extensively used by the Universitat Politecnica de Catalunya [17,103,104]. They used a specific receiver for GNSS-R, using vertically and horizontally polarized antennae instead of RHCP only. Thanks to this, they were able to see a notch in the oscillations of the SNR at a certain angle. This is linked to the Brewster angle, which is a particular angle where the reflection coefficient becomes null for vertical polarization [105]. As the Brewster angle depends on the permittivity of the layer, which itself depends on the soil moisture, identifying the position of the reflectance point and hence calculating the Brewster angle provides information about the soil moisture. They have shown promising results over bare and wheat fields, recovering the soil moisture with an error of less than 5%. The drawback of this method is the long recording of signal necessary to extract information (typically few hours).  
The interference technique has shown to be applicable also for geodetic antennas as shown by the University of Colorado, which is surprising since the aim of such antennas is to suppress multipath to minimize errors in positioning [92,106,107]. A high correlation between the estimates of the phase of the reflected signals and the height and the soil moisture content at the top centimeter has been shown, however it cannot represent the soil moisture content deeper because of the limited GPS signal penetration.