## 4 Future Perspectives
### GNSS-Reflectometry (GNSS-R)
#### 4.1.2 GNSS-R for Snow Cover Estimation
A summary of the techniques presented in this section is provided in Table 3.  
Although the previous investigations of the GNSS-R concept were promising, they are limited in application. Currently, the operation of existing GNSS-R instruments is restricted to \"flat surfaces\" [115] and cannot yet be applied over complex terrain such as the alpine environment or used to analyze surface properties under vegetation-cover. The reason for this is that the correlation of the reflected signal with the geometrical surroundings needs to be either known _a priori_ (_i.e._, existence of a flat surface like water-body) or is determined concurrently via long-observation times without moving the GNSS-R instrument. Moreover, a lot of research has been conducted with traditional GPS receivers that use a RHCP antenna. For the reflections, the polarization is affected by the properties of the soil, consequently, to process the signal reflections efficiently, both antenna polarizations should be used, either vertical and horizontal (as in [104]), or LHCP and RHCP. This, however, directly impacts the processing charge, as there are twice as many data to process. Finally, until recently, no substantial research had been conducted over heterogeneous fields or complex landscapes, as this would require consideration of its fine structure to identify the reflection's specular or diffuse point (s), which is not so easy to determine. Nevertheless, this challenge can potentially be overcome with the advent of new and modernized GNSS signals and signal processing as well as in view of the recent progresses in airborne and terrestrial laser scanning, as further discussed in the next sections. \begin{table}
\begin{tabular}{l l l l l l} \hline
**Technique** & **Measured** & **References** & **Spatial** & **Accuracy** & **Remarks** \\  & **Quantity** & & & **Range** & & \\  & & & **Resolution** & & \\ \hline Extraction of modulation & snow depth & [19, 109, 110] & 10’s m & 9–13 cm & Use of geodetic RHCP antennas. \\ parameters of the SNR & & & & Limitations for GNSS signals with & \\  & & & & high chipping rate. & \\ Extraction of modulation & soil moisture & [92, 106, 107] & 10’s m & N/A (good & Use of geodetic RHCP antennas. \\ parameters of the SNR & & & & agreement) & Limitations for GNSS signals with \\  & & & & high chipping rate. & \\ Geometry-free linear & snow depth & [111] & 10’s m & cm scale & Technique useful for stations that \\ combination (L4) & & & & do not record the SNR. & \\ Simple ray model & snow depth, & [112, 113] & 10’s m & cm scale & Lack of validation to better \\  & snow density & & & estimate the precision of the & \\ Fast Fourier Transform of & depth of snow & [114] & 10’s–100’s m & very low & Results in agreement with real data, \\ cross-correlation results & layers & & & (\(\sim\)10 m) & despite the low resolution. \\ (waveforms) & & & & Measurement performed in \\  & & & & Antarctic. \\ Measure of reflected & soil moisture & [91, 97, 98] & 10’s–100’s m & N/A & Good correlation between GPS \\ signals only & & & & reflectivity and soil moisture of the & \\  & & & & top 1 cm of the soil. & \\ Measure of direct and & soil moisture & [100–102] & 10’s–100’s m & N/A & Promising method, but not yet \\ reflected signals separately & & & & exploited enough to have \\  & & & & quantitative results. \\ Measure of direct and & soil moisture & [14, 103, 104] & 10’s m & 5% over & Use of vertical and horizontal \\ reflected signals & & & bare/wheat & polarization antennas instead of \\ simultaneously (Interference & & & fields & RHCP. Limitations for GNSS \\ Pattern Technique) & & & & signals with high chipping rate. \\ Measure of direct and & Snow & [108] & 10’s m & N/A (very & Use of vertical and horizontal \\ reflected signals & & & good & polarization antennas instead of \\ simultaneously (Interference & & & agreement) & RHCP. Limitations for GNSS \\ Pattern Technique & & & & signals with high chipping rate. \\ \hline \end{tabular}
\end{table} 
Table 3: Potential GNSS-R techniques for snow and soil moisture determination.