## 4 Future Perspectives
### GNSS-Reflectometry (GNSS-R)
#### 4.1.3 Future Options for the GNSS-R  
As discussed above, the potential of using reflected GNSS signals for studying surface environmental properties has been previously identified, but the exploitation of such signals is still in its infancy. Only a few GNSS-R research instruments have so far been conceived and investigations were usually limited to GPS signals (single frequency L1, or L2C). However, in addition to the USA-owned GPS, the Russian GLONASS constellation of 24 satellites resumed full operation in 2011; the European Union's Galileo positioning system is in the initial phase of deployment (four In-Orbit Validation satellites are already orbiting); and the People's Republic of China has announced that it will expand its regional navigation system (named BEIDOU) into a global system (named COMPASS) by 2020. While the signals transmitted by the existing GPS and GLONASS constellations are being modernized with the launch of new satellites, the new Galileo and COMPASS constellations will similarly offer modern signals including wideband modulation, data and pilot channels, longer codes, and transmissions of open service signals over different frequency bands. Among the properties of these new signals, the first one that will lead to improved signal detection and estimation is a higher power. For example, the future Galileo E1 and GPS L1C signals will have a power 1.5 dB higher than the GPS L1 C/A, and the Galileo E5a and GPS L5 signals will have a power 3.5 and 4.5 dB higher, respectively, which is clearly not negligible. Since the reflected signal is usually weaker than the direct signal, such power increase will proportionally improve the current threshold of detectability. In the same idea, the second property that will help to improve the detectability threshold is the existence of a pilot channel. Such channel enables the use of very long coherent integrations, while the existing GPS L1 C/A signal, which includes data, requires the use of non-coherent or differentially coherent integrations, which are far less efficient at low SNR [116]. The last major improvements will be the better spatial coverage and the higher number of measures, since more satellites will be visible and each satellite transmits several signals. By exploiting all these novelties, it is expected that the resolution and the accuracy of the measurement will significantly improve. However, these improvements will come at the price of additional signal processing and research challenges. For example, the chipping rate of the signals at the L5/E5 band is 10 times higher than for the GPS L1 C/A signal, a chip representing a distance of 29.1 m instead of 291 m. This has a direct consequence for some of the proposed GNSS-R techniques, such as those relying on interference due to the carrier only where the direct and reflected codes are assumed aligned [19, 104], which is not true anymore even for small extra paths (\(\leq\)10 m). Therefore, when considering these signals, both the carrier phase and the code alignment have to be considered; otherwise the interference methods discussed previously may provide unreliable results or at least the possible range for the antenna height will be decreased [104]. The current experiments are focused on planar or almost planar terrains, which is an ideal context, but not always representative of the area of interest; typically not applicable in the alpine environment. For such mountainous areas, numerous multiple reflections are likely to happen simultaneously, and not just one. A solution may come from the use of array beamforming, to isolate the different reflections, and apply the traditional algorithms, as well as identify their direction. As the proper calibration of an adaptive antenna array is cumbersome [117, 118] and space time adaptive algorithms are verycomputationally intensive; a possible substitute would be to use a virtual array, such as a synthetic uniform circular array (UCA).