# Latest Advances in the Global Navigation Satellite System--Reflectometry (GNSS-R) Field
## 7 Discussion  
GNSS-R was demonstrated from an airborne platform more than 20 years ago, and since then, several spaceborne missions have proven GNSS-R capabilities for Earth observation using small satellite platforms. GNSS-R is a key technology developed to perform forward-scattered radar measurements, a form of bistatic radar in which the transmitter and the receiver are aligned in the same incident plane. This method has several advantages as compared to a traditional radar. First, GNSS-R instruments do not require transmitters, as they collect the signal emitted by the GNSS constellation, available worldwide, reducing instrument complexity and power consumption, which consequently reduces their mass and volume. Second, the physics of forward-scattered signals are complementary toback-scattered signals. Over water, the radar back-scattered signal presents a very small amplitude, as most of the energy is reflected. This allows GNSS-R to perform ocean wind studies, or inland waterbody detection in forested areas, as in the Amazonian rainforest. Additionally, forward-scatter configurations limit the amount of double-bounce reflection received from the canopy, as double bounces are not possible in forward-scatter geometries.  
Current ground-based instruments are focused on the execution of the well-known GNSS-IR or interference pattern method, in which the interference between the direct and the reflected signal is used to estimate the moisture content of the soil, or the waves over the ocean. This technique is mature and ready for industry-wide deployment based on user needs. However, even though this technique is well known, there is a lack of studies of reflectivity data using low-altitude platforms, such as drones. Potential future research lines may include the evaluation of the bistatic radar cross section over different types of vegetation using very low-altitude platforms (<20 m) for its suitability in commercial drones.  
Current airborne studies are focusing on the exploration of new signals, such as GPS L5 or Galileo E5a, with a higher bandwidth, allowing for finer-range resolutions and increased signal-to-noise ratios. Present studies are also exploring the possibility of including RHCP and LHCP antennas with coherent receivers to detect the polarimetry signature of the surface. In this sense, there is a lack of polarimetric studies using airborne platforms. Multi-band, multi-constellation, and polarimetric instruments are the logical step to maximize the science return on the GNSS-R field. Several studies are pointing out the usability of two polarization antennas to directly estimate soil moisture without the need for vegetation attenuation or surface roughness correction.  
Current spaceborne missions are integrating GNSS-R as part of their primary payloads. This is a tremendous achievement for the entire GNSS-R community, providing more data with which to perform valuable science. Thanks to the electronics miniaturization, integrating GNSS-R receivers is now easier than ever. Current approaches to real-time DDM generation in orbit help to drastically decrease mission data budget, relaxing mission requirements. Future missions may require the integration of multi-channel correlators for different polarizations, as well as cross-correlation channels to generate the full-Stokes parameters of the reflected wave.  
In terms of applications, the future of GNSS-R is to combine the very valuable forward-scattered reflection with other remotely sensed sources, such as radar or radiometry, to provide an enhanced estimate of the Earth's surface. Polarimetric applications of GNSS-R are yet to be employed over the cryosphere and densely vegetated areas. The integration of all types of GNSS constellations with new polarimetric schemes and higher bandwidth signals would allow for better retrievals with which to estimate several geophysical parameters of very high relevance, such as inland waterbodies, soil moisture, vegetation, ocean winds, sea ice extension and type, etc.