# Latest Advances in the Global Navigation Satellite System--Reflectometry (GNSS-R) Field
## 4 Land Applications
### Land Reflection Point  
Unlike ocean applications, topography complicates the geolocation of the scattered GPS signal over land. Accurate geolocation of the specular point is an essential step for geophysical parameter retrievals over land, which has brought different studies in the field. In [105], the authors presented a methodology for geolocating and calibrating CYGNSS level 1 products over land. In addition, the authors provided an analysis of the spatial resolution link to coherent returns. The precise geolocation was achieved by employing a smooth ellipsoid model from the Earth combined with the information of the transmitting GNSS satellite and the precise location of the receiver, together with the terrain topography from the digital elevation map (DEM) derived from the shuttle radar topography mission (SRTM). The DEM was used to create a grid of points in the surroundings of the specular point location. Each grid pixel was then associated with a delay and Doppler value that were computed from the delay and Doppler value of the signal peak. Finally, the DEM was combined with the incident and reflection angle estimations to determine the suitability of the specular reflection. In [106], the authors developed an on-board reflection point prediction algorithm for GNSS-R satellites using a topographically accurate map. The algorithm allows researchers to predict the reflection point location over land. This is also useful for compression and calibration, making it ideal for small satellites or CubeSats.  
The study in [107] focused on understanding the Earth's topographic effects in order to obtain calibrated datasets. As in [99; 106], the authors in [108] employed topographical parameters derived from a DEM. The study considered the impact on grazing angles and nadir angles on a global scale. Some of the GNSS-R observables commonly used by the science community, such as the trailing edge slope and reflectivity, were evaluated in the manuscript. Flat surfaces with low topographic heterogeneity showed small trailing edge slopes and high reflectivity values, as expected, because of the increased coherency of the GPS signals. Interestingly, the findings in [108] showed that topographic features of rough surface contained information for a parametric analysis. The sensitivity of trailing edge slope and reflectivity to soil moisture was almost negligible, while the sensitivity of those metrics showed a stronger correlation with topographic wetness index. The authors in [109] investigated altimetry using spaceborne GNSS-R data which use different topographic models to consider the effects of large-scale slope for geometry computation. The methodology of [109] presented a novel geometry computation strategy based on inverse calculations of path geometries on an ellipsoid. The work was implemented using the DEM product from TanDEM-X 90 m, and the results showed that a surface slope of 0.6% at an elevation angle of 54 degrees resulted in a geolocation error of 10 km and a 50 m error determining the specular point height. The results of this study showed a large reduction on the standard deviation of specular point location errors from 4758 m to 367 m, and also a decrease in height error from 28 m to 5.8 m.