Spatial Resolution in GNSS-R Under Coherent Scattering

Abstract

GNSS-R can be understood as a multi-static radar using satellite navigation signals as signals of opportunity. The scattered signals over sea ice, flooded areas-even under dense vegetation-, and in some cases over land show a significant coherent component. Under coherent scattering conditions, it is usually stated that the coherent signal component comes from an area equal to the first Fresnel zone. This work analyzes in more detail the spatial resolution in this forward scattering configuration, showing that, when coherent scattering is non-negligible, the spatial resolution is mostly determined by the geometry, and not by typical surface roughness values. As the scattering area around the specular reflection point increases and encompasses the first Fresnel zone, the received power increases and then it fluctuates as higher order Fresnel zones are included (rapid phase changes due to the spherical waves). These contributions may explain in part the large scattering encountered over inhomogeneous land regions, as these different contributions add or subtract, depending on the phase of the electric field, and are weighted by different scattering coefficients (i.e. changes in the dielectric constant and/or surface roughness, such in water ponds, or some agricultural fields…). Finally, over homogeneous targets, when all Fresnel zones are included, the received power tends asymptotically to the value obtained using the free-space propagation with a total path length equal to the sum of the path lengths, weighted by the reflection coefficient. This value can also be interpreted as coming from an effective region that is actually ~0.6 times the first Fresnel zone.

Geolocation, Calibration and Surface Resolution of CYGNSS GNSS-R Land Observations

“…From the analysis presented in Figure 7 it is observed that this occurs, on average, for rivers with a minimum width exceeding 250 m and 200 m for 1 Hz and 2 Hz CYGNSS data, respectively. This places a minimum bound on the finest spatial features CYGNSS measurements are able to resolve, on average, suggesting that this is on the order of a first Fresnel zone and provides empirical support for previous theoretical treatments of this question [23]. However, there are several caveats to this analysis which assumes that rivers will generally satisfy the conditions to produce coherent CYGNSS forward reflections:…”

Desert Roughness Retrieval Using CYGNSS GNSS-R Data

“…The signal over land includes both coherent and incoherent scattering, with a relative weight that is subject to discussion [43,44]. The main observables related to the two mechanisms are the reflectivity and the Normalized Radar Cross Section, respectively.…”

An Aircraft Wetland Inundation Experiment Using GNSS Reflectometry

“…By mapping spatial changes in P cal , we can thus relate them to spatial changes in the surface reflectivity. Our dataset included GNSS-R reflections from the following PRNs: 3,7,8,9,11,14,16,22,23,26,27,28,30, and 31. Data were obtained from a range of incidence angles (min: 10.7 deg, max: 60 deg, mean: 34.7 deg).…”

“…It has been assumed that the received power from GNSS-R signals reflecting from water are coherent and follow the scattering model given by Equation 2 [11,23]. We attempted to verify this by selecting data over the open-water portions of Caddo Lake, which should show a relatively constant calibrated power level, and assessed whether P cal or P cal R 2 sr fit the data better.…”