Spatial Resolution in GNSS-R under Coherent Scattering

• Published in: IEEE Geoscience and Remote Sensing Letters
• Main Author: Camps, Adriano
• Other Authors: Ministerio de Ciencia, Innovación y Universidades (España), European Commission, Generalitat de Catalunya
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: 2020
• Subjects: Coherent scattering; Fresnel zone; Global Navigation Satellite Systems Reflectometry (GNSS-R); Spatial resolution; Sea ice
• Online Access: http://hdl.handle.net/10261/236265; https://doi.org/10.1109/LGRS.2019.2916164; https://doi.org/10.13039/501100002809; https://doi.org/10.13039/501100000780

Global Navigation Satellite Systems Reflectometry can be understood as a multistatic 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 letter analyzes in more detail the spatial resolution in this forward scattering configuration, showing that, when coherent scattering is nonnegligible, 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. This work was supported in part by the Spanish Ministry of Science, Innovation and Universities through SPOT under Grant RTI2018-099008-B-C21, in part by EU FEDER, ICREA Academia Award from the Catalan Government, and in part by ESA/ESTEC under Grant 4000120299/17/NL/AF/hh.