# First Evaluation of Topography on GNSS-R: An Empirical Study Based on a Digital Elevation Model

Centre Tecnòlogic de Telecomunicacions de Catalunya (CTTC/CERCA), 08860 Castelldefels (Barcelona), Spain

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Received: 16 September 2019 / Revised: 11 October 2019 / Accepted: 29 October 2019 / Published: 31 October 2019

(This article belongs to the Special Issue Applications of GNSS Reflectometry for Earth Observation)

Understanding the effects of Earth’s surface topography on Global Navigation Satellite Systems Reflectometry (GNSS-R) space-borne data is important to calibrate experimental measurements, so as to provide accurate soil moisture content (SMC) retrievals. In this study, several scientific observables obtained from delay-Doppler maps (DDMs) $\u27e8{{\displaystyle \left|{\mathrm{Y}}_{\mathrm{r},\mathrm{t}\mathrm{o}\mathrm{p}\mathrm{o}}(\mathsf{\tau},\mathrm{f})\right|}}^{2}\u27e9$ generated on board the Cyclone Global Navigation Satellite System (CyGNSS) mission were evaluated as a function of several topographic parameters derived from a digital elevation model (DEM). This assessment was performed as a function of Soil Moisture Active Passive (SMAP)-derived SMC at grazing angles ${\mathsf{\theta}}_{\mathrm{e}}$ ~ [20,30] ° and in a nadir-looking configuration ${\mathsf{\theta}}_{\mathrm{e}}$ ~ [80,90] °. Global scale results showed that the width of the trailing edge (TE) was small $\mathrm{T}\mathrm{E}$ ~ [100, 250] m and the reflectivity was high $\mathsf{\Gamma}$ ~ [–10, –3] dB over flat areas with low topographic heterogeneity, because of an increasing coherence of Earth-reflected Global Positioning System (GPS) signals. However, the strong impact of several topographic features over areas with rough topography provided motivation to perform a parametric analysis. A specific target area with little vegetation, low small-scale surface roughness, and a wide variety of terrains in South Asia was selected. A significant influence of several topographic parameters i.e., surface slopes and curvatures was observed. This triggered our study of the sensitivity of $\mathrm{T}\mathrm{E}$ and $\mathsf{\Gamma}$ to SMC and topographic wetness index ( $\mathrm{T}\mathrm{W}\mathrm{I}$ ). Regional scale results showed that $\mathrm{T}\mathrm{E}$ and $\mathsf{\Gamma}$ are strongly correlated with the $\mathrm{T}\mathrm{W}\mathrm{I}$ , while the sensitivity to SMC were almost negligible. The Pearson correlation coefficients of $\mathrm{T}\mathrm{E}$ and $\mathsf{\Gamma}$ with $\mathrm{T}\mathrm{W}\mathrm{I}$ are ~ 0.59 and ${\mathrm{r}}_{\mathrm{T}\mathrm{E}}$ ~−0.63 at ${\mathsf{\theta}}_{\mathrm{e}}$ ~ [20, 30] ° and ${\mathrm{r}}_{\mathsf{\Gamma}}$ ~ 0.48 and ${\mathrm{r}}_{\mathrm{T}\mathrm{E}}$ ~ −0.50 at ${\mathsf{\theta}}_{\mathrm{e}}$ ~ [80, 90] °, respectively.
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*Keywords:*GNSS-R; CyGNSS; SMAP; topography; digital elevation model; elevation angle; soil moisture content; topographic wetness index; bistatic scattering

This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited

**MDPI and ACS Style**

Carreno-Luengo, H.; Luzi, G.; Crosetto, M. First Evaluation of Topography on GNSS-R: An Empirical Study Based on a Digital Elevation Model. *Remote Sens.* **2019**, *11*, 2556.

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