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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) 〈∣∣∣Yr,topo(τ, f)∣∣∣2〉 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 θe~ [20,30] ◦ and in a nadir-looking configuration θe~ [80,90] ◦. Global scale results showed that the width of the trailing edge (TE) was small TE~ [100, 250] m and the reflectivity was high Γ~ [–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 TE and Γ to SMC and topographic wetness index (TWI). Regional scale results showed that TE and Γ are strongly correlated with the TWI, while the sensitivity to SMC was almost negligible. The Pearson correlation coefficients of TE and Γ with TWI are rΓ~ 0.59 and rTE~−0.63 at θe~ [20, 30] ◦ and rΓ~ 0.48 and rTE~ −0.50 at θe~ [80, 90] ◦, respectively.

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