# First Evaluation of Topography on GNSS-R: An Empirical Study Based on a Digital Elevation Model  
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Centre Tecnologic de Telecomunicacions de Catalunya (CTTC/CERCA), 08860 Castelldefels (Barcelona), Spain 1  
Centre Tecnologic de Telecomunicacions de Catalunya (CTC/CERCA), 08860 Castelldefels (Barcelona), Spain 1
mapping and spatial patterns of wetness can be generated in catchments under the assumption that groundwater tables follow topography holds. The future of Earth observation in SMC determination is strongly based on the ability of the next generation of space-borne sensors to improve the Earth's surface spatio-temporal sampling properties. Global Navigation Satellite Systems Reflectometry (GNSS-R) appears in this framework, with an intrinsic capability to advance over more classical remote sensing approaches. GNSS-R is a sort of L-band passive bistatic radar (as many transmitters as navigation satellites are in view) that provides a wide swath up to \(\sim\) 1500 km. It was first conceived as an interferometric technique or iGNSS-R to provide higher precision in ocean altimetry measurements [3; 4]. However, it can also be used for land surfaces applications. In this scenario, the conventional GNSS-R technique or cGNSS-R is more appropriate because lower coherent and incoherent integration times are required, so that the associated spatial resolution is improved [5]. At present, several works have demonstrated the feasibility of SMC estimation using space-borne cGNSS-R data from the United Kingdom (U.K.) TechDemoSat-1 [6; 7; 8], Soil Moisture Active Passive (SMAP) [9], and Cyclone Global Navigation Satellite System (CyGNSS) [10; 11] missions. CyGNSS was selected by the National Aeronautics and Space Administration (NASA) in 2012 as a highly innovative Earth Venture space system, and it was launched into space on 15 December 2016 [12]. CyGNSS is an 8-microsatellite constellation. Each single CyGNSS satellite carries on board two down-looking antennas with left hand circular polarization (LHCP). Both antennas point to the Earth's surface with an elevation angle \(\theta_{\mathrm{e}}\) - 62 \({}^{\circ}\) on each side of the satellite ground track (\(\theta_{\mathrm{e}}\) - 90 \({}^{\circ}\)at the normal direction to the Earth's surface). The gain of the antennas is \(\sim\) 14.7 dB (antenna boresight). Topographic inhomogeneities can distort GNSS-R measurements [13; 14]. They have to be compensated to improve the accuracy in the retrieval of SMC. In a bistatic radar geometry, GNSS scattered signals can be collected along the specular direction (elevation angle of the incident wave equals elevation angle of the scattered wave i.e., \(\theta_{\mathrm{e}i}\)=\(\theta_{\mathrm{e}s}\)=\(\theta_{\mathrm{e}}\)). The scattering is only strong over an area around the nominal specular point. On the other hand, the scattering area depends on the local topography as well as on the GNSS satellites' elevation angles. Local slopes modify the scattering area as compared to that corresponding to a flat Earth assumption. The scattering area may be a complex and sometimes non-contiguous area on ground. However, it remains uncertain to what extent topography influences GNSS-R data and how these effects can be corrected. The main purpose of the present investigation was to further understand the effects of rough topography using data collected by CyGNSS. The counterpart of the radar pulse width in CyGNSS is \(\sim\) 300 m (1 C/A code chip). As such, reflected delay Doppler maps (DDMs) are potentially sensitive to the relative orientation of the scattering surfaces, but also to the vertical relief distribution. In the first part of this study, the influence of six different topographic parameters derived from the Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) model on DDMs derived from CyGNSS Level 1 Science Data Record Version 2.1 [12; 15; 16] was evaluated as a function of SMC derived from SMAP-enhanced L3 Radiometer Global Daily 9 km Level L3 SPL3 SMP_E Version 1.0 product.