# Impact of the Elevation Angle on CYGNSS GNSS-R
## 1 Introduction  
Small satellites [1] are changing the paradigm in Earth remote sensing, taking advantage of innovative payloads (e.g., References [2; 3; 4]). As such, the operation of constellations of these instruments has the potential to observe Earth's dynamic processes with a higher spatiotemporal sampling than traditional techniques. In particular, the so-called Global Navigation Satellite Systems Reflectometry (GNSS-R) [5] is a sort of L-band passive multi-static radar (as many transmitters as navigation satellites are in view) that provides a wide swath up to \(\approx\)1500 km [6; 7; 8]. GNSS-R spatiotemporal sampling properties could open new process insights on mesoscale studies [6], wind speed determination [9], soil moisture content (SMC) determination [10], and vegetation water content (VWC) monitoring. There are several on-going research activities dedicated to improve the spatiotemporal sampling properties of GNSS-R. The main parameters being evaluated are the number of in-orbit receivers, the use of beam-forming strategies, and the directivity of the down-looking antennas [6]. Several methods have been proposed to extract the geophysical information added tothe GNSS signals in the scattering process over the Earth's surface: (a) The Passive Reflectometry and Interferometry System (PARIS) or interferometric GNSS-R (iGNSS-R) that can provide higher precision in ocean altimetry measurements, but requires the use of high-gain antennas [6, 8]; (b) the conventional GNSS-R (cGNSS-R) that requires only relatively low-gain antennas for scatterometry purposes [8]; and (c) the GPS interferometric reflectometry (GPS-IR) that can determine accurately SMC with potential application to the global GNSS network [11]. At present, several small satellites have been launched into space carrying a GNSS-R payload on-board, including the United Kingdom (U.K.)-Disaster Monitoring Constellation DMC-1 [2] in 2003, the U.K.-TechDemoSat TDS-1 [3] in 2014, and the Universitat Politecnica de Catalunya UPC's \({}^{3}\)Cat-2 in 2016 [4]. In 2012, the National Aeronautics and Space Administration (NASA) selected the CyGNSS mission led by the University of Michigan as a low cost and high science Earth Venture Space System [12]. CyGNSS, an eight-microsatellite constellation, was launched on December 15, 2016 into a low Earth orbit (LEO) with an inclination angle \(\approx 35^{\circ}\)[13]. The main objective of this mission is to estimate wind speed over tropical cyclones with an unprecedented spatiotemporal sampling of the ocean [14]. Each single CyGNSS microsatellite has two left-hand circular polarization (LHCP) down-looking antennas pointing to the Earth's surface with an inclination angle of \(\approx\)28\({}^{\circ}\) on each side of the satellite ground track. The antennas' gain is \(\approx\) 14.7 dB (antenna boresight), and the main payload is the so-called Delay Doppler Mapping Instrument (DDMI), which generates delay Doppler maps (DDMs) [8] on-board using a coherent integration time T\({}_{\rm c}\) = 1 ms, and applying an incoherent average over N\({}_{\rm inc}\) = 1000 samples. The application of active (e.g., synthetic aperture radar (SAR)) and passive (e.g., microwave radiometry and GNSS-R) remote sensing techniques over land-surfaces is based on the variability of the dielectric properties due to changes in SMC and biomass. However, surface roughness significantly affects SMC retrieval, especially in the case of GNSS-R [15, 16, 17] and SAR [18, 19, 20]. In this work, the sensitivity of GNSS-R to SMC, and the effect of surface roughness are assessed as a function of the elevation angle \(\theta_{\rm e}\) (\(\theta_{\rm e}\) = 90\({}^{\circ}\) at the normal direction to the Earth's surface) [21]. This is a first step towards SMC retrieval using GNSS-R multi-angular information [22] because the ratio of the coherent-to-incoherent scattering components [23] depends on the surface roughness through \(\theta_{\rm e}\).