# Impact of the Elevation Angle on CYGNSS GNSS-R
## 8 Conclusions  
The impact of \(\theta_{\mathrm{e}}\) on CyGNSS GNSS-R bistatic reflectivity \(\Gamma_{\mathrm{LHCP,CyGNSS}}\) was assessed over the Earth's surface with a focus on land as a function of soil moisture content (SMC) and effective surface roughness parameter h. Global scale results show that \(\Gamma_{\mathrm{LHCP,CyGNSS}}\) decreased with decreasing \(\theta_{\mathrm{e}}\) down to \(\approx\)55\({}^{\circ}\) because of the effect of the incoherent scattering term \(\sigma_{\mathrm{LHCP,incoh}}^{0}\). On the other hand, it increased for lower \(\theta_{\mathrm{e}}\). This could be explained because of a higher signal coherence that increased the coherent scattering term \(\sigma_{\mathrm{LHCP,coh}}^{0}\) in regions without a rough topography. This functional relationship depended on SMC and h. Also at a global scale, it appeared that the dynamic range of \(\Gamma_{\mathrm{LHCP,CyGNSS}}\) at a fixed \(\theta_{\mathrm{e}}\) was higher as h decreased. The sensitivity of \(\Gamma_{\mathrm{LHCP,CyGNSS}}\) to SMC was then studied as a function of these two parameters for different sparsely vegetated target areas. The optimum Pearson correlation coefficients were found to be \(\mathrm{r_{Pampas}}\approx 0.78\) at \(\theta_{\mathrm{e}}\approx\) [60,70]\({}^{\circ}\), \(\mathrm{r_{India}}\approx 0.72\) at \(\theta_{\mathrm{e}}\approx\) [50,60]\({}^{\circ}\), and \(\mathrm{r_{Sudan}}\approx 0.74\) at \(\theta_{\mathrm{e}}\approx\) [30,40]\({}^{\circ}\), for different mean levels of roughness \(\mathrm{h_{Pampas}}\approx 0.1\), \(\mathrm{h_{India}}\approx 0.11\), and \(\mathrm{h_{Sudan}}\approx 0.15\). At these angular ranges, the slopes of the linear regressions over these target areas were \(\approx\)26 dB/(m\({}^{3}\)/m\({}^{3}\)), \(\approx\)37 dB/(m\({}^{3}\)/m\({}^{3}\)), and \(\approx\)50 dB/(m\({}^{3}\)/m\({}^{3}\)), respectively. The empirically-derived impact of \(\theta_{\mathrm{e}}\) on the relationship of \(\Gamma_{\mathrm{LHCP,CyGNSS}}\) with SMC and h could provide useful information in the development of first order corrections of surface roughness. As a final remark, a better sensitivity of GNSS-R to SMC appeared over croplands when the coherent scattering term \(\sigma_{\mathrm{LHCP,coh}}^{0}\) became dominant over the incoherent one \(\sigma_{\mathrm{LHCP,incoh}}^{0}\). Therefore, SMC retrieval using GNSS-R could benefit on the higher spatial resolution of the coherent scattering regime, limited by the first Fresnel zone.  
CyGNSS is a good example of how a constellation of small satellites could provide complementary information to a classical satellite such as SMAP, with a higher spatiotemporal sampling in the latitudinal band \(\approx\) [-40,40]\({}^{\circ}\). The revolution of small satellites, such as CubeSats, allowing for the provision of scientifically valuable data has just started. Future research on deployable antennas would provide a chance to elucidate whether such low-cost, small-size satellites could provide an accuracy on the same order than a classical satellite. On the other hand, passive microwave Earth observation techniques (e.g., microwave radiometry, GNSS-R) fit quite well on a single CubeSat [56], enabling the use of low power-consumption instruments that pave the way to improve the operational duty-cycle. As a matter of fact, the spatial resolution of GNSS-R is \(\approx\)1 km under the coherent scattering regime. This can clearly improve the resolution of microwave radiometry (SMAP) \(\approx\)40 km.  
Conceptualization, Methodology, Software, Investigation, and Writing-Original Draft Preparation by H.C.-L; Supervision, by [PERSON] and M.C. This research received no external funding. This research received no external funding. This research was carried out with the support grant of a [PERSON] research fellowship from the Spanish Ministerio de Economia, Industria y Competitividad (MINECO), reference FJCI-2016-29356.  
The authors declare no conflict of interest.