# Impact of the Elevation Angle on CYGNSS GNSS-R
## 5 Global Scale Reflectivity over Land and Ocean  
The focus of this work is on the study of \(\Gamma_{\text{LHCP,CyGNSS}}\) over land surfaces. As a first step, the evolution of \(\Gamma_{\text{LHCP,CyGNSS}}\) is shown for three different ranges of \(\theta_{\text{e}}\) over the complete coverage of the Earth's surface provided by CyGNSS (Figure 2). Reflectivity data \(\Gamma_{\text{LHCP,CyGNSS}}\) were averaged over one month (from 20 September to 20 October 2017) using a 0.1\({}^{\circ}\)\(\times\) 0.1\({}^{\circ}\) latitude/longitude grid with a moving window of 0.2\({}^{\circ}\). The spatial scale of the roughness parameter h and SMC is \(\approx\)9 km. The size of the CyGNSS's footprint under the coherent scattering regime at \(\theta_{\text{e}}\approx 55^{\circ}\) was \(\approx\)1 km across-track, and \(\approx\)7.6 km along-track. Thus, the gridding strategy enables the analysis using different sensors with different spatial resolution. This strategy also allows to determine the evolution of \(\Gamma_{\text{LHCP,CyGNSS}}\) over a wider angular range and provides good sampling density at pixel level. This gridding was found to provide an improved correlation between CyGNSS reflectivity and the SMAP microwave brightness temperature [43]. Additionally, the application of this moving averaging filter minimized potential residual errors in the down-looking antenna gain pattern correction due to attitude determination and control system (ADCS) and in the estimation of the reflected Y\({}_{\text{r,Peak,LHCP}}\) and direct Y\({}_{\text{d,Peak,RHCP}}\) power waveforms peaks. Figure 2: _Cont._  
The averaged \(\Gamma_{\rm LHCP,CyGNSS}\) is displayed over land (Figure 2a,c,e) and ocean (Figure 2b,d,f) for different elevation angles in ranges of \(\theta_{\rm e}\approx\) [70,90]\({}^{\circ}\) (Figure 2a,b), \(\theta_{\rm e}\approx\) [45,65]\({}^{\circ}\) (Figure 2c,d), and \(\theta_{\rm e}\approx\) [20,40]\({}^{\circ}\) (Figure 2e,f). Over land-surfaces (Figure 2a,c,e), there was a strong coherent scattering \(\sigma^{0}_{\rm LHCP,coh}\) and the width of the waveforms was much narrower than over the ocean. In this situation, the power fluctuations of \(Y_{\rm r,Feak,LHCP}\) could be closely related to \(\sigma^{0}_{\rm LHCP,coh}\) at some areas. Additionally, the spatiotemporal variability of the permittivity over land surfaces was high. This explains the significant variability of \(\Gamma_{\rm LHCP,CyGNSS}\) (Figure 2a,c,e). Overall (Table 1), it appears \(\Gamma_{\rm LHCP,CyGNSS}\) increased from \(\theta_{\rm e}\approx\) [70,90]\({}^{\circ}\) to \(\theta_{\rm e}\approx\) [20,40]\({}^{\circ}\): \(\Gamma^{\rm ov^{\prime}<\theta_{\rm e}<\delta\
u}_{\rm Land}\approx-15.8\) dB, \(\Gamma^{\rm ov^{\prime}<\theta_{\rm e}<\delta\
u}_{\rm Ocean}\approx-12.1\) dB, \(\Gamma^{\rm ov^{\prime}<\theta_{\rm e}<\delta\
u}_{\rm Ocean}\approx-19.4\) dB, and \(\Gamma^{\rm ov^{\prime}<\theta_{\rm e}<\delta\
u}_{\rm Ocean}\approx-14.1\) dB. These experimental results can be explained because of the strong impact of \(\theta_{\rm e}\) on \(\Gamma_{\rm LHCP,coh}\). The coherent scattering was very high for grazing angle, because the effective surface roughness was low [51]. As such, the reflectivity was much higher at \(\theta_{\rm e}\approx\) [20,40]\({}^{\circ}\) than at \(\theta_{\rm e}\approx\) [70,90]\({}^{\circ}\). On the other hand, \(\Gamma_{\rm LHCP,CyGNSS}\) decreased from \(\theta_{\rm e}\approx\) [70,90]\({}^{\circ}\) to \(\theta_{\rm e}\approx\) [45,65]\({}^{\circ}\) because the incoherent scattering is dominant over the coherent one in this angular range: \(\Gamma^{\rm es^{\prime}<\theta_{\rm e}<\delta\
u}_{\rm Land}\approx-17.7\) dB and \(\Gamma^{\rm es^{\prime}<\theta_{\rm e}<\delta\
u}_{\rm Ocean}\approx-20.3\) dB. The incoherent or diffuse scattering was lower with decreasing \(\theta_{\rm e}\) because this term is generated by the effective surface roughness, which in turn decreased for lower \(\theta_{\rm e}\). This point explains the evolution of \(\Gamma_{\rm LHCP,CyGNSS}\) along this angular range.