# GNSS-R Observations of Marine Plastic Litter in a Water Flume: An Experimental Study
## 3 Results
### Clean Roughened Water Surface with and without Capillary Waves  
Figures 8 and 9 show the results for clean roughened water surfaces for 30\({}^{\circ}\) elevation angle, with and without capillary waves.  
In Figure 8 the sea surface follows the JONSWAP spectrum with a rms height of 5 cm. The time series of the RL and RR reflected signals exhibit a sharp periodic fading associated to the water surface waves, and the PSD shows a peak at T = 0.86 s, in agreement with the measured sea surface spectrum (Figure 5). Direct signal multi-path impact (Figure 8a, green line) is marginal as compared to the fluctuations of the RL and RR reflected signals (Figure 8b). Peaks above 50 Hz are due to EMI (Figure 8c). The PDF of the amplitude reflection coefficients (in dB) approximates to a log-normal (x-reversed and shifted, Figure 8d). Phase temporal series also show peaks (Figure 8e), but interestingly they are not correlated between polarizations, and histograms (Figure 8h) depart from a Gaussian shape observed in the flat surface case (not shown), and has a PDF reminiscent of a Rayleigh one.  
The impact of capillary waves is analyzed in the following figures. As in the previous figures, Figure 9 shows for clean roughed water surface and a significant wave height = 5 cm with capillary waves, and an elevation angle of 30\({}^{\circ}\) the time evolution (Figure 9a), the multi-path sanity check (Figure 9b), and the spectra (Figure 9c). In Figure 9c, a secondary peak (-200 Hz) appears, which can be attributed to EMI. The PDF of the reflection coefficients (Figure 9d) is \"flattened\" with respect to Figure 8d (no capillary waves case); its excursion is the nearly the same, and both the kurtosis and the skewness have notably decreased.  
Figure 10 shows the PDFs of the reflection coefficients amplitude [dB] and phase [deg] for clean roughed water surface (significant wave height = 9 cm) without capillary waves and elevation angles of 30\({}^{\circ}\), 45\({}^{\circ}\), and 60\({}^{\circ}\).  
In the amplitude (Figure 10a,c,e), the effect of the elevation angle is the opposite at RL and RR polarizations: while at RL polarization the kurtosis (k) and the skewness (sk) increase, most notably from 45\({}^{\circ}\) to 60\({}^{\circ}\) elevation angles (k from 3.2-3.6 to 6.1, and sk from \(-\)0.5--0.7 to \(-\)1.6), indicating a transition from a Gaussian behavior to a more Ricean one, i.e., with a stronger coherent component, at RR polarization it is the opposite, but it occurs mostly from 30\({}^{\circ}\) to 45\({}^{\circ}\) (k from 10.4 to 2.5-2.8, and sk from \(-\)1.9 to \(-\)0.2--0.8), showing a decrease of the coherent component and an increase of the incoherent one.  
In the phase (Figure 10d,f), the behavior is more similar at both RL and RR polarizations, showing a transition between 45\({}^{\circ}\) to 60\({}^{\circ}\) elevation angles, when the mean and standard deviation significantly increase, and the kurtosis and skewness rapidly drop (e.g., k drops from 26.6 and 31.2 at 45\({}^{\circ}\) elevation angle to 4.6 and 3.0 at 60\({}^{\circ}\) elevation angle).  
Finally, Figure 11 shows the time series and PDFs of the amplitude of the reflection coefficients [dB] as in Figure 11, but for a clean sinusoidal water surface (significant wave height = 9 cm) without capillary waves, and elevation angles of 30\({}^{\circ}\), 45\({}^{\circ}\), and 60\({}^{\circ}\). Although during the planning of the field experiment using these as reference profiles was considered, the observed quasi-periodic behavior is obviously too artificial, and clearly not representative of the natural conditions. In all the subsequent tests with floating debris, the JONSWAP spectrum was used.  
Figure 9: Clean roughed water surface (significant wave height = 5 cm) with capillary waves. Plots description are the same as Figure 8.