# GNSS-R Observations of Marine Plastic Litter in a Water Flume: An Experimental Study
## 5 Conclusions  
This manuscript has described the field experiment conducted by UPC at the Deltares (Delft, The Netherlands) \"Atlantic Basin\", as part of the GLIMPS project led by DEIMOS UK for ESA, to study the potential of GNSS-Reflectometry for marine plastic litter detection from space. The experimental set-up, data acquired, the data processing, and the main difficulties encountered have been presented. A novel approach to process GNSS-R observables has been presented. It is based on the change of some statistical descriptors of the amplitude and phase histograms of the measured instantaneous reflectivity. These observables are:  
* The standard deviation of the estimated reflectivity (or received signal power, or DDM peak), which decreases sharply when passing from flat to rough surface, increases a bit when there are capillary waves, and in general increases when there is marine litter;
* The standard deviation of the phase (phase of peak of DDM, computed at 1 ms coherent integration time, and without any incoherent averaging) can also be used;
* The kurtosis and skewness can also be used, as they vary very significantly. In the RL amplitude histogram the kurtosis and the skewness increase, while at RR the kurtosis decreases, and the skewness increases. \begin{table}
\begin{tabular}{l l l l} \hline Feature & Amplitude & & & \\ \hline \hline \(\Delta\) disturbance & - & Surface roughness \(\uparrow\Rightarrow\sigma(\Delta\Gamma_{\text{R}})\) - 9 dB & - & Sharp decrease \(\sigma_{\Delta\uparrow\text{RTI}}\) with increasing rms height and \\  & - & Capillary waves \(\uparrow\Rightarrow\sigma(\Delta\Gamma_{\text{R}})\) - 1.1-5 dB & presence of capillary waves \\  & - & Kurt varies in both cases & kurtosis and skewness \(\uparrow\) long waves \\ \hline \hline \(\Delta\) disturbance & - & Different temporal behavior & - & \(\sigma_{\Delta\uparrow\text{RTI}}\) smaller for sinusoidal \\  & - & \(\sigma_{\Delta\uparrow\text{RTI}}\) - 0.2-0.4 dB & - & No clear trend \\  & - & \(\sigma_{\Delta\uparrow\text{RTI}}\) - 2.5-3 dB with net and bottles, & \(\sigma_{\Delta\uparrow\text{RTI}}\) \(\uparrow\) 0.6-0.8\({}^{*}\) \\  & - & \(\sigma_{\Delta\uparrow\text{RTI}}\) - 0.4 dB with net only & - & Marginal effect on other observables \\  & - & Kurtosis \(\uparrow\) & \(\sigma_{\Delta\uparrow\  
The use of the first one requires an accurate absolute calibration, which may be difficult to achieve in real GNSS-R instruments. The second one is also impractical in spaceborne instruments, as significant incoherent averaging is required to increase the carrier-to-noise ratio. The kurtosis and skewness variations offer, in principle, the capability to detect anomalous reflectivity behavior with respect to the reference cases (same marine litter, same wave and observation conditions). Under the experimental conditions presented, the types of marine litter that can possibly be detected are bottles and fixed nets, food wraps and bags, and nets, as they are the ones that dampen the most the long waves of the water surface. The extrapolation of these results to the airborne and spaceborne cases is not straightforward and it has to be done with care, as the results presented here have been obtained for 1 ms coherent integration time, without incoherent averaging, and do not include the following effects:  
* The different surface roughness in the gyres, regardless of the presence-or not-of marine litter;
* The presence of biofouling and associated increased wave damping;
* The spatial averaging linked to the higher platform height. In the airborne case as the first Fresnel zone from a 1 km height GNSS-R receiver is \(\sim\)10 m, which is of the order of magnitude of the averaging time (seconds to tens of seconds) times the speed of propagation of the waves in the flume. However, in the spaceborne case, because from LEO the First Fresnel zone is \(\sim\)300-500 m (depending on the incidence angle), the averaging should be conducted over much longer periods of time.