# GNSS-R Observations of Marine Plastic Litter in a Water Flume: An Experimental Study
## 1 Introduction  
As part of the \"Global Monitoring of Microplastics using GNSS-Reflectometry\" (GLIMPS) project led by Deimos Space UK, the Universitat Politecnica de Catalunya conducted a ground-based GNSS-R experiment in a controlled water flume: the \"Atlantic Basin\" at Deltares' research premises (Delft, The Netherlands) under different wave conditions, and with different plastic types and concentrations.  
Some works have recently reported the potential of GNSS-Reflectometry to detect marine plastic litter [1]. Although it is highly unlikely that at the GNSS L1 frequency the change in the dielectric constant could produce a detectable effect in the received signal through a change in the reflection coefficient, the presence of the plastics themselves could produce a detectable effect through a change in the surface roughness, either by damping the longer waves, or even by creating new capillary waves (\"ripples\") when the wavefronts are distorted by the presence of the floating litter. This effect will likely be largely dependent on the size of the plastics used in the experiment, the wave's characteristics, and the geometry of observation. These effects have been the object of the controlled empirical study presented in this work, with the main limitation (or advantage) being that the spatialresolution is given by the size of the first Fresnel zone, because of the dominant coherent scattering conditions [2; 3].  
In an open ocean scenario, the high bistatic reflectivity signatures observed in CYGNSS GNSS-R data may not be only due to the presence of the microplastics themselves. There is evidence that, for example, low surface roughness areas are linked to the presence of phytoplankton on the ocean surface [4]. It is likely that the biofouling or accumulation of microorganisms, plants, or algae on the plastics' surfaces increases the concentration of surfactants, resulting in an increase of the wave damping. As plastics tend to accumulate in ocean current gyres, so do other surfactants and biofouling.  
All these effects: (1) different surface roughness in the gyres regardless of the presence or not of plastics (or even in a hurricane eye in an extreme case, e.g., Figure 1 in [5]); (2) presence of phytoplankton and associated increased wave damping [4]; and (3) presence of plastics damping large waves [1], but possibly creating new ripples, have to be evaluated to assess the feasibility of detecting marine plastic litter from space. In this controlled experiment in a water flume, the third item is addressed.  
This study describes the experiments conducted at the Atlantic Basin of the Deltares research institute (Delft, The Netherlands), during October 2021 [6]. It also describes the data processing, and the main conclusions, and it is structured as follows:  
* Section 2 contains a detailed explanation of the materials and methods: the experiment's setup, environment conditions (i.e., waves conditions and plastic types used),
* Section 3 presents the experimental results,
* Section 4 discusses the main results,
* Section 5 summarizes the main conclusions.