Directly remote sensing plastic pollution within the open ocean has proven to be challenging. Therefore, other methods of detecting marine plastics through a proxy should be explored. Radar satellites are sensitive to surface roughness and can therefore detect dampening of wind-driven capillary waves on the sea surface. Plastics within the ocean have been found to be colonized by microbes, which can produce surfactants, substances which can dampen the short capillary waves on the ocean. This research investigates whether a ground radar instrument is capable of detecting plastisphere-based surfactant dampening within a semi-natural environment setting. We find that we can observe reduced backscatter from dampening effects that are occurring within our experiment setting. We also find that the backscattering from experiments involving plastisphere-based surfactants are significantly different from control experiments where microbial production is stopped.

Detection of marine litter is an urgent task given the large amount of plastics that end up in all water systems. This manuscript provides the description and results obtained from a ground-based Global Navigation Satellite System - Reflectometry (GNSS-R) system built in a controlled indoor environment intended to test the capabilities of GNSS-R techniques for marine litter detection, when different wave conditions and plastic litter were used. Results show that it may be possible to detect the damping of waves produced by the accumulations of some types of plastic.

Over 14 million tons of plastic end up yearly in the ocean, originating from polluted waterways such as rivers, sewers, or storm waters, or from industrial and nautical activities with improper garbage disposal methods. Due to its long decomposition time, and the presence of currents and gyres, most of plastic debris freely floating in the ocean end up forming large extensions of marine litter known as garbage patches. An example is the Great Pacific garbage patch located in the central North Pacific Ocean covering an area larger than 1.6 million squared kilometers. Some works have recently reported the potential of GNSS-Reflectometry to detect marine plastic litter from space. This study presents the results of a controlled field experiment conducted under the auspices of ESA at the "Atlantic Basin" at the Deltares research institute (Delft, The Netherlands).

The remote sensing of marine plastics is a relatively new field and research into radars capabilities for monitoring / detection is mostly limited. Here, we utilize the use of a C- and X-band ground radar to understand the capabilities for monitoring marine plastic pollution. Results show that backscattering differences can be found between reference acquisitions of clean water and test water filled with plastics, in both C- and X-band frequencies. With X-band detecting significant differences in backscattering in 48/68 test cases, and C-band detecting differences in 20/67 test cases.

Plastics in the riverine environment are of major concern due to their potential pathways into the wildlife and more generally the ocean. Dams are capable of trapping marine plastics within the riverine environment. This entrapment changes the surface roughness of the area with the marine debris. Radar satellites are sensitive to surface roughness and can therefore detect these changes. This research investigates areas in Serbia and Boznia & Herzegovina using ESA Sentinel-1 polarimetric SAR data. This study shows the feasibility of detecting large accumulations of plastic near dams, with detectors capable of achieving 75-85% positive detection ratings with a 0.1% false alarm rate. Additionally, we find the use of single VV polarization is inadequate for this task and PolSAR data are needed.

An approach that may be suitable to detect and quantify plastics in the ocean is the use of Global Navigation Satellite System-Reflectometry (GNSS-R). Measurements of the roughness of the sea surface, known as Mean Square Slopes (MSS) can be derived from GNSS-R. The measured MSS can be compared to the expected surface roughness derived from an empirical model, with the difference being computed as the ‘MSS anomaly’. It may be possible to use this MSS anomaly to identify regions where the sea surface roughness is lower than expected. A lower-than-expected sea surface roughness can be due to a variety of different physical phenomena, but may under certain circumstances be indicative of the location of floating matter, including plastic. We will present the preliminary analysis of the sensitivity of this MSS anomaly to the concentration of ocean plastic measured in-situ and predicted by ocean circulation models. This analysis of sensitivity to plastic will be carried out in the context of sensitivity to other parameters such as the wind stress and ocean swell.

Plastics within the ocean have been found to be colonised by microorganisms that, as a by-product of their metabolism, produce surfactants. Short capillary waves on the sea surface can get dampened due to the increased surface elasticity of these surfactants. Radar satellites are sensitive to surface roughness and can therefore detect the dampening of these waves. This research investigates areas inside the Atlantic, Pacific and Indian Ocean gyres using ESA Sentinel-1 and DLR TerraSAR-X data. We found out that we can observe several surfactant instances in the gyres and these are not correlated to medium or high level of chlorophyll. We can exclude that they have origin in biogenic slicks. Among other possible unknown origins, we hypothesise that these surfactants are produced from plastic concentrations within the ocean.

Microwave radiometers are widely used for remote sensing and radio astronomical observations. Their absolute accuracy depends on the performance of the blackbody targets which are used for the radiometric calibration. In this paper we summarize the performance aspects of such calibration targets and present different designs for upcoming ESA missions.

Polarimetric synthetic aperture interferometric radiometers are analyzed, including the polarization leakage and other front-end non-idealities that can be measured and represented with S-parameters. The analysis utilizes the noise wave concept in order to account for the amplitude and phase properties of the front-end. The method is applied to a model of an entire synthetic aperture interferometric radiometer. The simulations can be used to examine and retrieve requirements for the system parameters. The feasibility of the method is demonstrated and examples of end-to-end simulation results are also given in this paper.

Sub-millimetre remote sensing instruments can provide critical information on cirrus clouds and an alternative way of measuring precipitation with a much smaller antenna than those which microwave sensors currently use. Two satellite concepts CIWSIR and GOMAS were proposed as ESA Earth Explorer missions; these were not funded, however they were recommended for an aircraft demonstrator. ESA studies have been performed to identify the optimum instrument and platform to demonstrate these satellite concepts. This paper reports on one of these preparatory activities; the design of a sub-millimetre wave airborne demonstrator for both ice cloud and precipitation observations which will be able to prove the feasibility of the scientific principles of both satellite missions. The paper will describe the derivation of the demonstrator requirements, consideration of the available platform and instrument options, the design of the selected concept, performance prediction and the outline of a proof of concept flight campaign. It will present the outcome of the study which describes a demonstrator design based upon the new Met Office International Sub-Millimetre Airborne Radiometer (ISMAR).

The GEO atmospheric sounder (GAS) is a potential European future instrument, which utilizes interferometric synthetic aperture radiometry to obtain the desired spatial (30 km) and temporal (30 minutes) resolution for measurement of atmospheric temperature and humidity profiles under all weather conditions. This paper presents a simulation model for the entire imaging process. It enables detailed simulation of the impact of the instrument characteristics on the imaging performance. The model has been used as a tool to predict the performance of an instrument demonstrator under development.

This paper presents the concept and initial breadboarding results of geostationary atmospheric sounder (GAS), which is being developed by Saab Space AB and Omnisys AB, Sweden, and funded by European Space Agency (ESA). GAS utilizes interferometric synthetic aperture radiometry to obtain desired spatial (30 km) and temporal (nowcasting) resolution for measurement of atmospheric temperature and humidity profiles under all weather conditions. These parameters are decisively important to meteorological and climate models at all time scales.

Synthetic aperture radiometers with a rotating thinned array can effectively reduce the number of antenna units required for the applications with very high spatial resolution, like GEO atmospheric sounding. The rotating array results in non-regular distributed visibility samples. A grid-based measurement approach is proposed in this paper to facilitate the image reconstruction and system performance evaluation, especially the radiometric sensitivity computation.

This paper presents an overview of the microwave instruments currently in development under the Future Programmes of the ESA's Earth Observation Envelop Programme. The description of the instruments will be structured according to two broad categories of future missions, i.e. the Earth Explorer and Earth Watch candidates.