Italian Research Makes Important First Steps in Detecting Floating Plastic Pollution with Satellite SAR Imagery

By Kevin P. Corbley

Plastic debris floating in the oceans and other water bodies is a dangerous and growing problem. Even though water currents tend to conglomerate smaller plastic objects, such as bottles and packing materials, into large “islands” of debris, finding this form of water pollution in an effective way has proved elusive…until now. Italian researchers have taken an important first step in developing a practical technique for detecting floating plastics in open water.

“The idea was to try to understand whether we can observe floating plastic using satellite instruments,” said Prof. Ferdinando Nunziata at Sapienza University in Rome. “It’s very challenging.”

Although challenging, plastic pollution is a problem that must be dealt with. Plastic objects typically end up in water bodies either by direct littering or storm runoff from land. As is true with any pollutants, they cause chemical, physical, and biological stress on the marine ecosystem. Most alarmingly, recent studies indicate plastic objects in water are breaking down into microplastics that can enter the food chain.

Prof. Nunziata explained that satellite remote sensing is an ideal method of finding floating plastics in oceans and seas because these water bodies are so vast, and satellites can capture data over broad areas relatively quickly. He noted that experiments with optical imagery are severely limited by weather conditions.

The Italian research team theorized that satellite SAR imagery could complement optical at detecting floating plastics through a multi-sensor and multi-platform approach. Working in a Southern Italy lake, they placed a series of 2×2-meter plastic targets in the water. They then acquired X-band SAR data with the COSMO-SkyMed Second Generation constellation along with optical drone-based measurements over the study area.

The COSMO-SkyMed SAR instruments operate in several resolution-swath modes, and for this research, the Spotlight mode was selected. It provides the highest level of spatial resolution, nominally about 30 centimeters for civilian users, while orbiting at 600-kilometer altitude. This was considered ideal for finding small objects in specific areas of oceans and seas. The team also experimented with different polarizations of the emitted/returned radar signals.

“The plastic target appears as a brighter spot in the surrounding water in both the co-polarized and cross-polarized normalized radar cross section,” said Nunziata. “The co-polarized had the largest plastic-to-water contrast and the largest separation between the plastic and natural targets.”

Drones were used to validate the findings in the research areas. Since completion of the initial Southern Italy project discussed in the paper below, the team duplicated its success with co-polarized X-band SAR data in two other Italian lakes and in the ocean. However, they cautioned that additional studies will be needed to determine the accuracy with which the X-band SAR differentiates plastic from other natural debris floating in the water.

An interesting aspect of the projects is the process behind the ability of X-band microwave sensors to detect the plastics in water. Plastic, after all, is usually transparent to microwave signals, however, the researchers believe it is water droplets coating the floating objects that make them visible in the SAR image plane.

Much work remains to be done to build on the research results, explained Anna Verlanti, a PhD student at Università degli Studi di Napoli “Parthenope,” in Naples. Ultimately, the team conceives an operational version of their technique will involve satellites, drones, and ships. During the fieldwork completed to date, the drone used for validation of the satellite data was equipped with three different sensors – LiDAR, optical multispectral, and thermal.

“With the different sensors, we are able to understand the properties of the targets…we used heterogeneous targets made of different kinds of plastic,” she explained, adding the goal is to eventually assess floating debris by composition, whether natural or synthetic.

GRSS members can learn more about the floating plastics research by reading “Detecting Coastal Aggregation of Macro-Plastic Litter Using Satellite X-Band SAR Imagery,” which can be found at this link: ieeexplore.ieee.org/document/11441416/authors#authors.