Experimental observations of marginally detectable floating plastic targets in Sentinel-2 and Planet Super Dove imagery

Remote sensing applications are garnering much attention as a promising solution for detection, tracking and monitoring of floating marine litter (FML). With an increasing number of studies portraying the technical feasibility of FML detection, we attempt here to experimentally observe a minimum detectable abundance fraction of floating plastic (white HDPE sheets), in a Sentinel-2 and PlanetScope SuperDove pixel. Such a threshold can set a baseline for detectability in terms of pixel-based spectral classification methodologies, and can be especially relevant for low-FML-concentration areas such as the Northeastern Mediterranean. We constructed and deployed artificial targets comprising of 1, 2 and 3 m² of floating white HDPE sheets. We acquired Sentinel-2 and SuperDove data of the target deployment area, along with ancillary data which assists with imagery interpretation. The data is atmospherically corrected (ACOLITE v.20221114) and a spectral separability analysis is performed using the spectral angle distance metric, to assess the possibility of spectrally discriminating the FML targets from water pixels in the scene. Results show that the detection threshold is above 3 m² for the Sentinel-2 satellite, while the SuperDove’s higher spatial resolution results in spectral angles between the FML targets and water pixels in the scene which show marginal separability for the 2 and 3 m² HDPE targets. When applying a partial unmixing detection algorithm using a previously acquired signature, we could detect the 3 m² target in both the Sentinel-2 and SuperDove images, but with commission errors that render the feasibility of practical application of such low FML concentrations detection questionable.