# Aerial Remote Sensing of Aquatic Microplastic Pollution:
## 4 Discussion and Recommendations
In the case of satellite remote sensing, there is also the question of what happens to such a weak signal as it travels all the way up through the atmosphere. Despite two theoretical studies expressing skepticism about the feasibility of detecting aquatic MPs using satellite-borne optical sensors [23, 24], it is encouraging that two more recent studies published in 2024 [25, 26] reported having succeeded in doing so by way of correlation analyses between remotely sensed reflectance levels and in situ-sampled MP concentrations. Significant correlations were found in spectral bands near known absorption peaks of MPs in the NIR and SWIR regions, which seems to lend credence to the results. Conspicuously absent from the published literature is any reported attempt to remotely sense aquatic MPs from closer proximity to the Earth's surface in much finer spatial resolution imagery, as could be acquired by low-altitude aircraft. Such attempts would be worthwhile given the steep challenges of detecting aquatic MPs with satellite-borne optical sensors and the need to better verify and validate findings such as those reported by the recent correlative studies. [PERSON]'s [23] estimate of 0.005% pixel coverage by aquatic MPs in satellite imagery assumed an average particle size of 2.5 mm and a uniform distribution of particles across the water surface area at their highest reported ocean concentration. If these assumptions are maintained while decreasing pixel size (i.e., increasing spatial resolution by decreasing image acquisition height), the % pixel coverage will only begin to increase when the spatial resolution becomes fine enough that a single 2.5-mm particle exceeds 0.005% of the pixel area--corresponding to a \(\sim\)35-cm pixel size--beyond which the coverage will progressively increase with further decreasing pixel size. A 5-cm pixel size would result in 0.25% coverage, exceeding [PERSON]'s [23] estimate of at least 0.2% coverage required for aquatic MP detection by high-fidelity visible-NIR sensors. A 1-cm pixel size would result in 6.25% coverage, far exceeding the detection threshold. With the additional use of a SWIR-capable sensor, the plausibility of MP detection begins to seem quite likely. Such spatial resolutions can most conveniently and economically be achieved by means of small drone-based remote sensing, as systems carrying high-performance visible-SWIR hyperspectral sensors are newly operational [40, 41]. Thus, a sensible next step in investigating the feasibility and developing the capability of aerial optical remote sensing of aquatic MPs would be to conduct experiments at a much finer scale than reported to date, using drone-borne hyperspectral imaging. Such experiments could conceivably be performed outdoors under semi-controlled conditions (e.g., artificial reservoirs in which MPs are introduced), followed by trials in natural systems. The use of drones would enable convenient assessment of the detectability and spectral signal characteristics of MPs at varying heights above ground (subject to local airspace regulations) and, by extension, provide a currently lacking empirical baseline to model the upscaling of aquatic MP optical remote sensing from higher altitudes, helping to pave the way for the use of crewed aircraft and satellites that can survey much larger areas. Such experiments may also prompt development of the operational capacity to perform drone-based remote sensing of aquatic MPs in the environment over modest-size areas, for example, in freshwater systems, short distances offshore, or small areas at sea if launched from boats. It should be emphasized that this approach is limited to detecting MPs at or near the water surface (down to \(\sim\)15 cm in depth), which represent only a portion of environmental MPs in the water column [42]. The depth at which plastics occur depends on their density relative to that of water (or seawater), which varies by plastic type.