Development of detection indexes based on MTG-FCI data with explicit consideration of aerosols
Task 1: Development of the sargassum detection indexes
Sargassum algae, like all photosynthetic species, exhibit specific reflectance spectra around the red- edge and near-infrared bands. Around 600 nm (red-edge bands), these species absorb almost all of the radiation while displaying a reflectance peak around 800 nm (near-infrared). On the contrary, the ocean’s surface water exhibits quasi-constant reflectance over these bands.
The construction of a sargassum detection index is based on these properties and typically computes a difference between the sargassum and ocean reflectance, referred to as the ‘Floating Area Index’ (FAI) described by Hu (2009). This index is based on a combination of 3 bands, one in the red (640 nm for MTG-FCI), one in the NIR (865 nm for MTG-FCI) and one in the SWIR (1610 nm for MTG-FCI) with the advantage of being less sensitive to aerosols, to changes in observation conditions, and makes it possible to observe sargassum through fine clouds. In addition, there is a proportional relation between the index value and the fractional coverage, giving a quasi-direct estimate (after calibration) of a quantitative coverage variable. It will therefore be possible to evaluate and compare with other sensors such as MODIS or OLCI. Minghelli et al (2021) showed the possibility of using a NDVI index to retrieve sargassum beds from geostationary GOES-East with satisfactory results. This widely used index is computed from the combination of the surface reflectance in the red (640 nm on MTG-FCI) bands and near-infrared (NIR; 865 nm on MTG-FCI) bands with the advantage of reducing the temporal noise due to directional effects but with the limitations of being very sensitive.
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- Hu, C. (2009). A novel ocean color index to detect floating algae in the global oceans. Remote Sensing of Environment. https://doi.org/10.1016/j.rse.2009.05.012
- Minghelli, A., Chevalier, C., Descloitres, J., Berline, L., Blanc, P., & Chami, M. (2021). Synergy between Low Earth Orbit (LEO)—MODIS and Geostationary Earth Orbit (GEO)—GOES Sensors for Sargassum Monitoring in the Atlantic Ocean. Remote Sensing. https://doi.org/10.3390/rs13081444