CERTO scientists have developed an online system to improve the accuracy of above-water measurements of remote sensing reflectance (Rrs) by removing residual surface-reflected skylight.
Remote sensing reflectance (Rrs) is an essential data for water colour remote sensing studies to support algorithm development and atmospheric correction validation. Rrs measured using the above-water approach usually suffer the influences from reflected skylight from water surface, which will largely influence the accuracy of measured Rrs. Therefore, correction and removal of the residual reflected skylight for Rrs measured from above-water approach is necessary before using it for algorithm development and satellite product validation.
During the CERTO project, an online bio-optical processing system (https://fortheralandingzone.stir.ac.uk/bio_optic/) was developed providing a method to remove the residual reflected skylight from above-water measured Rrs, this online tool is free for any researchers. This method is based on the Rrs peak at 810 nm, which is induced by a reduction of the absorption of pure water at this wavelength. When there are more particles (phytoplankton, sediments) in the water, this peak will be higher. Studies have shown that there is a very good non-linear relationship between the height of this peak and Rrs at ~750 nm, and this relationship is used for Rrs correction in the method in the online processing system. More details can be found in Jiang et al. (2020).
Below is an example of the Rrs before (top left) and after (bottom left) correction for a transect measured on 13 September 2021 during CERTO Plymouth campaign. It can be seen that after correction Rrs from same location cluster together, which is reasonable because water in the same location should have similar bio-optical properties. Legend of time in the map means the boat was moving from the North to the South, points are where the Rrs sampled.
