Analysis of high frequency of remote sensing reflectances in optically complex waters for the next generation of hyperspectral sensors

The complex aquatic ecosystems, such as lakes, are generally characterized by a high degree of spatial and temporal changes. In particular, hourly and daily dynamics are evident due, for examples, to the growth/decrease of phytoplankton depending on light availability and to the variation of the suspended solids in wind-induced resuspension of the bottom sediments. Such a variability is then changing depending on location, so that variation in spatial patterns is also a typical feature of these ecosystems. Satellite images have been used widely since many decades to observe and to understand spatial and temporal variability of water constituents, while the exploration of hourly temporal variability is still limited to geostationary sensors, whose spectral and spatial resolutions are anyway limited for resolving the optically complexity of inland waters. To fill this gap, in this contribution we present the results obtained by acquiring Remote Sensing Reflectance (Rrs) from two hyperspectral spectroradiometers mounted on fixed platforms and / or on floating buoys. These sensors allow to gather continuous measurements during the day and for several consecutive days from two different yet comparable devices. A set of Rrs measures were taken by a WispStation (manufactured by Water Insight) in Lake Trasimeno (in the April-September 2018 range). The system measures every 10 minutes the radiance and irradiance in the spectral range of 350-900 nm with a spectral resolution of 3 nm. The set-up is based on an automatic system so that the instrument detect the water surface at optimal azimuth angles for most of the day. A second set of Rrs spectra was instead acquired by a ROX sensor (manufactured by JB Hyperspectral devices) operated on a floating buoy in Lake Maggiore at the end of June 2018. The ROX system is assembling Ocean Optics spectroradiometers and it operates in the range 400-950 nm with a spectral resolution of 1.5 nm and with an acquisition time of 1 minute. The analysis aims to adapt the state-of-the art algorithms for retrieving biogeochemical parameters to the next generation of hyperspectral satellite (apart a continuous testing with OLCI).