Hyperspectral field and satellite radiometric matchups in Mediterranean oligotrophic waters

The HyperBOOST return leg mission, aboard the Tara sailing research vessel, included three in situ hyperspectral radiometric systems (above-water: HYPERNETS and So-Rad; in-water: HyperPRO) operating near-simultaneously in oligotrophic waters. This allowed for numerous high-quality matchups with Level-2 (L2) satellites over the 11 days of the leg. This study aimed to determine the level of agreement in terms of radiometric measurements between the three in situ sensors and between in situ and satellite-based multi- and hyperspectral sensors in oligotrophic waters. The resulting water-leaving reflectance (ρ_w) in the blue-green wavebands (i.e. 400 - 600 nm) derived from the three systems matched closely, with differences (ɛ) of 6.4% between So-Rad and HYPERNETS, and 6.6% between So-Rad and HyperPRO. Differences of 12.2% were found between the HYPERNETS and HyperPRO. The differences between satellite-derived values and field measurements of ρ_w averaged across all in situ matchups for blue-green wavebands also proved to be reasonable, ranging from 7.1% for Sentinel-3A OLCI to 18.7% for MODIS-Aqua, with PACE OCI v2.0 (8.8%) a close second. In the red wavebands (i.e., 600-700 nm), Sentinel-3A OLCI again performed the best, with an absolute median difference of 0.35 (unitless), with Sentinel-3B OLCI, the poorest performer at 0.75. These results demonstrate the viability of underway hyperspectral radiometry from a moving vessel for validation of multi- and hyperspectral satellite ocean color products in oligotrophic waters.