Full mission evaluation of EnMAP water leaving reflectance products using three atmospheric correction processors.

This study presents what we believe is the first extensive assessment of the water reflectance products from the German hyperspectral Environmental Mapping and Analysis Program (EnMAP). We evaluate EnMAP's standard normalized water leaving reflectance [ρ_W]_N over 17 water sites in the first two years of the mission. The EnMAP [ρ_W]_N standard product is generated by a dedicated water atmospheric correction (AC) called the Modular Inversion Program (MIP). The quality of the [ρ_W]_N retrievals was assessed using in situ hyperspectral measurements and Aerosol Robotic Network - Ocean Colour (AERONET-OC) multispectral measurements. The results showed very good agreement between in situ hyperspectral match-ups and EnMAP [ρ_W]_N, with an underestimation of EnMAP of -17.37% (bias, β) and an error (ϵ) of 23.75% at 418 - 797 nm. Two other AC processors were also investigated: the polynomial based algorithm applied to MERIS (Polymer) and the atmospheric correction for OLI lite (Acolite). The intercomparison exercise between the three AC methods applied to EnMAP data using the hyperspectral match-up dataset showed better statistical metrics for MIP (ϵ=23%,β=-17.37%) compared to Polymer (ϵ=42.20%,β=-2.43%) and Acolite (ϵ=97%,β=97%). The superior performance of MIP was further confirmed by the validation results obtained with the multispectral match-up dataset; MIP retrievals show good agreement with in situ measurements at the majority of study sites. Conversely, Polymer and Acolite retrievals tended to overestimate, especially in clearer waters as the Lampedusa study site.