Hyperspectral EnMAP Data Processing for aquatic science and applications

Abstract

Challenge EnMAP, the Environmental Mapping Analysis Program, is a German satellite mission that was successfully launched in April 2022. EnMAP is an optical (VNIR/SWIR) remote sensing mission with high spatial (30 m GSD) and spectral (FWHM ~6-12 nm) resolution. Although primarily designed for terrestrial applications, EnMAP exhibits substantial potential for aquatic studies due to its hyperspectral capabilities. In comparison to other sensors, EnMAP ground segment processing is pioneering by providing separate processing chains for land and water, allowing for specialized data processing for each application. It also includes a sunglint avoidance algorithm using EnMAP’s tilting capability for mitigating the impact of sunglint-contaminated pixels in specific geographical constellations. This research focuses on how EnMAP can be used for studying aquatic ecosystems, with emphasis on EnMAP water processing and applications in aquatic research.

Methodology The EnMAP satellite is equipped with a VNIR-SWIR push-broom hyperspectral imager that covers a spectral range from 420 to 2450 nm and comprises 224 contiguous spectral bands. EnMAP follows a sun-synchronous orbit with a standard revisit period of 27 days, which can be reduced to less than four days through its off-nadir pointing capabilities. The EnMAP ground segment, managed by the German Aerospace Center (DLR), oversees the satellite control, data reception, storage, and processing. Users have access to three different product levels: L1B, L1C, and L2A. What stand out is its separate processing chains for land and water pixels within the L2A processing. Besides the standard processing using land atmospheric correction algorithms (PACO), the aquatic processing is done using the Modular Inversion and Processing System (MIP) developed by EOMAP GmbH, offering two types of water reflectance data: subsurface irradiance reflectance and normalized water leaving reflectance. To enhance the acquisition of glint-free data, a sunglint mitigation strategy has been implemented by selecting optimal image acquisition geometries on user requests. All EnMAP products are processed on-demand using the most up-to-date processor versions and undergo rigorous radiometric and spectral calibration. The ground segment conducts regular assessments of EnMAP products to ensure mission requirements, whereby these products are additionally subject to independent validation by the science team.

Expected results EnMAP's specific water-related data products play a significant role in advancing aquatic research, offering a substantial advantage, especially for researchers who were previously dependent on terrestrial sensors that required custom atmospheric corrections and thus when working in aquatic environments limiting their ability to fully exploit the potential of the data. EnMAP's global coverage, which extends to high latitudes (+/- 74°) and incorporates off-nadir capabilities, also facilitates observations in remote and challenging areas, including polar regions. The hyperspectral data is particularly advantageous for coastal and inland waters, allowing detailed examination of optical components within the water column and on the seafloor. The advanced spectral resolution in the visible, near-infrared, and shortwave-infrared enables the improved assessment of e.g. phytoplankton functional types and dissolved organic carbon levels. In optical shallow and clear waters, it further allows improved monitoring and differentiating of benthic substrate types and derive bathymetry. The detailed quantification of these constituents can contribute significantly to our understanding of various processes such as carbon cycling, nutrient availability, and preservation of threatened ecosystems like coral reefs, seagrass meadows, and mangrove forests. This paper will discuss in detail the advantages of the EnMAP processing chain and will present various applications that make use of EnMAP's hyperspectral water reflectance data.

Outlook for the future Delivering accurate water reflectance data is crucial for improving our knowledge of coastal and inland water environments. Achieving such accuracy is a complex task because of the difficulties related to correcting for atmospheric effects, the complex properties of water and aerosols, and the added complications when dealing with areas close to land, ice, or human-made structures. Optimized processing can help obtaining valuable data over aquatic ecosystems to make terrestrial Earth observing sensors significantly more useful for aquatic ecosystem monitoring. EnMAP provides a unique opportunity to gain a deeper understanding of aquatic ecosystems and the influence of human activities on aquatic environments, and thus be a pathfinder for future operational hyperspectral missions.