Scenario and overview

Agricultural production is under increasing pressure by global anthropogenic changes, including rising population, diversion of cereals to biofuels, increased protein demands and climatic extremes. Through a fleet of Earth observation (EO) satellites, Europe is dedicated to keep fingers on the pulse of its agricultural lands. Among the most innovative vegetation monitoring concepts involves FLEX (FLuorescence EXplorer), which was recently selected as ESA's 8th Earth Explorer and is by design timely and ground-breaking: it is the first mission concept specifically dedicated to monitoring the `breathing' of terrestrial vegetation. FLEX will fly in tandem with Sentinel-3 and will globally measure Sun-Induced chlorophyll Fluorescence (SIF) spectral emission from terrestrial vegetation. Together with Sentinel-3, these two new-generation European EO missions offer huge possibilities to increase our knowledge on the basic functioning of the Earth's vegetation, i.e. the photosynthetic process of plants resulting in carbon uptake. With the selection of FLEX, Europe reinforces a forefront role in fostering SIF research. However, for now ESA's commitment stops at Level-2c, i.e. delivering geometrically corrected SIF data. Hence, it is to the responsibility of the scientists to interpret and process these data into relevant products that gain insights into terrestrial vegetation dynamics. With this proposal named SENTIFLEX, we aim to unravel satellite-based SIF-photosynthesis relationships given heterogeneous spatiotemporal conditions on a per-pixel basis. Consolidated relationships will serve the development of a European vegetation productivity monitoring facility that eventually will be based on assimilation of Sentinel-3 surface reflectance with FLEX SIF data.

(© ESA/ATG medialab)



FLEX is the first mission specifically designed to capture the complete broadband SIF signal as well as complementary reflectance spectra in the green to NIR region (500-780 nm). The SIF emission is characterized by two peaks, one in the red (SIFred) and the other in the NIR (SIFNIR). FLEX will acquire imagery at a spatial resolution of 300 m, which is suitable for discriminating individual plots and stands. Moreover, FLEX is planned to fly in tandem with Sentinel-3, which enables parallel retrieval of relevant atmospheric and vegetation properties (e.g., canopy structure and temperature, non-photochemical energy dissipation). Given the availability of Sentinel-3 data and already-acquired or simulated FLEX-like data from FLEX science studies, the aim of SENTIFLEX is to go beyond the current state of the art in satellite-based SIF-photosynthesis research and overcome current limitations. This will be accomplished by: (1) development of a theoretical SIF-photosynthesis relationship no longer exclusively relying on indirect spectral proxies (e.g., FAPAR, APAR) but instead incorporating quantifiable biophysical variables and associated uncertainties that allow uncoupling vegetation and atmospheric dynamics from the SIF signal on a per-pixel basis; and, (2) implementation of the consolidated theoretical basis into a prototype FLEX-SENTINEL-3 data processing chain for vegetation photosynthesis and productivity monitoring.

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