GREENSCALE and ESA: towards a synergy to better interpret vegetation fluorescence using the FLEX satellite?

FLEX relies on the measurement of chlorophyll fluorescence, a very faint light signal emitted by plants and directly linked to photosynthesis. Monitoring this fluorescence makes it possible to estimate the gross primary productivity (GPP) of vegetation canopies, and thus to indirectly characterise CO₂ exchanges between the biosphere and the atmosphere.

Indeed, fluorescence acts as a proxy for photosynthetic activity: when a plant absorbs light, a fraction of the energy is re‑emitted as this subtle optical signal.

Thanks to its cutting‑edge instrument, the FLORIS imaging spectrometer, FLEX will passively measure sun‑induced chlorophyll fluorescence (SIF) from space, at global scale and with a 300‑m resolution—significantly higher than that of existing missions estimating SIF, and suitable for a wider diversity of terrestrial ecosystems. This approach will make it possible to estimate the gross primary productivity (GPP) of vegetated ecosystems, thereby improving our understanding of large‑scale photosynthetic processes.

It is important to note that FLEX does not directly measure net ecosystem carbon fluxes (NEE) or ecosystem respiration. The mission therefore cannot, on its own, quantify carbon sinks or sources, but it provides a key variable to constrain the models that estimate them.

However, interpreting SIF measured from space remains a major scientific challenge. Unlike controlled laboratory or field conditions—where different photosynthetic regimes can be probed using controlled light stimuli—passive remote sensing only provides access to steady‑state fluorescence. This natural signal exhibits substantial variability, modulated by many factors (instantaneous light intensity, biotic and abiotic stress, temperature, canopy structure), which complicates its interpretation at the ecosystem scale.

The scientific community working around FLEX is therefore part of a broader effort to better understand and model this variability, in order to fully exploit the mission’s potential.

In this context, complementary scientific projects such as GREENSCALE play a key role in improving our understanding of how the steady‑state fluorescence signal is generated at the scale of individual canopy elements, and in clarifying the extent to which it reflects plant physiology and carbon assimilation. GREENSCALE investigates chlorophyll fluorescence and carbon and nitrogen gas exchanges in barley crops, with the aim of better understanding the mechanisms governing plant carbon assimilation. By measuring fluorescence at the scale of agricultural fields and comparing it with other in situ measurements, GREENSCALE helps strengthen the link between SIF and photosynthetic processes under real‑world conditions.

Ultimately, this work could help ESA and the scientific community refine their models for interpreting fluorescence signals measured from space, and improve assessments of the contribution of agricultural ecosystems to the carbon cycle.

The development of FLEX has recently reached a major milestone with the integration of the FLORIS instrument onto the satellite platform by Thales Alenia Space in Cannes. This phase marks the beginning of the final testing campaign before launch, scheduled between April and May 2026 aboard the Vega‑C rocket from the European spaceport in French Guiana. An exceptional visit of the satellite is planned for 16 April, offering a preview of the mission before its departure.

Once in orbit, FLEX will follow a trajectory that allows it to revisit the same areas regularly: its sun‑synchronous orbit ensures that landscapes are observed at the same local time each day, and the satellite will take about one month to cover the entire planet. This global and repeated observation capability is essential for tracking vegetation dynamics across multiple timescales while avoiding diurnal variability in the signal.

FLEX will not be alone in orbit: it will fly in tandem with Sentinel‑3C, which will provide crucial complementary information on land surfaces.

Through the GREENSCALE project, the FairCarboN programme contributes not only to improving the measurement, understanding and modelling of nitrogen mobilisation mechanisms to optimise net carbon assimilation per unit of agricultural nitrogen applied. It will also directly help refine the methodological and phenomenological assumptions used to compute and interpret the satellite products generated by ESA’s FLEX mission for global monitoring of photosynthetic activity. This work paves the way for a more detailed understanding of the carbon cycle and more accurate anticipation of climate change.