Mission
The FLuorescence EXplorer (FLEX) mission will collect global chlorophyll fluorescence data from vegetation, providing clues to photosynthetic activity and plant health. This information is essential to better understand and characterize the global carbon cycle. This understanding can support developments in agriculture and contribute to food security.
That is why FLEX was selected as the eighth Earth Explorer Mission of the European Space Agency (ESA) Living Planet program in 2015 ( mission selection report). FLEX will fly in tandem with the Coperincus Sentinel-3 mission so that data from the OLCI and SLSTR instruments can contribute to the interpretation of the FLEX data.
Data about FLEX:
- Start: 2026
- Launcher: Vega-C
- Mission duration: 3.5 years
- Size: 1.5 x 1.2 x 1.2 m (LxWxH), 5.2 m wide after unfolding the solar panels
- Satellite mass: 460 kg (of which 140 kg instruments, 30 kg fuel)
- Orbit: sunsynchronous, at 814 km, inclination 98.64°
- Revisit time: 27 Tage
- Spatial resolution: 300 m
- Spatial coverage: Global; land surfaces between 56° S and 75° N, including islands ≥100 km² and coastlines 50–300 km in length
Instrument:
The Fluorescence Imaging Spectrometer (FLORIS) is a hyperspectral line scanner in a spectral range of 500-780 nm with 0.1 nm spectral sampling in the oxygen absorption bands (759-769 and 686-697 nm) and 0.5-2 nm in the red-edge bands, of chlorophyll absorption and in the area of the photochemical reflection index (PRI).
FLEX Data products
ℹ The list of data products below is based on currently available documents and is updated regularly.
L1C
TOA radiance
Products for FLORIS, OLCI, and SLSTR, as well as a synergy product that is projected on common grid. [1]
L2A - L2 Atmospheric products
TOC radiance + irradiance
Atmospherically corrected top-of-canopy values + cloud screening included for FLORIS. [1]
L2B - L2 Fluorescence products
L2C - L2 Vegetation products
LAI
Leaf area index (LAI) is defined as the one-sided green leaf area per unit ground surface area (leaf area/ground area, m²/m²). LAI changes with species composition and phenology and provides information about canopy structure and functional characteristics of vegetation cover. It is retrieved through a combination of Gaussian process regression (GPR) and a 1D radiative transfer model. Thus, the retrieved LAI is the effective LAI, which does not account for leaf clumping. It can be used in Beer-lambert models e.g. used in GPP estimation. [6]
fAPAR
Fraction of absorbed photosynthetically active radiation (fAPAR) [m-2 s-1] is calculated as the fraction of photosynthetically active radiation (PAR; the solar radiation reaching the surface at 400-700 nm) that is absorbed by vegetation. [6]
LCC
Leaf chlorophyll content (LCC) [µg m-2] controls leaf optical properties and is directly linked to fluorescence emission. It is used for the retrieval of CO2 flux. LCC is representative for the pixel footprint and is likely lower that the mean LCC of all scences in one pixel. [6]
LCARC
Leaf carotenoid content (LCARC) [µg m-2] provides the total carotenoid content and is relevant to understanding photosynthesis, especially in phenology studies. [6]
APAR chlorophyll
Absorbed PAR by chlorophyll molecules (APAR chlorophyll) [m-2 s-1] is the amount of blue-sky APAR (blue sky = under the mix of diffuse/direct light) absorbed by chlorophyll a + b pigments. [6]
LST
Land surface temperature (LST) will be derived from the Sentinel 3 SLSTR instrument with an uncertainty of ±1 K (G) and of ±2 K (T). [1]
FQE
Fluorescence quantum efficiency (FQE) is defined here as the effective FQE at canopy level. That means it is the probability that a PAR photon absorbed by chlorophyll is re-emitted as a fluorescence photon. It can be described as the ratio of the emitted fluorescence (spectrally and hemispherically integrated and corrected for (re-)absorption) over aPAR_chl. [6]
Fesc
Fluorescence escape probability (Fesc) is defined as the ratio of TOC fluorescence (in observation direction times pi) divided by the fluorescence radiance produced in the canopy by chlorophyll. It is provided spectrally resolved. [6]
RED
Regulated energy dissipation (RED) [photon m-2 s-1] describes the probability (the so-called yield) that APAR is dissipated as heat through collective reversible mechanisms that plants use to protect themselves from excessive illumination. [6]
ETR
Electron transport rate (ETR) [electrons m-2 s-1] s the rate by which excited electrons travel to the reaction centers of photosystems. It here is defined as the product of the photochemical yield (phiP) and fAPAR_chl. In a first attempt it will be calculated from APAR_chl, LST and FQE. [6]