Carbon exchanges between the surface and the atmosphere mediated by plant photosynthesis are a key component of the terrestrial carbon balance, as photosynthesis is the mechanism for atmospheric carbon uptake by terrestrial vegetation and assimilation at the surface as accumulated biomass. Vegetation photosynthesis is highly variable according to environmental factors, particularly when plants are exposed to variable stress conditions, and such variability is enhanced under climate change and human pressure. A better quantitative estimation of the actual carbon assimilation by plants is needed to improve the accuracy of current terrestrial carbon models and to improve the predictability of such models towards future scenarios.
While Earth Observation (EO) techniques have been used extensively to model and quantify terrestrial vegetation dynamics, by determining the structure and functioning of plants, a direct estimate of actual photosynthetic activity by vegetation is only possible by quantifying not only the actual absorbed light by plants but also how the absorbed light is internally used by the plants. In fact, only a fraction of such absorbed light is used for photosynthesis, but such fraction is highly variable in space and time as a function of environmental conditions and regulation factors. Measuring chlorophyll fluorescence together with the total absorbed light, in addition to other key plant variables, provides a unique opportunity to quantify vegetation photosynthesis and its spatial and temporal dynamics by satellite observations.
The Fluorescence Explorer (FLEX) mission was selected in 2015, by the European Space Agency (ESA), as the 8th Earth Explorer within the Living Planet Programme, with the key scientific objective of a quantitative global mapping of actual photosynthetic activity of terrestrial ecosystems, with a spatial resolution adequate to resolve land surface processes associated to vegetation dynamics. FLEX will also provide quantitative physiological indicators to account for vegetation health status and environmental stress conditions, to better constraint global terrestrial carbon models.
Spatial coverage is driven by the need to globally observe all terrestrial vegetation, while optimal observation time around 10:00 is driven by the diurnal cycle of photosynthetic processes. The spatial resolution of 300 m is driven by the need to resolve land processes at appropriate scales relevant for the identification and tracking of stress effects on terrestrial vegetation, covering at least several annual cycles. The targeted uncertainty for photosynthesis derived from instantaneous measurements ranges from about 5% in unregulated conditions up to 30% in case of highly variable regulated heat dissipation, in line with model requirements and improving current capabilities provided by other techniques.
Given the four main pathways for light absorbed by plants (photochemistry, constitutive heat dissipation, chemically regulated heat dissipation and fluorescence) FLEX measurements include not only fluorescence emission, but also vegetation temperature and estimates of regulated heat dissipation, which is highly variable and drives the relation between fluorescence and photosynthesis. In addition, FLEX has also to acquire all the necessary information to determine vegetation conditions to properly interpret the photosynthesis variability, and all the necessary information needed for appropriate cloud screening, compensation for atmospheric effects and proper analysis of the measured signals.
For the retrieval of vegetation fluorescence, very high spectral resolution (better than 0.3 nm) is needed, with also very high signal-to-noise ratio given the weakness of the fluorescence signal as compared to the background reflected radiance. To be able to accomplish such objective, the FLEX mission carries the FLORIS spectrometer, with a spectral sampling in the order of 0.1 nm, specially optimized to derive, spectrally resolved, the overall vegetation fluorescence spectral emission in the full range 650-780 nm, and also measuring the spectral variability in surface reflectance in the range 500-650 nm indicative of chemical adaptations in regulated heat dissipation.
FLEX is designed to fly in tandem with Copernicus Sentinel-3. Together with FLORIS, the OLCI and SLSTR instruments on Sentinel-3 provide all the necessary information to retrieve the emitted fluorescence, and to allow proper derivation of the spatial and temporal dynamics of vegetation photosynthesis from such global measurements. OLCI and SLSTR data also help in the compensation for atmospheric effects and the derivation of the additional biophysical information needed to interpret the variability in fluorescence measurements.
The science products to be provided by the FLEX mission are not restricted to the basic chlorophyll fluorescence measurements, but include also the estimates of regulated and non-regulated heat dissipation, needed to quantify actual photosynthesis. Together with canopy temperature and other variables characterizing vegetation status, such as chlorophyll content and fraction of light absorbed by photosynthetic pigments, Level-2 products include instantaneous photosynthesis rates and estimates of vegetation stress based on ratios between actual versus potential photosynthesis and variable PSI/PSII contributions tracking photosynthesis dynamics. Level 3 products are derived by means of spatial mosaics and temporal composites, giving also as a temporal product the activation / deactivation of photosynthesis, growing season length and related vegetation phenology indicators. Finally, by means of data assimilation into advanced dynamical models of FLEX time series and ancillary information, Level-4 products are also provided, including time series of Gross Primary Productivity (GPP) and more advanced dynamical vegetation stress indicators.
Such science products can be directly used by vegetation dynamical models, climate models and different applications. In particular, with the increase population and food demand, usage of agriculture will need optimization of crop photosynthesis in such variable conditions, and FLEX is expected to contribute not only to the carbon science but also in associated applications. Efforts are put in place to guarantee proper cal/val activities and dedicated validation network for FLEX products. Particular efforts are in place to provide each product with realistic and properly estimated uncertainties, and also to propagate the derived uncertainties from the original satellite data until the final high-level products, accounting in each step for the covariance matrices associated to each set of intermediate variables, and using a combination of Montecarlo and analytical error propagation tools along the whole FLEX data processing chain.
The availability of validated ready-to-use high level science products will allow an extensive scientific usage of FLEX data, with also a high potential for derived applications. The open availability of FLEX products, and the accessibility through open exploitation platforms where users can themselves validate the products or even make changes in the algorithms to optimize the products towards their specific needs, is intended to offer a large versatility in FLEX exploitation approaches. FLEX is also expected to be used in conjunction with many other sources of EO data, including high spatial resolution time series from Sentinel-2. The FLEX Level-2 products are already provided in the same geographical grid as Sentinel-2 products to facilitate such exploitation developments. Usage of common global multi-resolution spatial grids for high-level products, and compatibility of data formats, are also taken into account to maximize the inter-operability of FLEX products with other EO products in global data assimilation approaches and multiple applications.
FLEX (FLuorescence EXplorer) is the 8th Earth Explorer mission currently being developed by ESA with the objective to perform quantitative measurements of the solar induce vegetation fluorescence. It will advance our understanding of the functioning of the photosynthetic machinery and the actual health of terrestrial vegetation. The fluorescence signal measured from space is so faint that additional information is required for an accurate retrieval and interpretation of the vegetation Fluorescence emissions. Hence the FLEX satellite will fly in convoy with a Sentinel-3 satellite for close temporal coregistration of its OLCI and SLSTR measurements.
The FLEX project development started in 2016 with the technologically challenging FLORIS Instrument by the instrument contractor Leonardo (Italy) with OHB (Germany) in its core team. The platform development was then kicked-off in 2019 with ThalesAleniaSpace (France) as the overall satellite prime contractor. A major milestone has been achieved by completing the instrument and satellite Critical Design Reviews beginning of 2022, allowing now to move into the phase of flight hardware manufacturing and integration.
An overview of the development progress will be provided including an outlook of the future project activities to get ready for launch in 2025.
The FLEX instrument Performance Simulator (FIPS) is a software allowing the simulation of synthetic raw data representative of the FLEX instrument radiometric, spectral and geometric performances. The FIPS simulates the optical performance of the instrument telescope and the two spectrometers. Particular emphasis was put in simulating the straylight performance of the instrument. The full acquisition chain from detectors to onboard data generation is also simulated allowing the generation of instrument source packets.
The Ground Prototype Processor (GPP) processes both synthetic and real instrument Earth Observation data, from the instrument source packet up to the Level 1B user product. The processing includes dark signal removal, smearing correction, non-linearity correction, straylight correction, absolute radiometric calibration and flat field equalization. The resulting Level 1B product includes geolocated top-of atmosphere radiances, associated data quality information, meteorological data and instrument characteristics required for further processing of the data to the Level 2.
In addition, the GPP is also designed to process data from the instrument whilst operating in various calibration modes. This functionality will enable in-flight characterization and calibration of the instrument.
The instrument radiometric calibration will be performed in-flight using a sun diffuser. It will be further monitored and validated using regular observations of the moon and deep convective clouds. The non-linearity of the instrument detector chain will be characterized on ground and then verified in-flight using natural targets at various level of signal and associated instrument integration times.
The in-flight spectral characterization will be based on the measurements of atmospheric absorption features as well as solar absorption lines observed on the onboard sun diffuser.
The absolute geometric performance will be monitored and corrected for through spatial feature matching of nominal EO data with a database of georeferenced high spatial resolution (30 m) Landsat images. The spatial coregistration between the high resolution and low spectral resolution spectrometers will be ensured by spatial feature matching between the two spectrometers data.
ESA’s 8th Earth Explorer mission FLEX aims at mapping Sun-induced fluorescence (SIF) as a proxy to quantify photosynthetic activity of terrestrial vegetation. The mission consists of a single platform (FLEX) carrying a hyperspectral imaging spectrometer (FLORIS). Flying in tandem with Copernicus’ Sentinel-3 satellite, FLEX will exploit the synergies with OLCI and SLSTR instruments. The complexity of the FLEX mission concept, its stringent mission requirements, and the large data volume impose important challenges to the Level-2 processing algorithms.
Within ESA/ESTEC FLEX Level-2 Study, a consortium made of remote sensing scientists and industry is joining forces to tackle these challenges, developing a Level-2 data processing prototype that can accurately retrieve SIF emission from space. The Level-2 data processing chain consists in four modules:
Level-1C: creating a synergistic product that includes co-registered FLEX and Sentinel-3 instrument data, quality flags and pixel classification, as well as a refined FLORIS spectral/radiometric calibration.
Level-2A: characterizing the atmospheric conditions (water vapor and aerosol optical properties) with a state-of-the-art synergistic approach using Sentinel-3 and FLEX data. This module provides an accurate inversion of the surface (apparent) reflectance within 1% error.
Level-2B: deriving the full spectrum of SIF emission from terrestrial vegetation by an efficient spectral fitting method within 0.2 mW/m2/sr/nm error.
Level-2C: providing vegetation biophysical parameters and a measurement of the non-photochemical quenching in order to interpret the SIF signal and its link to vegetation photosynthesis.
The FLEX Level-2 Study just finished its 3rd year of activities with a fully implemented software that it is functionally and scientifically validated. The current validation results show high accuracy of Level-1C calibration, atmospheric correction and subsequent SIF retrieval. In this presentation we will give an overview of the project activities and the current status of the developed algorithms, validation results and on-going challenges. The goal is to promote discussion with the scientific community and industry to consolidate the Level-2 mission products and data processing algorithms. In this presentation, we will describe the entire Level-2 data processing chain, and demonstrate the current accuracy of the implemented algorithms through the latest performance assessment results.
FLEX-E is the FLEX Level-2 end-to-end mission performance Simulator for ESA’s FLEX Earth Explorer-8. It is a key tool to demonstrate the feasibility of the whole FLEX mission concept and the baseline for the mission Ground Segment. It is also a versatile scientific tool, allowing to simulate the variability of ground, atmospheric and observation conditions that the FLEX – Sentinel-3 tandem mission will face, as well as to test the impact, on the L2 scientific products, of the constraints imposed by the technical solutions adopted for the FLEX instrument/platform.
FLEX-E has a modular architecture, based on the concept defined in ESA’s ARCHEO-E2E project. All FLEX-E modules are integrated within the OpenSF generic simulation framework. The “core” is the Scene Generation Module (SGM), designed to provide reference scenes in ground coordinates, with subpixel resolution, in a consistent manner for all the FLEX / Sentinel-3 tandem mission instruments. The SGM allows the different instruments to “fly” over the same scene (with different viewing geometries and spectral configurations), as it will be the case in real FLEX operations over a given geographical area. The interaction of the incoming solar radiation with vegetation and soils, and its propagation up to the Top of the Atmosphere (TOA), is performed with two coupled radiative transfer codes: SCOPE and MODTRAN. Through this, the SGM is able to generate the TOA radiance hypercube that is the input of the FLORIS and Sentinel-3 instrument modules. The orbit, attitude and observation geometry, needed for the scene generation, is provided by two Geometry Modules (GM and S3G), for the FLORIS and Sentinel-3 spectrometers, respectively.
The FLORIS instrument module, developed in a parallel project, comprises two chained submodules. The FLEX Instrument Performance Simulator (FIPS) models first the conversion of TOA radiances to digital numbers, including all the instrument spatial, spectral and radiometric effects and errors. Then, the Ground Processor Prototype (GPP) generates the L1b data products from the raw data, by implementing the calibration and correction of systematic errors. For the Sentinel-3 data flow, the Instrument+L1 Processing Module (S3M) models a simplified S3 sensors’ behaviour (OLCI and SLSTR), both in the spatial and spectral domain, and the L1 Ground Processing for the generation of Level-1 products.
At the end of the chain, the Level-2 Retrieval Module (L2RM), also developed in a parallel project, ingests the L1b inputs from GPP and S3M and implements all the retrieval algorithms for the FLEX L2 products, including Top-of-canopy reflectance, Sun-Induced Chlorophyll Fluorescence (SICF) and high-level photosynthesis products.
The Mission Performance Assessment is finally done by comparing the L2 products with the L1b data and the reference data produced by the SGM assessing the performance with respect to the Mission requirements provided in MRD document. This is achieved through two dedicated Performance Assessment Modules (PAM) for L1 and L2 mission requirements’ evaluation, and a parallel generic PAM configured for FLEX’s MRD. FLEX-E allows the independent evaluation of the impact of different instrumental configuration and effects, as well as “real world” scenarios (ground, atmosphere, observational) in the L2 retrieval.
The overall design and architecture of FLEX-E is presented, together with the status of its implementation and the latest results of the FLEX mission performance assessment.
In 2025 the European Space Agency (ESA) will launch the FLuorescence EXplorer (FLEX) mission, which will provide global maps of vegetation fluorescence that can reflect photosynthetic activity and plant health and stress. In turn, this is not only important for a better understanding of the global carbon cycle, but also for agricultural management and food security. FLEX will fly in tandem with the Copernicus Sentinel-3 mission, in particular working in combination with the OLCI and SLSTR instruments Sentinel-3 carries.
FLEX being in its implementation phase, the preparations for the later product validation started by means of mainly ground and airborne instrumentation. The gathered data provide fundamental information about the underlying processes and even more important confidence of data products and their required uncertainties. One challenge in this context is a comprehensive understanding and characterization of measurement uncertainty of the proposed validation datasets and the spatial and temporal support or representativity of these.
These data also form the basis for defining future validation activities and identifying potential key Cal/Val issues.
We will provide an overview of the future validation strategies for FLEX and how these integrate into a broader validation strategy for the Sentinel-3 and FLEX tandem and earth observation science strategy for the carbon cycle. In addition, we will highlight recent activities and outline planned activities for the coming years.