Plant photosynthesis is a key component of the terrestrial carbon cycle. Modulation of vegetation photosynthesis under climate change, human pressure and environmental factors is a key driver in future climate trends, as photosynthesis is the mechanism for atmospheric carbon uptake by terrestrial vegetation and assimilation at the surface as accumulated biomass. Moreover, with the increase population and demand for more and quality food, usage of agriculture and forest resources will need optimization of crop photosynthesis in such variable conditions.

While remote sensing techniques have been used extensively to model and monitor terrestrial vegetation dynamics by determining the absorption of solar light by plants, a direct estimate of actual photosynthetic activity by vegetation is only possible by quantifying how such absorbed light is internally used by the plants. 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 / temporal dynamics by remote sensing observations.

The objective of FLEX is to provide quantitative estimates of the actual photosynthetic activity of terrestrial ecosystems globally, as it varies spatially and temporally according to vegetation health status and environmental stress conditions. FLEX will also provide quantitative physiological indicators for actual vegetation health status and stress conditions.

Based on such science objectives, specific mission requirements are derived. Spatial coverage is driven by the need to observe all terrestrial vegetation, while optimal observation time is driven by the diurnal cycle of photosynthetic processes. Spatial resolution 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. 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 to provide 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. 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.

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. With all the additional measurements needed in different spectral ranges, implementation of the mission as a single satellite, covering all the measurements simultaneously needed, would be difficult to afford. Implementation as a tandem mission with Copernicus Sentinel-3 is the selected option, as Sentinel-3 OLCI and SLSTR instruments provide all the information needed by FLEX, in addition to the specific information provided by the FLORIS instrument on board the FLEX satellite, which deliver full-spectrum of total fluorescence emission plus the spectral variability in surface reflectance indicative of chemical adaptations in regulated heat dissipation.

The science products to be provided by the FLEX mission are not restricted to the basic chlorophyll fluorescence measurements, but includes 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 (LAI, chlorophyll content) 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. Finally, by means of data assimilation into advanced dynamical models of FLEX time series and ancillary information, Level-4 products are also provided, such as 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. Efforts are put in place to guarantee proper cal/val activities and dedicated validation network for FLEX products. 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 Fluorescence Explorer (FLEX) mission has been selected as ESA’s 8th Earth Explorer mission. The primary objectives of the mission are to provide global estimates of vegetation fluorescence, actual photosynthetic activity, and vegetation stress. FLEX will fly in tandem formation with Sentinel-3 providing ancillary data for atmospheric characterization and correction, vegetation related spectral indices, and land surface temperature. The purpose of this paper is to present its scientific payload, FLORIS, which is a push-broom hyperspectral imager, flying on a medium size platform. FLORIS will measure the vegetation fluorescence in the spectral range between 500 nm and 780 nm at medium spatial resolution (300 m) and over a swath of 150 km. It accommodates an imaging spectrometer with a very high spectral -resolution (0.3 nm), to measure the fluorescence spectrum within two oxygen absorption bands (O2A and O2B), and a second spectrometer with lower spectral resolution to derive additional atmospheric and vegetation parameters. A compact opto-mechanical solution is the current instrument baseline. A polarization scrambler is placed in front of a common dioptric telescope serving both spectrometers to minimize the polarization sensitivity. The telescope images the ground scene onto a double slit assembly. The radiation is spectrally dispersed onto the focal planes of the grating spectrometers. Special attention has been given to the mitigation of stray-light which is a key factor to reach good accuracy of the fluorescence measurement. The absolute radiometric calibration is achieved by observing a dedicated Sun illuminated Lambertian diffuser, while the spectral calibration in flight is performed by means of vicarious techniques. The thermal stabilization is achieved by using two passive radiators looking directly to the cold space, counterbalanced by heaters in a closed loop system. The focal planes are based on custom developed CCDs. The opto-mechanical design is robust, stable vs temperature and easy to align. The optical quality is very good as recently demonstrated by the latest tests of an elegant breadboard. The scientific data products comprise the Top Of Atmosphere (TOA) radiance measurements as well as fluorescence estimates and higher-level products related to the health status of the vegetation addressing a wide range of applications from agriculture to forestry and climate.

ESA’s FLEX/Sentinel-3 mission aims at mapping Sun-induced fluorescence (SIF) as a proxy to quantify photosynthetic activity of terrestrial vegetation. Through synergy between the various mission instruments (FLORIS, OLCI and SLSTR), the tandem mission concept offers possibilities to improve quality of the final mission products. This is achieved by highly accurate algorithms such as (1) refinement of FLORIS radiometric calibration and geolocation, (2) atmospheric correction and cloud screening and (3) retrieval of biophysical products and temperature for a complete interpretation of the fluorescence signal.
Due to the complexity of the mission concept and stringent requirements for the data processing algorithms, ESA developed an End-to-End Mission Performance Simulator (E2ES) tool to reproduce the expected mission performance and consolidate the whole data processing chain. As part of the E2ES, the development and optimization of the Level-2 data processing chain plays an important role in the preparatory activities for the future Ground Segment development. An on-going 2-years ESA/ESTEC project, leaded by Magellium (France) with the universities of Valencia (Spain), Bologna and Milano-Bicocca (Italy) as subcontractors, aims at developing this Level-2 processing chain for its integration and validation into the FLEX E2E simulator. The L2 data processing chain consists in four modules:
L2A: creating a synergistic product by co-registering and cross-calibrating FLORIS (FLEX) and OLCI/SLSTR (Sentinel-3) instruments, and refining the FLORIS in-flight spectral calibration;
L2B: characterizing the atmospheric state (water vapor and aerosol optical properties) and inverting the surface apparent reflectance for all mission instruments (FLEX and Sentinel-3);
L2C: retrieving SIF emission from terrestrial vegetation by a state-of-the-art spectral fitting method; and
L2D: determining vegetation biophysical parameters for a complete interpretation of the SIF signal.


The FLEX L1B to L2 Algorithms Development study concludes its mid-term review (January 2019) with a first version of the software, fully implemented, functionally and scientifically validated. Preliminary results show high accuracy performance of L2 algorithms. Cross-calibration between FLORIS and OLCI reduce absolute radiometric calibration errors <0.5%, and spectral calibration errors are on the order of pre-flight characterization requirements. Atmospheric correction and subsequent SIF retrieval achieve errors <10% in SIF retrieval. Retrieval of biophysical products is improved by using the synergy between OLCI and FLORIS, with high computation time performance.

The goal of this presentation is to give an overview of the project activities and current status in order to promote synergies between the scientific community and private sector. In this presentation, we will describe the entire L2A-to-L2D data processing chain, and demonstrate the current accuracy of the implemented algorithms through the latest performance assessment results.

Hundreds of FLUXNET towers are presently operational as standalone projects and as parts of regional networks. Many have weather, radiation and soil data to help clean, analyze and interpret the flux data. However, the vast majority of these towers do not allow straight-forward coupling with remote sensing (drone, aircraft, satellite, etc.) and modeling workflows, and even fewer have optical sensors for validation of remote sensing products, ecosystem modeling, and upscaling from the field to regional and global levels.

In 2016-2019, new tools to collect, process, and share time-synchronized flux data from multiple towers were developed and deployed globally. These new tools can be effective in fostering scientific interactions, joint grant writing, data sharing and collaborations among the flux and remote sensing and modeling communities.

It is because these tools allow relatively easy matching of tower data with remote sensing data due to the following capabilities:

* The fully automated FluxSuite system combines hardware, software and web services, and does not require an expert to run it.
* The system can be incorporated into a new flux station or added to a present station, using a weatherized remotely-accessible microcomputer, SmartFlux3.
* It utilizes EddyPro software to calculate fully-processed fluxes and footprints in near-real-time, alongside radiation, optical, weather and soil data.
* All site data are merged into a single quality-controlled file timed using PTP time protocol.
* Data from optical sensors can be integrated into this complete dataset via compatible data loggers.
* Multiple stations can be linked into the time-synchronized network with automated reports and email alerts visible to PIs in real-time.
* Flux researchers can cross-share station access to specific stations and data access with each other.
* Flux data analysis software, Tovi, seamlessly ingests the data from the flux stations and allows a non-expert to quality control and gap-fill the flux data, partition NEE into GPP and Reco, and select exact perimeters on the map around the station to assess fluxes coming from just these areas.
* Remote sensing researchers and modelers without actual physical stations can form “virtual networks” of stations by collaborating with tower PIs from different physical networks.

As a result, the proposed concept can be utilized to couple flux tower data with remote sensing and modelling workflows as follows:

1. GPS-driven PTP protocol synchronizes instrumentation within the station, different stations with each other, and all of these to remote sensing data to precisely align remote sensing and flux data in time.
2. Footprint size and coordinates computed and stored with flux data help correctly align tower flux footprints and drone, aircraft or satellite motion to precisely align optical and flux data in space.
3. Areas selectable within the total station footprint allow matching flux tower data to the exactly same areal coverage as satellite data or modeling grid.
4. A full snapshot of the remote sensing pixel or modeling grid can then be constructed including leaf-level, ground optical sensors, and flux tower measurements from the same area closely coupled with the remote sensing measurements to help interpret remote sensing data, validate models, and improve upscaling.

Additionally, current flux stations can be augmented with advanced ground-based optical sensors and can use standard routines to deliver continuous products (e.g. SIF, PRI, NDVI, etc.) based on automated field spectrometers (e.g., FloX and RoX, etc.) and other optical systems.

Hundreds of new towers already operational globally can be readily used for the proposed workflow. Over 500 active traditional flux towers can be updated to synchronize their data with remote sensing measurements.

This presentation will show how the new tools are used by major networks, and describe how this approach can be utilized for matching remote sensing, modeling and tower data to aid in ground truthing, improve scientific interactions, and promote joint grant writing and other forms of collaboration between the flux, remote sensing and modeling communities.

In this contribution, we present activities and results obtained in recent campaigns conducted in the context of the future spaceborne FLuorescence EXplorer (FLEX) mission. In summer 2018, several test sites encompassing agricultural ecosystems (Le Rogaie farm Italy; Selhausen and the Agricultural Research Campus Klein-Altendorf Germany; the CALMIT Field Research Facility, Nebraska), temperate mixed deciduous forest (Laegeren, Switzerland), Mediterranean oak forests, evergreen open woodland (Majadas de Tietar, Spain) and deciduous forest of Downy oak (Observatoire de Haute-Provence, France) were investigated. At all sites, field, airborne and satellite data have been collected to demonstrate the feasibility of fluorescence retrievals and to outline the large potential of fluorescence information for an advanced understanding of the functioning of vegetation from space.
All sites were equipped to acquire in situ measurements of top of canopy radiance and atmospheric properties. Sun-induced fluorescence at ground level was estimated from spectral data collected with the FloX system, to get both continuous time series across full vegetative periods, and to characterize the spatial representativeness of the sites. Aerial images were acquired with the HyPlant airborne imaging spectrometer, operating at different times of the day and with several spatial resolutions. HyPlant data were acquired at 1 m spatial resolution for specific studies at each investigated site and by common dedicated flight lines in the solar principal plane at 4.5 m, to cover areas of about 5x5 km within 15’ around the Sentinel 3 overpasses, for scaling perspectives in a FLEX-like configuration. During the Sentinel-3 A/B tandem Phase, OLCI has been re-programmed in the S09 configuration to mimic FLORIS band settings and this potentially allows generating fluorescence maps at 300 m spatial resolution and testing cal/val schemes for different managed and unmanaged ecosystems.
We particularly present the results for the retrieval of the full fluorescence spectra at ground and airborne level and the evaluation of different fluorescence metrics for better interpreting time courses and spatial pattern dynamics of fluorescence under natural and controlled conditions. An overview and key results obtained at the different sites will be presented. In addition, progress of the scaling study from ground to satellite level, toward a validation scheme of FLEX fluorescence products, will be reported for the investigated sites.