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Une évolution de la Composante Spatiale de Copernicus (CSC) est prévue pendant la seconde moitié des années 2020 pour répondre aux besoins prioritaires des utilisateurs non couverts par l'infrastructure existante. La mission CHIME (Copernicus Hyperspectral Imaging Mission for the Environment) couvrira une partie de ces besoins en fournissant des mesures hyperspectrales pour des domaines applicatifs comme la sécurité alimentaire, l'agriculture et la gestion de matières premières. Cet article fournit une description de la mission CHIME et de son état de développement actuel.

The accurate retrieval of Solar-Induced chlorophyll Fluorescence (SIF) is a pivotal target for Earth Observation since SIF can be easily monitored through optical remote sensing and provides unique information concerning the vegetation health status. Here, we propose i-φ-MaLe (metti il nome per esteso), a novel algorithm, which couples the Fourier analysis with a supervised machine learning-based procedure trained with the atmosphere-canopy radiative transfer (RT) SCOPE model. i-φ-MaLe is the first method able to simultaneously retrieve, from the vegetation reflectance spectra, the Top Of Canopy SIF spectrum, the SIF spectrum corrected for leaf/canopy reabsorption (i.e. at photosystem level), the quantum efficiency (Fqe) and three canopy-related biophysical parameters (Leaf Area Index - LAI, Chlorophyll content - Cab and APAR) in few milliseconds. Validation procedures, based on the analysis of RT simulations, demonstrated that i-φ-MaLe, in experimental conditions (signal to noise ratio – SNR >= 500), estimates each biophysical parameter and SIF spectrum with a relative root mean square error (RRMSE) lower than 5%. In order to investigate the seasonal and daily dynamics of SIF, LAI, Cab, Fqe and APAR, the method has been also applied to field experimental data collected in the context of the AtmoFLEX and FLEXSense ESA campaigns, both at top-of-canopy (TOC) and tower (~100 meters) levels. Concerning the TOC scenario, the retrieved annual dynamic for SIF spectra has been compared with the results obtained by inversion-based methods, showing a good consistency amongthe different approaches (RRMSE ~ 10%). Moreover, SIF daily and annual dynamics, retrieved by excluding the oxygen spectral bands affected by the atmospheric reabsorption, have been investigated for high tower measurements. . In this context, i-φ-MaLe provided promising results that can integrate and possibly overcome complex and computationally expensive atmospheric compensation techniques actually needed to retrieve fluorescence from oxygen absorptions bands. This study demonstrates a promising potential to exploit ground and tower spectral measurements with advanced processing algorithms, for improving our understanding on the link between canopy structure and physiological functioning of plants. Moreover, i-φ-MaLe can be straightforwardly employed to process reflectance spectra and open new perspectives in fluorescence retrieval at different scales.

During the tandem phase of Sentinel-3A and Sentinel-3B in summer 2018 the Ocean and Land Colour Imager (OLCI) mounted on the Sentinel-3B satellite was reprogrammed to mimics ESA's eighth Earth Explorer, the FLuorescence EXplorer (FLEX). The OLCI in FLEX configuration (OLCI-FLEX) had 45 spectral bands between 500 and 792 nm. The new data set with high-spectral-resolution measurements (bandwidth: 1.7–3.7 nm) serves as preparation for the FLEX mission. Spatially co-registered measurements of both instruments are used for the atmospheric correction and the retrieval of surface parameters, e.g. the fluorescence or the leaf area index. For such combined products, it is essential that both instruments are radiometrically consistent. We developed a transfer function to compare radiance measurements from different optical sensors and to monitor their consistency. In the presented study, the transfer function shifts information gained from high-resolution “FLEX-mode” settings to information convolved with the spectral response of the normal (lower) spectral resolution of the OLCI sensor. The resulting reconstructed low-resolution radiance is representative of the high-resolution data (OLCI-FLEX), and it can be compared with the measured low-resolution radiance (OLCI-A measurements). This difference is used to quantify systematic differences between the instruments. Applying the transfer function, we could show that OLCI-A is about 2 % brighter than OLCI-FLEX for most bands of the OLCI-FLEX spectral domain. At the longer wavelengths (> 770 nm) OLCI-A is about 5 % darker. Sensitivity studies showed that the parameters affecting the quality of the comparison of OLCI-A and OLCI-FLEX with the transfer function are mainly the surface reflectance and secondarily the aerosol composition. However, the aerosol composition can be simplified as long as it is treated consistently in all steps in the transfer function. Generally, the transfer function enables direct comparison of instruments with different spectral responses even with different observation geometries or different levels of observation. The method is sensitive to measurement biases and errors resulting from the processing. One application could be the quality control of the FLEX mission; presently it is also useful for the quality control of the OLCI-FLEX data.

During the tandem phase of Sentinel-3A and B the channels of the spectrometer of the Ocean and Land Imager (OLCI) on Sentinel 3-B were reprogrammed to imitate measurements of ESA’s 8th Earth Explorer mission FLEX [1]. FLEX is designed to retrieve the complete fluorescence spectrum from the high resolutive visible spectrum and to quantify the contribution of the two photocycles in green plants [2]. Fluorescence is a valuable proxy of plant photosynthesis activity [3], [4]. By measuring the emitted light of plants on satellites, plant distribution and state can be monitored on a global scale.The reprogrammed OLCI-B measurements consist of 45 bands between 500 and 800 nm with a bandwidth of about 1.8 nm (FWHM). The spectral and radiometric calibration of the 45 bands has not the same level of maturity as the one for the nominal setting, thus their radiometric uncertainty needs to be quantified. This is done by comparing the 45 FLEX-like bands with the co-located nominal 21 bands of OLCI on Sentinel-3A. The comparison is realised using a transfer function based on radiative transfer simulations. In a first step surface and atmosphere parameters are estimated from the OLCI-B FLEX-like measurements, that explain the measurements. The second step simulates the according OLCI-A measurements at nominal band settings and compares them with the real measurements made by OLCI-A.This study serves also as a precursor experiment for the FLEX mission, where the radiometric calibration of FLEX will be verified using co-registered OLCI-A (or B) measurements. Based on this study the strategy for the uncertainty of the FLEX intensity measurements will be developed and tested.The uncertainty estimation is a key factor of the Fluorescence retrieval as the Fluorescence contribution to the top of atmosphere signal is very small. To distinguish between signal and noise, the uncertainty must be kept as small as possible. Additionally, the uncertainty serves as input parameter for subsequent retrieval algorithms.[1] M. Celesti et al., ‘In prep.: Sentinel-3B OLCI in “FLEX mode” during the tandem phase: a novel dataset towards the future synergistic FLEX/Sentinel-3 mission’, p. 20, 2020.[2] M. Drusch et al., ‘The FLuorescence EXplorer Mission Concept—ESA’s Earth Explorer 8’, IEEE Trans. Geosci. Remote Sensing, vol. 55, no. 3, pp. 1273–1284, Mar. 2017, doi: 10.1109/TGRS.2016.2621820.[3] W. Verhoef, C. van der Tol, and E. M. Middleton, ‘Hyperspectral radiative transfer modeling to explore the combined retrieval of biophysical parameters and canopy fluorescence from FLEX – Sentinel-3 tandem mission multi-sensor data’, Remote Sensing of Environment, vol. 204, pp. 942–963, Jan. 2018, doi: 10.1016/j.rse.2017.08.006.[4] L. Guanter, L. Alonso, L. Gómez‐Chova, J. Amorós‐López, J. Vila, and J. Moreno, ‘Estimation of solar-induced vegetation fluorescence from space measurements’, Geophysical Research Letters, vol. 34, no. 8, 2007, doi: https://doi.org/10.1029/2007GL029289.

The recognized evidence of global warming demands assessment of the present and future water cycle in Europe and worldwide. Recently, evidence of modified hydrological regime in the Alps under climate change has been documented. In particular, several studies (e.g. Bocchiola, 2014; Soncini et al. 2016) indicated an increase in hydrological flows in autumn and winter in response to snowfall trading with intense rainfall, shorter snow cover during winter, as well as decreased flows during dry spring and summer and large shrinking of glaciers at high altitude. However, according to the IPCC Fifth Assessment Report, it is still necessary to deepen our understanding of the impact of climate change and land use on groundwater recharge and levels in the alpine catchment areas (Cochand et al. 2019).For this purpose, a water balance of the last three hydrogeological years (March 2017 - March 2020) was carried out on the Valtellina catchment (northern Italy, Central Italian Alps). This basin is a perfect case study for its wide unconfined aquifer in the floodplain, which makes it highly sensitive to this type of change. Moreover, the management of the water resource is of considerable importance, being crucial in a wide range of sectors (tourism, irrigation, domestic use, energy and industry).Due to the extensive and diversified study area (26,000 km2) and the low ground data density (7 meteorological stations, 4 surface-water monitoring points, and 9 groundwater monitoring points), the water balance terms were estimated by exploiting and combining Earth Observation data products with ground data, also taking into account the geological and geomorphological characteristics of the basin. In particular, the evapotranspiration and the snow cover were provided, by MOD16A2 (MODIS/Terra Evapotranspiration 8-Day Level-4 Global 500m SIN Grid) and MOD10A2 (MODIS/Terra Snow Cover 8-Day L3 Global 500m SIN Grid, Version 6) satellite data, respectively.As a result, the groundwater storage of a wet hydrogeological year compared with the groundwater storage of a dry hydrogeological year allowed analysing the sensitivity of groundwater resources to climate change. Bocchiola, D.: Long term (1921–2011) Hydrological regime of Alpine catchments in Northern Italy. Advances in Water Resources, 70, 51-64, 2014.Cochand, M., Christe, P., Ornstein, P., & Hunkeler, D.: Groundwater storage in high alpine catchments and its contribution to streamflow. Water Resources Research, 55(4), 2613-2630, 2019.Soncini, A., Bocchiola, D., Confortola, G., Minora, U., Vuillermoz, E., Salerno, F., Viviano, G., Shrestha, D., Senese, A., Smiraglia, C. and Diolaiuti, G.A.: Future hydrological regimes and glacier cover in the Everest region: The case study of the upper Dudh Koshi basin. Science of the Total Environment, 565, 1084-1101, 2016.

In recent years, technological progress in high-resolution field spectrometers have enabled the use of alternative tracer for constraining ecosystem-scale photosynthesis, i.e. sun-induced fluorescence (SIF). The principle underlying the use of SIF as a proxy of gross primary productivity (GPP) is based on the fact that the light energy absorbed by chlorophyll molecules can proceed into three different pathways: photochemistry, heat dissipation, and chlorophyll fluorescence. Since these processes directly compete for the same excitation energy, measurements of SIF and non-photochemical quenching (NPQ) are expected to provide information on photosynthetic performance.However, SIF signal measured at the leaf level or beyond is affected by several processes, including wavelength dependent scattering and reabsorption, which may need to be considered when linking SIF data and photosynthetic CO2 assimilation.To address this question, we conducted a multi-scale and multi-technique study that considered measurements of photosynthetic (GPP), optical (SIF, reflectance - R and transmittance - T), physiological (NPQ) and biophysical (the amount of absorbed photosynthetically active radiation - APAR) parameters of two soybean varieties: the MinnGold mutant, characterized by significantly reduced chlorophyll content (Chl), and the wild type, non-Chl deficient Eiko. We further used the “Soil-Canopy Observation Photosynthesis and Energy fluxes” (SCOPE) model to investigate the reabsorption and scattering of SIF. The measured leaf R, T and SIF and top-of-the-canopy R were used to retrieve biochemical and structural parameters of both varieties by inversion of the SCOPE model, while its forward mode was used to determine and correct for the scattering and reabsorption of SIF at both leaf and canopy level.Our study revealed that despite the large difference in Chl content (the ratio of Chl between MinnGold and Eiko was nearly 1:5), similar leaf and canopy photosynthesis rates were maintained in the Chl‐deficient mutant. This phenomenon was captured neither by traditional spectral vegetation indices related to canopy greenness, nor by SIF measured in-situ. However, the modelling simulations revealed that when correcting for leaf and canopy scattering and reabsorption processes both varieties presented similar SIF yield (SIF/APAR). Furthermore, field measurements showed that APAR and NPQ in MinnGold were lower than in Eiko. This together explains the similar measured GPP and simulated SIF yield between the two varieties, and indicates that interpretation and application of SIF as a GPP tracer requires understanding and quantification of all these processes.

[EN] The interest of the scientific community on the remote observation of sun-induced chlorophyll fluorescence (SIF) has increased in the recent years. In this context, hyperspectral ground measurements play a crucial role in the calibration and validation of future satellite missions. For this reason, the European cooperation in science and technology (COST) Action ES1309 OPTIMISE has compiled three papers on instrument characterization, measurement setups and protocols, and retrieval methods (current paper). This study is divided in two sections; first, we evaluated the uncertainties in SIF retrieval methods (e.g., Fraunhofer line depth (FLD) approaches and spectral fitting method (SFM)) for a combination of off-the-shelf commercial spectrometers. Secondly, we evaluated how an erroneous implementation of the retrieval methods increases the uncertainty in the estimated SIF values. Results show that the SFM approach applied to high-resolution spectra provided the most reliable SIF retrieval with a relative error (RE) 6% and, This article is based upon work from COST Action ES1309 OPTIMISE, supported by COST (European Cooperation in Science and Technology; www.cost.eu).Cendrero-Mateo M.P. and Alonso L. are currently funded by AVANFLEX project (Advanced Products for the FLEX mission), no ESP2016-79503-C2-1-P, Ministry of Economy and Competitiveness, Spain. Wieneke S. is currently funded by the individual fellowship of the European Union's H2020 Marie Sklodowska-Curie actions under the grant agreement ReSPEc no [795299]. Goulas Y. is currently funded by the ECOFLUO project (In-situ Remote Sensing of Chlorophyll Fluorescence to Monitor Carbon Dynamics in Ecosystems in support of the FLEX mission), no 4500058116 and 4500058229, Centre National d'Etudes Spatiales (CNES), France. Mac Arthur, A. was funded by the UK's NERC/NCEO Field Spectroscopy Facility, at the University of Edinburgh. Pacheco-Labrador, J. was supported by the EnMAP project MoReDEHESHyReS "Modelling Responses of Dehesas with Hyperspectral Remote Sensing" (Contract No. 50EE1621, German Aerospace Center (DLR) and the German Federal Ministry of Economic Affairs and Energy) and the Alexander von Humboldt Foundation via Max-Planck Prize to Markus Reichstein.

Remote sensing of solar-induced chlorophyll fluorescence (SIF) is of growing interest for the scientific community due to the inherent link of SIF with vegetation photosynthetic activity. An increasing number of in situ and airborne fluorescence spectrometers has been deployed worldwide to advance the understanding and usage of SIF for ecosystem studies. Particularly, a number of sites has been instrumented with the FloX (J&B Hyperspectral Devices, Germany), an automated instrument that houses two high resolution spectrometers covering the visible and near infrared spectral regions, one specifically optimized for fluorescence retrieval, the other for plant trait estimation.In this contribution we explore the feasibility to consistently retrieve plant traits and SIF from canopy level FloX measurements through the numerical inversion of a light version of the SCOPE model. The optimization approach was specifically adapted to work with the high- frequency time series produced by the FloX. In this context, a strategy for optimal retrieval of plant traits at daily scale is discussed, together with the implementation of an emulator of the radiative transfer model in the retrieval scheme. The retrieval strategy was applied to site measurements across Europe and the US that span a variety of natural and agricultural ecosystems.The full spectrum of canopy SIF, the fluorescence quantum efficiency, and main plant traits controlling light absorption and reabsorption were retrieved concurrently and evaluated over the growing season in comparison with site-specific ancillary data. Improvements and challenges of this method compared to other retrievals are discussed, together with the potential of applying a similar retrieval scheme to airborne datasets acquired with e.g. the HyPlant sensor, or the reconfigured “FLEX mode” data acquired with the recently launched Sentinel-3B during its commissioning phase.

Compact multi-spectral sensors that can be mounted on lightweight drones are now widely available and applied within the geo- and environmental sciences. However; the spatial consistency and radiometric quality of data from such sensors is relatively poorly explored beyond the lab; in operational settings and against other sensors. This study explores the extent to which accurate hemispherical-conical reflectance factors (HCRF) and vegetation indices (specifically: normalised difference vegetation index (NDVI) and chlorophyll red-edge index (CHL)) can be derived from a low-cost multispectral drone-mounted sensor (Parrot Sequoia). The drone datasets were assessed using reference panels and a high quality 1 m resolution reference dataset collected near-simultaneously by an airborne imaging spectrometer (HyPlant). Relative errors relating to the radiometric calibration to HCRF values were in the 4 to 15% range whereas deviations assessed for a maize field case study were larger (5 to 28%). Drone-derived vegetation indices showed relatively good agreement for NDVI with both HyPlant and Sentinel 2 products (R2 = 0.91). The HCRF; NDVI and CHL products from the Sequoia showed bias for high and low reflective surfaces. The spatial consistency of the products was high with minimal view angle effects in visible bands. In summary; compact multi-spectral sensors such as the Parrot Sequoia show good potential for use in index-based vegetation monitoring studies across scales but care must be taken when assuming derived HCRF to represent the true optical properties of the imaged surface.

The contribution presents the development and testing of a fluorescence retrieval scheme based on the ESA's FLuorescence EXplorer mission concept. The algorithm employs on a coupled surface-atmosphere forward model at oxygen absorption bands: i) the atmospheric effect is computed by MODTRAN5; ii) the surface reflectance and fluorescence are modeled by means of the Spectral Fitting approach. The algorithm, previously tested on numerical simulations, was further implemented and optimized to process real observations collected by the FLEX airborne demonstrator HyPlant. The retrieval scheme has been tested on a number of flight lines collected in several locations, different ecosystems types, atmospheric conditions and instrument observation conditions. For the first time, this work shows the capability of retrieving canopy fluorescence from real airborne observations by means of a physically-based algorithm as envisaged for FLEX. The results achieved on the large core data sets of imageries show the consistency of the physical retrieval algorithm for a wide range of scenarios and fluorescence values are in line with ground observations.

Leaf fluorescence can be used to track plant development and stress, and is considered the most direct measurement of photosynthetic activity available from remote sensing techniques. Red and far-red Sun Induced chlorophyll Fluorescence (SIF) maps were generated from high spatial resolution images collected with the HyPlant airborne spectrometer over even-aged loblolly pine plantations in North Carolina (USA). Canopy fluorescence yield (i.e., the fluorescence flux normalized by the light absorbed) in the red and far-red peaks was computed. This quantifies the fluorescence emission efficiencies that is more directly linked to canopy function compared to SIF radiances. Fluorescence fluxes and yields were investigated in relation to tree age to infer new insights on the potential of those measurements in better describing ecosystem processes. The results showed that red fluorescence yield varies with stand age. Young stands exhibited a nearly 2-fold higher red fluorescence yield than mature forest plantations, while the far-red fluorescence yield remained constant. We interpreted this finding in a context of photosynthetic stomatal limitation in aging loblolly pine stands. Current and future satellite missions provide global datasets of SIF at coarse spatial resolution, resulting in intra-pixel mixture effects, which could be a confounding factor for fluorescence signal interpretation. To mitigate this effect, we propose a surrogate of the fluorescence yield, namely the Canopy Cover Fluorescence Index (CCFI) that accounts for the spatial variability in canopy structure by exploiting the vegetation fractional cover. It was found that spatial aggregation tended to mask the effective relationships, while the CCFI was still able to maintain this link. This study is a first attempt in interpreting the fluorescence variability in aging forest stands and it may open new perspectives in understanding long-term forest dynamics in response to future climatic conditions from remote sensing of SIF. This article is protected by copyright. All rights reserved

In this contribution we present activities and selected results obtained in recent studies and campaigns conducted in the context of the FLuorescence EXplorer (FLEX) mission. FLEX is a candidate mission for the ESA 8th Earth Explorer and large efforts are currently dedicated to the development of an implementation scheme for an accurate mapping of fluorescence from the selected spaceborne sensor and mission configuration. Field and airborne data collected in different experimental campaigns, together with simulated data, have been used to demonstrate the feasibility of fluorescence retrievals and the potential of exploiting high spatial resolution fluorescence maps for a better understanding of the environment from space.