FLEX (Fluorescence Explorer)
ESA's Earth Explorer missions are developed in response to Earth observation priorities identified by the scientific community. These reports form the basis for selection of the eighth Earth Explorer mission. Two candidates, FLEX and CarbonSat, have undergone extensive feasibility studies. CarbonSat aims to quantify sources and sinks of carbon dioxide (CO2) and methane (CH4) by measuring their distribution in the atmosphere. FLEX aims to quantify photosynthetic activity and plant stress by mapping vegetation fluorescence.
On November 19, 2015, ESA announced that its Member States had selected FLEX as the eighth Earth Explorer mission, upon recommendation from the Earth Science Advisory Committee. Following a rigorous selection process, the satellite will be ESA's eighth Earth Explorer, planned for launch by 2022. 1) 2) 3)
The FLEX mission in ESA's Living Planet Program aims to 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.
The conversion of atmospheric carbon dioxide and sunlight into energy-rich carbohydrates through photosynthesis is one of the most fundamental processes on Earth - and one on which we all depend. Information from FLEX will improve our understanding of the way carbon moves between plants and the atmosphere and how photosynthesis affects the carbon and water cycles. In addition, information from FLEX will lead to better insight into plant health and stress. This is of particular relevance since the growing global population is placing increasing demands on the production of food and animal feed.
The FLEX satellite will orbit in tandem with one of the Copernicus Sentinel-3 satellites, taking advantage of its optical and thermal sensors to provide an integrated package of measurements.
The concept of the FLEX mission is to measure fluorescence from space without needing an active system, just using sunlight. To do this a very sensitive instrument is needed. As part of the development process, we have to check that this can first be done from the air with the appropriate instrumentation. For this reason FZ-Juelich in Germany with SPECIM in Finland developed the HyPlant sensor. In principle HyPlant spans the scales from single leaves (cm resolution, when used on ground) to the ecosystem using it from an airborne platform. 4)
Although most people have heard of the process that describes how plants convert energy in sunlight into organic compounds, photosynthesis involves an extremely complex chain of events. In fact, it is so involved that no fewer than 10 Nobel prizes have been awarded for efforts unravelling the secret that is so fundamental to sustaining life on Earth.
From space, FLEX proposes to measure fluorescence as an indicator of photosynthetic activity.
Working in sequence, there are two different 'solar power systems' inside plant and algae cells. They collect energy in sunlight and produce – subject to environmental conditions and the health of the plant – chemical energy for photosynthesis, heat, and a faint glow called fluorescence. Depending how efficiently the plant works, both solar power systems glow with different intensities at different wavelengths.
As part of the development of the FLEX mission concept, scientists have been carrying out field campaigns to study the efficiency of these two systems by using the Hyplant instrument on an aircraft to measure fluorescence emitted from vegetation.
Hyplant has two 'imaging spectrometers' – essentially cameras that see at different wavelengths. One covers from red to near-infrared and splits up the wavelengths finely to pick out the fluorescence signals. 5) 6) 7) 8)
Figure 1: Artist's concept of the FLEX satellite, in formation with a Sentinel satellite in the background, measuring plant fluorescence from orbit (image credit:: ESA, ATG medialab)
The Earth Explorer Element: The Earth Explorer missions are designed to address key data needs or gaps in knowledge identified by the science community, while demonstrating breakthrough technologies and observing techniques. For each new mission, a number of candidate ideas or concepts are proposed, which go through a phased selection process and feasibility studies until one candidate is selected. From the outset, the science community is involved in the definition of these new missions and in the peer-reviewed selection process to ensure that each mission is developed efficiently and provides the exact data required by users. This approach provides excellent opportunities for international cooperation, both in the scientific domain and in the technological development of new missions (Figure 2). 9)
The first three Earth Explorers, CryoSat, GOCE and SMOS, have met earlier scientific challenges in the areas of Earth's cryosphere, gravity, soil moisture and ocean salinity, respectively. These missions have exceeded their original objectives, and scientists are now exploiting their data for new applications.
Figure 2: Overview of ESA's Earth Explorer missions (image credit: ESA)
The fourth Earth Explorer, Swarm, was launched on 22 November 2013. This three-satellite constellation aims to unravel one of the most mysterious aspects of our planet: the magnetic field. The future ADM-Aeolus will be the first space mission to profile the wind on a global scale, and EarthCARE will probe the relationship between clouds, aerosols and radiation. Biomass will map the distribution of biomass and the amounts of carbon stored in the world's forest ecosystems with greater accuracy than ever before. FLEX, recently selected as ESA's eighth Earth Explorer mission, will map vegetation fluorescence to quantify photosynthetic activity. This information will improve our understanding of the way carbon moves between plants and the atmosphere and how photosynthesis affects the carbon and water cycles. It will also lead to better insight into plant health and stress.
With GOCE's mission completed, the seven Earth Explorer missions currently in orbit or in development are:
• SMOS (Soil Moisture and Ocean Salinity mission): ESA's water mission
• CryoSat-2: ESA's ice mission
• Swarm: ESA's magnetic field mission
• ADM-Aeolus (Atmospheric Dynamics Mission): ESA's wind mission
• EarthCARE (Earth Cloud, Aerosol and Radiation Explorer)
• Biomass: ESA's forest mission
• FLEX (Fluorescence Explorer): ESA's planet health mission.
Photosynthesis and Fluorescence:
Photosynthesis is the process by which plants utilize sunlight, water, nutrients and CO2 to produce complex energy-rich biomolecules. It is the fundamental mechanism underlying plant growth and productivity and, thus, energy and mass exchange. Overall, photosynthesis has an efficiency of just 3–6% in converting solar energy into biochemical energy. Nonetheless, it is the basic process that enables biomass accumulation in plants. Because plant growth and productivity require the availability of nutrients and water, in addition to adequate sunlight and warmth, variations in these conditions affect photosynthetic rates and, therefore, are reflected as changes in plant productivity. This is why photosynthesis can be considered to be an integrative indicator of biosphere dynamics (Ref. 9).
When illuminated, green plants reflect, transmit and absorb light, but they also re-emit light in the form of fluorescence. When chlorophyll molecules in a leaf absorb photons, electrons are energized to an excited state. The fate of these 'excitons' depends on the physiological status of the plant. For example, under optimal conditions, approximately 82% of the absorbed light is used for carbon assimilation (photochemistry) while the remaining light is lost as heat and dissipated as chlorophyll fluorescence emissions. Fluorescence is therefore the most direct measurable reporter for photosynthetic efficiency.
The fluorescence signal originates from the core complexes of the photosynthetic machinery where energy conversion of absorbed photosynthetically active radiation occurs. The emission of the light referred to as chlorophyll fluorescence emanates directly from two photosystems, PS II and PS I. Both photosystems operate in a reaction chain and are commonly measured as a two-peak signal (Figure 3). By quantifying information on the full fluorescence emission spectra, we can improve our understanding of how light is used by plants. This is the approach used by plant physiologists in laboratories and in the field. FLEX will provide such measurements from space and allow retrieval of the relevant parameters of fluorescence emission as the most direct indicator of photosynthesis and vegetation stress.
Apart from light absorption to initiate photosynthesis, plants also need special mechanisms to protect them from light intensities in excess of that required for photochemistry in a given species or situation, otherwise there is a risk of photo-damage to molecules and tissues. Various mechanisms serve to minimize damage from excessive irradiation, including conformational changes within the chlorophyll pigment bed, and chemical conversion between two forms of the carotenoid pigment xanthophyll (such as violaxanthin and zeaxanthin).
The latter mechanism is considered important in the dissipation of excess light energy as heat, a key process of the phenomenon known as nonphotochemical quenching. The PRI (Photochemical Reflectance Index) has been shown to be responsive to the action of this xanthophyll cycle mechanism. The PRI is measured in the spectral reflectance range 500–600 nm, in particular around 531 nm, and normalized to a reference value taken at 570 nm. The FLEX mission will also provide measurements in this spectral range.
Figure 3: Total fluorescence emission spectrum based on the contributions from the two photosystems, PS II and PS I. Both photosystems operate in a reaction chain and are commonly measured as a two-peak signal, which are identified by their wavelength positions: F685 (originating mostly from PS II) and F740 (from both PS II and PS I), image credit: Forschungszentrum Jülich
In an experiment to support the development of the FLEX candidate Earth Explorer mission, the HyPlant instrument was used on an aircraft to detect vegetation under stress. The experiment involved rolling out two fields of turf and applying one with a common herbicide and leaving the other untreated. As the image (Figure 4) shows, the treated field on the left glows red, emitting more fluorescence compared to the control field on the right, and, in fact, more than the surrounding vegetation. 10)
In general florescence is an indicator of photosynthetic activity, but in this case the herbicide interrupted the plant power systems so that absorbed solar energy could not be used for photosynthesis. To get rid of this energy the plants emitted more fluorescence. By detecting these abnormal peaks of florescence from space, FLEX could offer early warning of stress in plants that may appear healthy to the eye.
Figure 4: Vegetation under stress (image credit: University of Milano-Bicocca)
Retrieval of vegetation fluorescence from space observations: 11)
Although fluorescence techniques are routinely used in laboratory and field conditions to quantify the dynamics of vegetation photosynthesis, the application of these techniques to space measurements requires combination of high spectral resolution and high signal-to-noise ratios, which have been available only in recent years. With technological improvements and advances in scientific research, specifically dedicated missions to map globally vegetation fluorescence have been proposed and some results have been already obtained by using data from other missions not initially designed for this purpose, exploiting the available high spectral resolution in the measured data.
The remote sensing of vegetation fluorescence is based on the usage of high spectral resolution (typically in the order of 0.3 nm or better) around strong absorption features in the solar radiation spectrum, either solar Fraunhofer lines or terrestrial atmosphere absorptions, like oxygen absorptions, and exploiting the fact that reflectance and fluorescence vary smoothly with wavelength while solar radiation has strong spectral variation around such absorption features.
When measured from space, vegetation fluorescence contributes only a tiny fraction of the signal coming on top of the reflected radiance by the terrestrial surface, so that specific retrieval algorithms are needed to disentangle fluorescence from reflectance. The retrieval of vegetation fluorescence is based on the spectral contrast between atmospheric transmittance / solar irradiance at the surface (with strong spectral structure) and the surface reflectance (spectrally smooth) what allows the decoupling of the emitted fluorescence from the reflected solar light, typically by spectral fitting methods using the full spectral information. Also, retrievals of fluorescence from space have to make corrections for atmospheric effects, which imply the need to also measure atmospheric information.
Global mapping of vegetation photosynthesis:
Vegetation photosynthesis can be derived from fluorescence measurements thanks to the fact that when a chlorophyll molecule is excited by the absorption of light, the electrons moved to higher energy levels can return to the ground state by means of four main deactivation mechanisms, as illustrated in Figure 5, which are competing among them, so that knowing one of the components (namely fluorescence) and having some information about the heat dissipation status, photosynthesis can be estimated.
Figure 5: Energy dissipation mechanisms for an excited chlorophyll molecule after absorption of light (image credit: FLEX collaboration)
While fluorescence is the best proxy to vegetation photosynthesis, the measurement of vegetation fluorescence alone is not enough to quantitatively determine vegetation photosynthesis as a function of environmental conditions. For such purpose, additional measurements about canopy temperature, solar irradiance at the surface, vegetation conditions (LAI, chlorophyll content), are essentially needed for a proper interpretation of fluorescence levels, and for the mapping of photosynthesis the mission must include additional measurements of instantaneous environmental conditions and vegetation dynamical status.
Since the relationship between photosynthesis and fluorescence depends on the status of the other energy dissipation mechanisms, particular non-photochemical energy dissipation (which follows a diurnal cycle), a critical issue is the selection of the observation time along the day to properly interpret the measurements. As indicated in Figure 6, the fluorescence efficiency, ΦF (ratio between emitted fluorescence and absorbed radiation) and photochemical efficiency, ΦP (ratio between energy used for photosynthesis and absorbed radiation) are positively correlated only over some time along a typical diurnal cycle, which is the optimal satellite overpass time selected for FLEX.
Figure 6: Typical diurnal course of fluorescence efficiency (ΦF) and photochemical efficiency (ΦP). Satellite measurements are done along the time where both ΦF and ΦP behave as positively correlated, marked in green (image credit: FLEX collaboration)
The FLEX space segment consists of a single three-axis stabilized satellite flying in tandem with Sentinel-3 to make optimal use of existing relevant space observation capabilities, providing a suite of complementary measurements acquired within 6– 15 s of each other in order to minimize the effects of moving clouds.
Note: The spacecraft will be described when the information becomes available.
• August 8, 2017: Teledyne e2v has been awarded a multimillion euro contract by OHB System AG to supply customized CCD (Charge Coupled Device) image sensors for the FLEX (Fluorescence Explorer) satellite mission, under a program funded by ESA. 12)
- Teledyne e2v will design, develop and carry out the qualification of high performance CCDs and their customized package for integration into the FLEX instrument, which is under development by Leonardo S.p.A, Florence, Italy and OHB System AG, Wessling, Germany.
• On Nov. 7, 2016, ESA awarded a contract to the Italian company Leonardo to build the main instrument (FLORIS) for the upcoming FLEX satellite to study the health of Earth's vegetation. The contract was signed in Florence, Italy between Josef Aschbacher of ESA and Gianfranco Terrando of Leonardo. A leader in electrooptical instruments, Leonardo will lead a consortium of European companies, including the primary partner OHB System AG. 13)
Launch: A launch of the FLEX mission is planned for 2022.
Orbit: Sun-synchronous orbit, altitude of 815 km.
Tandem mission with Sentinel-3:
ESA's FLEX mission will orbit in tandem with one of the Copernicus constellation satellites. Taking advantage of Sentinel-3's optical and thermal sensors will lead to an integrated package of measurements to assess plant health. With the Sentinel-2 satellites also in orbit, there is a unique opportunity of using this data synergistically from all three missions for vegetation studies. 14)
Sentinel-3 instruments OLCI and SLSTR provide the needed atmospheric information, as well as contribute to retrieve vegetation information (LAI, chlorophyll) useful for the interpretation of the fluorescence signal.
Figure 7: The spectral observation scheme with the FLEX and Sentinel-3 missions in tandem (image credit: FLEX collaboration) 15)
Figure 8: FLEX and Sentinel-3 joining forces (image credit: ESA)
FLORIS (FLuORescence Imaging Spectrometer)
FLORIS is a high-resolution imaging spectrometer acquiring data in the 500–780 nm spectral range, with a sampling of 0.1 nm in the oxygen bands (759–769 nm and 686–697 nm) and 0.5–2.0 nm in the red edge, chlorophyll absorption and PRI (Photochemical Reflectance Index) bands. Sentinel-3's OLCI (Ocean and Land Color Instrument) and SLSTR (Sea and Land Surface Temperature Radiometer) instruments provide complementary information to retrieve the fluorescence signal from OCLI's camera 4 and the SLSTR nadir view respectively. The observations made by FLEX and Sentinel-3 instruments of the same target on the ground must be acquired within 6 seconds (goal) to 15 seconds (threshold) of each other (Ref. 2).
FLORIS operates in pushbroom mode with a spatial sampling of about 300 x 300 m2 and a swath width of 150 km, which is entirely contained within the Sentinel-3 OLCI camera-4 swath, the one closest to the nadir direction. The instrument calibration relies on both cross-calibration with the equivalent bands of OLCI and a dedicated onboard calibration device. The FLEX and Sentinel-3 images are coregistered by on-ground processing through correlation algorithms. An overview of the FLORIS bands and those of the Sentinel-3 OLCI and SLSTR instruments is given in Figure 9.
Figure 9: Comparison of FLORIS spectral ranges and Sentinel-3 instrument bands (image credit: ESA)
The mission concept consists in a single platform that carries the FLORIS instrument which is being designed and optimized for discrimination of the fluorescence signal in terrestrial vegetation, providing images with a 150 km swath and 300 m pixel size. By using two combined imaging spectrometers, FLORIS will measure the radiance between 500 and 780 nm with a bandwidth between 0.1 nm and 2 nm, with high spectral resolution of 0.3 nm in particular at the Oxygen-A (755 -780 nm) and -B bands (677-697 nm). It will also cover the photochemical reflectance features between 500 and 600 nm, the chlorophyll absorption region between 600 and 677 nm, and the red-edge in the region of 697 to 755 nm, which allow a highly accurate measurement of the spectral distribution of vegetation fluorescence in absolute terms as needed by physically-based retrieval methods. 16)
The atmospheric correction is one of the main steps in the data processing chain of a satellite mission in order to derive surface properties free from the influence from the atmosphere. Implementing a highly accurate atmospheric correction algorithm is particularly important for instruments measuring in deep absorption bands at high spectral resolution. This is the case of ESA's FLEX mission, whose instrument FLORIS measures within the deep Oxygen absorption bands (O2-B at 684 nm and O2-A at 760 nm) at a spectral resolution of 0.3 nm. Therefore, a precise atmospheric correction of FLORIS data over the O2 bands is required in order to properly correct the atmospheric radiative perturbations. Particularly important is the compensation of atmospheric water vapor, surface pressure and aerosol properties. While water vapor and surface pressure can be univocally determined e.g. through differential absorption techniques and meteorological pressure fields, the complete characterization of aerosol optical properties (i.e. extinction, absorption and scattering) and vertical distribution can only be unambiguously determined by instruments that can provide multi-angular, multispectral and polarimetric capabilities (e.g. 3MI/MetOp-SG). However, FLEX fluorescence retrieval only needs to compensate the net effect of aerosol properties, which can be partially characterized through the use of parametric approximations describing these optical properties. 17) 18)
Figure 10: Spectral information provided by the FLORIS instrument (image credit: FLEX collaboration)
Combining FLORIS, OLCI and SLSTR, a full set of spectral measurements from the blue up to the thermal infrared will be exploited by FLEX in the Level-2 retrieval algorithms to derive information about photosynthesis rates and vegetation stress conditions (Ref. 18).
The input to Level 2 products are FLORIS TOA radiances geo-rectified and resampled to the same spatial grid as Sentinel-3/OLCI data, in a combined FLORIS/Sentinel-3 (OLCI/SLSTR) dataset. Level-2 products include O2-A and O2-B emission values (F687 and F760), and peak emission values (λ<red>,F<red> and λ<far-red>,F<far-red>), with an accuracy 0.2 mW m-2 sr-1 nm-1 at 300 x 300 m2, plus the total spectrally integrated fluorescence emission (Ftot) with an accuracy of 10% at 300 x 300 m2. In addition, FLEX determines the non-photochemical energy dissipation (NPQ), with an accuracy ΔPRI<0.003 for relative changes, and derives surface temperature with an absolute accuracy 1-2 K (derived from Sentinel-3 SLSTR). Higher level products include fluorescence quantum efficiency, PSI - PSII contributions, photosynthesis rate, and vegetation stress indicators. In addition, Level 3-4 products are derived by means of spatial mosaics and temporal composites, giving also as a temporal product the activation / deactivation of photosynthesis and growing season length. Finally, by means of data assimilation of data time series and ancillary information, higher level products are also obtained, such as GPP (Gross Primary Productivity).
Figure 11: FLEX Level-2 data processing scheme (image credit: FLEX collaboration)
The data processing for the FLEX / Sentinel-3 tandem mission is complicated by the fact that the data stream used as inputs by the retrievals consists of the combination of 5 different "instruments" (FLORIS HR, FLORIS LR, OLCI and SLSTR-N and SLSTR-B), each one with different viewing geometry, but also different spatial resolutions and image acquisition procedures (pushbroom systems, whiskbroom conical scanning systems), and covering the full spectral range from the blue to the thermal. An End-to-End Simulator has been developed to assess the performance of the mission/instrument configuration as well as the retrieval algorithms, the influence of instrumental errors on the retrieval accuracies, and to consolidate the requirements for Level-2 products (Figure 12). 19)
Figure 12: FLEX products and applications as a function of spatial scales (image credit: FLEX collaboration)
Plant photosynthesis is a key component of the global carbon cycle, and fluorescence provides actual (not just potential) photosynthesis rates to constraint models of CO2 assimilation (GPP), with great potential for applications in agriculture / food production. While most of the information that has been acquired by remote sensing of the Earth's surface about vegetation conditions and photosynthetic activity has come from "reflected" light in the solar domain, ESA's Earth Explorer FLEX mission is the first space mission focused on the estimation of fluorescence emission by terrestrial vegetation on a global scale with high spatial resolution and spectrally resolving the fluorescence emission.
The FLEX mission not only includes the measurement of the full spectrum of fluorescence emission, but also includes explicit measurement of photochemical changes in reflectance (i.e., PRI), canopy temperature measurements and all the relevant variables (chlorophyll content, Leaf Area Index, etc.) needed to assess the actual physiological status of vegetation and to provide quantitative estimates of photosynthetic rates and vegetation stress conditions.
The development of new field / tower instruments and airborne sensors, atmospheric correction procedures and advanced retrieval schemes, theoretical models to interpret the signal (leaf/canopy level, ecosystem level, global level), ground validation networks, plus a sophisticate end-to-end mission simulator, and techniques for the assimilation of the fluorescence information in Dynamical Vegetation Models, are some of the activities being carried out towards the launch of FLEX planned for 2022.
FLORIS instrument design:
- Low Resolution (LR): (500 nm–677 nm) & (697 nm–740 nm)
- O2-B: (677 nm–697 nm)
- O2-A: (740 nm–780 nm)
Two spectrometers have been implemented to cover the required spectral range: the LR (Low Resolution) spectrometer for the Low Resolution bands (range 500–758 nm) with un-binned SSI (Spectral Sampling Interval) of 0.6 nm/pixel and the HR (High Resolution) spectrometer for the two oxygen bands (range 677–697, 740–780 nm) with un-binned SSI of 0.0933 nm/pixel. A spectral binning x 3 and x 5 can be selected in some specific bands to meet the SSI requirement (Table 2). The spectral range 677–697 nm and 740–758 nm is also acquired by the LR spectrometer for HR-LR spectral inter-band coregistration.
Table 1: FLORIS spectral requirements and performance
Legend to Table 1: SSD = Spatial Sampling Distance, ARA =Absolute Radiometric Accuracy, RSRA = Relative Spectral Radiometric Accuracy, RXRA = Relative Spatial Radiometric Accuracy, ISRF = Instrument Spectral Response Function, FWHM = Full Width Half Maximum, ALT/ACT = ALong/ACross-Track, DOP = Degree Of Polarization, SEDF = System Energy Distribution Function, p = ACT detector pitch, F = focal length, ts = sampling time, H = satellite altitude, GTO = Ground To Orbit, BOL = Begin Of Life, EOL = End Of Life., TBC = To Be Conﬁrmed).
Table 2: FLORIS spectral requirements and performance
Legend to Table 2: SR = Spectral Resolution = FWHM of ISRF, SSI = Spectral Sampling Interval, SNR = Signal to Noise Ratio. LR = Low Resolution, HR = High Resolution, req. = requirement, per. = performance. SNR req. at reference radiance (Lref) and SSI req.
Radiometry is a key driver to measure the small fluorescence signal variation with an accuracy of 10%. Therefore a high SNR (Signal to Noise Ratio, SNR > 100) is needed for each spectral channel with low reference radiance (e.g., Lref = 7.5 W/m2/sr/µm at 761 nm) within the narrow O2 absorption bands (Spectral Sampling Interval < 0.1 nm), Table 2).
Another fundamental requirement is straylight, which shall be limited (and further reduced with on ground correction algorithms) to avoid pollution of the fluorescence signal. The straylight signal within the O2A and O2B bands at a distance larger than 40 SSD (Spatial Sampling Distance) from the edge of a transition between a bright (e.g., a cloud) and a dark zone (e.g., the reference vegetation) (Figure 1a) shall be less than 0.04 mW/m2/sr/nm at level 1b (after straylight correction) and 0.2 mW/m2/sr/nm at level L0 (without correction). An important contribution is the spectral straylight, which comes from the light scattered by the spectrometer optical elements (and the gratings in particular) from the continuum bands into the oxygen bands (Figure 1b). This is affecting measurements even without the presence of bright targets in the scene.
Figure 13: Non uniform scene (a) for spatial stray-light and non-uniform spectra, (b) for spectral stray-light evaluation. Lbright = Lcloud, Ldark = Lref. (image credit: FLEX collaboration)
Instrument Architecture and Optical Design:
The instrument architecture is based on a pushbroom hyperspectral imager with a common telescope and two spectrometers, separated in field. The operational temperature range is 293.15 K ± 2 K. The pupil diameter to comply the SNR requirement is 75.6 mm for HR and 36.1 mm for LR.
A single telescope (234.5 mm of focal length and pupil diameter of 75.6 mm, f-number 3.1) in Figure 14 is implemented to collect the radiation coming from two single strips of 300 m x 150 km onto 2 identical slits (84 µm x 44.1 mm) placed in its focal plane and separated by a distance of 4.5 mm (Figure 15), corresponding to an on-ground separation of 15.9 km.
The coregistration between the HR and LR bands and the Sentinel 3 data is performed on ground. In front of the telescope an external baffle is placed in order to reduce the out of ﬁeld straylight. A dual Babinet scrambler is needed to meet the polarization sensitivity requirements. The wedge angle of the scrambler plates is the result of a trade-off between the strength of depolarization and the image quality degradation. The required spectral resolution is obtained by using two modified Offner spectrometers (Figure 14). Holographic gratings are used due to the relatively high groove density and because they can guarantee low grating imperfections (residual roughness, proﬁle errors, etc.). This is very important because the gratings are the major contributors of spectral straylight, which directly impact on the fluorescence measurement accuracy within the O2 absorption band. Three identical low noise backside illuminated frame transfer 1060 x450 (42 µm spatial x28 µm spectral) CCD detector units, cooled (-35 ºC) by a radiator, allow to cover the three spectral range (O2B, O2A and LR bands). The required SNR is achieved by means of a 42.8 ms integration time (300 m on ground) without saturation during cloud observations (full well capacity of 1.25 Me- for each pixel), 1.7 MHz readout frequency, four ports and x 2 pixel binning along the spatial direction at the CCD level. A further binning along the spectral direction (x 3 or x 5) can be provided by the FEE (Front End Electronics).
Figure 14: Instrument optical layout (image credit: FLEX collaboration)
Figure 15: Double slit assembly: HR and LR slits separated by 4.5 mm (image credit: FLEX collaboration)
The design approach foresees no aberration compensation between the telescope and the spectrometers to simplify the separate procurement and test of each subsystem. A stabilization of the optical bench of ±1 ºC is implemented in order to achieve the required spectral stability. All the materials selected are common for space projects:
• Optics: F2G12, Lak9G18, CaF2, BK7G18 for lenses, N-BK7, S-FPL51 for mirrors and Fused Silica for gratings.
• Mechanics: Aluminum 6061, Titanium and Invar.
A Petzval objective with 5 lenses (2 aspherical lenses) is used as fore optics (Figure 16). It has a real entrance pupil located 75 mm in front of the ﬁrst lens, where the instrument aperture stop is accommodated and it is telecentric in the image focal plane. It guarantees good image quality up to F# = 3.1 in the HR spectral band (FOV =± 5.4º x 0.0º) and to F# = 6.5 in the LR spectral band (FOV =±5.4º x 1.1º). A smaller F# is needed in the HR spectral band for radiometric constraints (lower input radiance and higher spectral resolution with respect to the LR spectrometer). Chromatic aberration correction is achieved with only three radiation resistant glasses (lateral color <4.7 µm in the HR range and <10.5 micron in the LR one). Radial distortion is less than 0.2% and WFE rms is less than 0.23 waves in HR (0.14 waves in LR) channel by design.
Figure 16: Telescope optical layout (image credit: FLEX collaboration)
High Resolution Spectrometer:
The proposed solution is based on the Lobb's theory of concentric designs for grating spectrometer. It works in the spectral range 670–780 nm with a pixel size of 28 µm in the spectral direction and 84 µm in the spatial one. The F# is the same of the telescope and the magnification is +1. The starting point is the Offner relay. Performances of the relay are improved by the addition of a concentric spherical lens through which the light passes twice. The spectrometer is obtained from the relay design by changing the convex spherical mirror with a convex spherical grating. An additional ﬂat mirror (High Resolution Flat mirror) placed after the HR entrance slit is needed to make easier the HR spectrometer mechanical accommodation and in particular the separation between the slit assembly and the HR detector.
The convex holographic grating has a groove density of 1450 grooves/mm and the average incidence angle on the grating is approximately 38.2 º. The -1 order of the diffracted beam is back reflected close to the incident beam, with diffraction angles between 21.3º (at 677 nm) and 30.9 º (at 780 nm). This beam is then guided towards the focal plane by means of a second reflection of the concave mirror and the second transmission through the meniscus (Figure 17). Apart from the zero-order (specular direction) beam which is stopped by a light trap, all other diffraction orders are evanescent. The use of the high frequency in the grating has two main advantages:
• Spectrometer compactness: the larger are the dispersion angles the smaller is the required grating radius of curvature (it corresponds to a lower focal length)
• High grating efficiency. Average efficiency greater than 60% can be achieved with an optimized saw-tooth groove proﬁle.
Grating polarization sensitivity is less than 20% and the required grating BRDF (Bidirectional Reflectance Distribution Function), describing the angular distribution of the scattered signal, shall be better than the one of a mirror with a surface roughness of 2 nm rms. The system gives excellent correction for chromatic aberrations and distortions. This means that the slit images on detector are straight and parallel, falling on detector columns without significant spatial and spectral co-registration errors (keystone less than 0.4 µm and smile less than 0.5 µm by design).
Figure 17: High Resolution Spectrometer layout (image credit: FLEX collaboration)
Legend to Figure 17: HRF = High resolution Flat mirror, HRL = High Resolution Lens, HRM = High Resolution curved Mirror, HRG = High Resolution Grating, HR = High Resolution Detector.
Low Resolution Spectrometer:
The LR spectrometer optical design is of the same family of the HR one but with a different design of the corrector lens. It works in the spectral range 500–758 nm with a pixel size of 28 µm in the spectral direction and 84 µm in the spatial one. The LR spectrometer works with magnification -1 and its aperture is reduced down to 36.1 mm (f/6.5) by means of a pupil stop placed at the grating plane.
Starting from a standard 2- mirror Offner spectrometer, a grating groove density of 500 lines/mm has been selected in order to achieve the required spectral sampling minimizing the spectrometer size and ghosts due to grating non-operational orders. In order to improve the image quality, a spherical lens has been added. Due to the envelope constraints the design approach of this lens is different from the one used for the HR spectrometer.
The incidence angle on the grating is approximately 17.4º. The beam is diffracted by the first order between 33.2 º (at 500 nm) and 41.9º (at 740 nm). The grating is holographic and with a saw-tooth proﬁle. After a second reflection on the primary mirror (M2 has the same radius of curvature of M1) the beam is folded by M3 mirror towards the lens L2 before reaching the focal plane (Figure 18). The optical quality is almost diffraction limited (WFE rms < 0.13 waves) and the correction of distortions is excellent (keystone less than 0.5 µm and smile less than 0.1 µm).
Figure 18: Low Resolution Spectrometer layout (image credit: FLEX collaboration)
Detector and FEE (& Front End Electronics):
Three identical CCDs (2 for HR and 1 for LR) (Figure 19 a,b), backside illuminated, split frame transfer, radiation tolerant, 4 output ports, 1072 x 460 format with 42 µm x 28 µm (spatial x spectral direction) pixel size and x 2 spatial on-chip binning have been selected. 1050 x 430 (rows x columns) will allow the acquisition of 150 km swath and of 40 nm/20 nm spectral range for the O2A /O 2B bands of HR spectrometer (258 nm for LR). Further 10 x 2 columns and 5 x2 rows are used as margin for alignment, while 5 x 2 columns and 6 x 2 rows, respectively, for dark current and smearing signal corrections. The selected format (Figure 19) with a shorter device width along the spectral direction is selected for a fast row driver (row transfer time < 1.25 µs) in order to reduce the smearing. A 35 µm active silicon thickness with a HfOx 97.9 nm anti-reflection coating has been selected in order to maximize quantum efficiency and minimize reflectivity in the O2A-O2B bands, with a max 0.65% of etalon effect (fringing) at 780 nm, which can be corrected by means of a ﬂat field measurement during calibration. A 1.7 MHz readout frequency for each port allows the download of all pixels at 43.5 ms pixel reading time. The selected gain (0.75 µV/e- for 2.5 Me- CHC (Charge Handling Capacitance) for the x 2 binning pixels) allows measurement of clouds signals both in HR and LR bands for possible inflight straylight estimation. The readout noise is reduced with a CDS (Correlated Double Sampling) device in the FEE (Front End Electronics). A dumping gate has been implemented to skip part of swath during the diagnostic mode. In this case all pixels of the swath for a homogeneous target are acquired without binning at different successive times, by using the same CCD timing as the operative mode. The operative detector temperature of 238 K ±0.10 K (to mitigate the CTE (Charge Transfer Efficiency) — degradation due to traps and to increase radiometric stability) is achieved by using an external radiator & heater/sensor near the package. No window is foreseen in order to avoid multi-reflections with the anti-reflection coating placed on the detector sensible area. Two adjacent detectors (Figure 7b) are placed in the HR focal plane along the spectral direction and separated of a distance less than 12.06 mm in order to cover the two O2 absorption bands (677–697 nm and 740–780 nm).
The FEE (Front End Electronic) is split into two parts: the FPPE (Focal Plane Proximity Electronics) devoted to pre-amplification and CCD bias conditioning, and the VAU (Video Acquisition Unit) for gain/offset drift compensation, bias and clocks generation/driving, FEE Telemetry (TM)/Telecommand (TC) and digital processing/serializing. The FPPE is placed at 5 cm from the detector while the VAU is at a distance of about 20–30 cm from FPPE. A 3 MHz ADC (Analog to Digital Converter) working at 16 bit with about 1.6 LSB total noise and three spacewire links versus nominal/redundant ME (Main Electronics) are used.
Figure 19: Detector configuration: (a) 460 columns and 536 x 2 rows are used in the Image Areas IA I-II (incl. alignment pixels), 5 x 2 columns are used for blind pixels (dark estimation) while 6 x 2 rows for smearing corrections. Two different Storage Areas (SA I-II) are used for the reading-while integrating process though 2 output ports for each one. (b) geometry of the HR1 (O2B) and HR2 (O2A) detectors with 12 mm of separation between the image pixels (image credit: FLEX collaboration)
The optical Bench is in Al6061 T6 with optics holders in Titanium or Aluminum for high thermal insulation & stability (Figure 20a). The Earth nadir baffle is also in aluminum and it is divided in three parts: an external fixed one, an internal one within the calibration unit and another internal fixed one in front of the scrambler/BP (Band Pass) ﬁlter device. Three white painted radiators cooling the detectors and the VAU's are mounted on the upper side of instrument (Figure 20b), with a full view to the cold space for optimum Spacecraft (S/C) accommodation & thermal efficiency. The isostatic mount concept with three titanium bipods at 120 degrees assures the mechanical stability (Figure 20a). The ME is separated by the instrument and is mounted on the S/C payload module.
Figure 20: (a) FLORIS Mechanical layout: internal view; (b) FLORIS Mechanical layout: external view (image credit: FLEX collaboration)
The spectral calibration is performed on ground during pre-launch activities and will be checked/updated in-flight by means of vicarious techniques using the measurement of the atmospheric gas and/or the sun Fraunhofer lines. In flight radiometric calibration is obtained with a two points calibration scheme. A Sun illuminated reflection diffuser (of Spectralon with ρ=100%, θ inc. = 65º) is observed every 7–15 days at the South Pole, while a dark reference, useful for dark current and electronic offset data, is measured every 2–3 orbits. A satellite slew of 30º is planned for the sun observation. The calibration unit is composed by a carousel (Figure 21a) which can rotate in three different positions. The ﬁrst one, relevant to the nominal observation (Nadir), allows telescope pupil illumination by the light coming from the front aperture through the nadir baffle (Figure 21b), the second one presents black ﬂat target at the telescope input for instrument dark calibration (Figure 21c), and the last one allows the sun calibration by observing a Sun-illuminated reference stable reflection diffuser with the Sun entering into the instrument by the solar port placed on the lateral side (Figure 21d). During almost all operations after instrument delivery (calibration and satellite AIV (Assembly, Integration and Verification) activities, the launch, the Earth observation and the dark calibration, the diffuser remains enclosed in a small room, protected from possible external and internal contaminations. Only during the solar calibration the diffuser is placed in the calibration position, with sun entering through the solar port.
Figure 21: Rotating Calibration Unit internal view (a) and in three different positions: (b) Earth Nadir Observation, (c) Black target observation, (d) Sun diffuser observation through the solar port (image credit: FLEX collaboration)
On Ground Calibration/Validation Activities:
The methodology used to verify instrument performance on-ground encompasses the characterization of components and subsystems during the instrument build/integration and the calibration/characterization/validation of the instrument.
The key performance criteria of components, items, and subsystems is established at various stages of instrument development so that the relevant geometric, radiometric, spectral, polarization and straylight properties are determined to verify all governing requirements during the pre-flight assembly, integration, test and calibration of the instrument. On-ground instrument calibration/characterization/validation will be performed in facilities designed to simulate the on-orbit environment using specific GSEs (Ground Support Equipments). Instrument level alignment and a reduced test campaign before and after instrument qualification will be performed at Leonardo site. Full geometric calibration as well as spectral, polarimetric, radiometric and straylight test and calibration will be performed at the calibrator facility. The calibration data, generated during on-ground calibration, will be used as the reference calibration of FLORIS when it begins its on-orbit mission. Subsequent in-flight calibration campaigns will be used to monitor and update the calibration parameters throughout the mission.
The geometric calibration includes the instrument SEDF (Spatial Response (called System Energy Distribution Function) characterization and the determination of alignment parameters relevant for instrument pointing knowledge when integrated on S/C and then in flight with respect to earth/sun target. Also spatial coregistration measurements for each and between HR and LR channels will be performed.
The spectral calibration includes the measurement of ISRF (Instrument Spectral Response Function) shape,SSI ( Spectral Sampling Interval ), SR (Spectral Resolution), Smile, and CU (Calibration Unit) spectral features.
Radiometric Calibration measures the instrument radiometric response in all spectral bands. It includes radiance calibration coefficients, response linearity, noise, detector PRNU (Pixel-to-pixel Response Non-Uniformity), detector DSNU (Dark Signal Non-Uniformity), etc. Radiometric calibration shall also include a characterization of the instrument response through the sun calibration port versus sun angle at instrument level. The radiometric calibration is performed using fully and partial linearly polarized light and fully unpolarized light.
Straylight is an important characterization and calibration parameter as the levels of straylight at various scene/radiance conditions contribute to errors in the radiometric accuracy of the instrument and in the fluorescence retrieval algorithm. Straylight assessment is one of the most challenging tasks of calibration and requires a combination of methods including point source and/or extended target stimulus in order to simulate extended non-uniform scenes.
The more stringent requirement for the Spectral Resolution (SR = 0.3 nm) is relevant to the two HR O2 absorption bands 686–697 nm and 759–769 nm (Table 2). This has been achieved by using a slit width of three times (84 µm) the pixel sampling interval (28 µm). A similar concept has been applied for the LR 697–758 nm band. For these HR and LR bands the Spectral Resolution is principally driven by the slit width (Figure 22a). The other HR bands (677–686 nm, 740–759 nm and 769–780 nm) have been obtained by means of x5 spectral binning performed at VAU level, with a spectral resolution principally driven from the aggregated pixel size (140 µm) (Figure 22b, Table 2). The remaining LR bands (500–697 nm) have been obtained by means of x3 spectral binning at VAU level, with a spectral sampling equal to the slit with (84 µ ) (Figure 22c). Finally the ISRF (Instrument Spectral Response Function) is a quasi-trapezoidal function for almost all bands and a quasi-triangular function for the LR 500–697 nm bands (Figure 22a–c). In fact, considering the achieved optical quality, the ISRF can be principally obtained by a convolution between two rectangular functions, representing the slit width of 84 µm and the spectral pixel size after binning x1, x3 or x5, corresponding respectively to 28 µm (Figure 22a), 84 µm (Figure 22c) and 140 µm (Figure 22b).
Figure 22: ISRF at focal plane (x-scale: 84 µm = 0.28 nm for HR and 1.8 nm for LR) for a slit width of 84 µm and (a) a spectral pixel of 28 µm (LR = 697-758 nm, HR = 686-697, 759-769 nm) with SR = 0.28 nm (HR) and SR = 1.8 nm (LR), (b) a x5 binned spectral pixel of 140 µm (HR = 677–686, 740–759, 769-780 nm) with SR = 0.47 nm, (c) a x3 binned spectral pixel of 84 µm (LR = 500-697 nm) with SR = 2.03 nm (image credit: FLEX collaboration)
The image quality has been evaluated from the FWHM (Full Width Half Maximum) of the SEDF (System Energy Distribution Function), which represents the spatial response of the optical system to a spectrally uniform point source. A system MTF (Modulation Transfer Function) budget has been performed taking into account the following EOL (End-Of-Life) contributions for the across and/or the along track directions: scrambler, telescope, slit, spectrometer, detector pixel response (including cross-talk and Charge Transfer Efficiency), pixel integration along motion and jitter (instrument and satellite). Then the SEDF has been obtained (Figure 23a) from the inverse Fourier Transform of the system MTF. The FWHM of SEDF in the across and along track directions (Figure 23b) are respectively 95.6 µm (along-track) and 88.7 µm (across-track) corresponding respectively to 1.14 SSD (Spatial Sampling Distance) and 1.06 SSD (with SSD = 84 µm).
Figure 23: (a) 2D SEDF at the detector focal plane; (b) Along- and across-track sections of the SEDF (SSD = 84 µm = 288 m at 805 km altitude), image credit: FLEX collaboration
The combination of a high SNR and a high spectral sampling requirement asks for a demanding instrument in terms of the smearing noise (row transfer frequency = 0.8 MHz), pupil diameter (75.6 mm) and optics transmission (37%). The pupil diameter is about three times higher with respect to that of the OLCI spectrometer embarked on the Sentinel-3 platform and the previous MERIS (on Envisat), which has a similar on ground sampling (300 m) and integration time (45 ms) but a more relaxed spectral sampling interval and spectral resolution (about 10–20 times). The comparison between SNR requirement and performance are reported in Figure 24. The minimum SNR performance is 125 at 760.7 nm with SSI of 0.093 nm, compliant with the requirement (SNR = 111) with a margin of +12%.
Figure 24: Comparison between SNR requirements and performance: blue curve = SNR requirement, green curve = SNR performance, pink curve = Reference input radiance (W/m2 /sr/µm). (a) Total Band, (b) O2A band (image credit: FLEX collaboration)
Considering that the slit represents an intermediate field stop of the system, the total instrument stray-light can be divided in two terms, one coming from the telescope and the other coming from the spectrometer. In both cases the main stray-light contributions are the scattering coming from the surface roughness of the optomechanical elements, the scattering coming from the distributed PAC (PArticle Contamination) and the ghosts. The optical design is based on very demanding roughness values (0.5 ÷1.5 nm rms for lenses and mirrors, 2.5 nm for scrambler and 2 nm for grating), low AR coatings reflectivity (0.5-1.5%), including the detector retina, high grating efficiency (>60%) and high mirror reflectivity (98.5%). Moreover, contamination will be taken under control for the overall AIT (Assembly, Integration and Test) and AIV (Assembly, Integration and Verification) activities in order to achieve challenging values of EOL PAC (250 ppm for the ﬁrst surface, 35 ppm for scrambler assembly and telescope inner surfaces and 150 ppm for all others components).
Mitigation actions will be adopted by preventing the increase of contaminants: operations in ISO 5 classroom with contamination control and cleaning of exposed surfaces (if necessary) for all the main assemblies up to the delivery to satellite prime, use of a quasi-hermetic cover and closer of carousel during almost all AIT/AIV activities at satellite site, purging with dry nitrogen during activities in which the instrument will not be in a controlled environment (long storage periods, vibration tests, route from a cleanroom to another, and other activities at satellite site), use of dedicated snorkels (cold space oriented) in the design in order to facilitate on-orbit de-contamination.
Actual performance (Figure 25) are higher with respect to level L0 requirement (0.2 mW/m2/sr/nm) of about a factor 4, meaning that the requirement at level 1b (0.04 mW/m2/sr/nm) will be obtained only after a dedicated on-ground stray-light measurement campaign and model correction activity (similar to that performed for OLCI on Sentinel-3). In-flight verification by measuring the transition between a cloud and a vegetated target and/or by observing the transition between moon and cold space are also planned.
Figure 25: Comparison between straylight radiance evaluation and requirement at level L0 (image credit: FLEX collaboration)
FLORIS Elegant Breadboard:
The FLEX pre-development has been addressed from the very beginning by ESA toward an experimental validation of the optical concept, the related alignment and test procedures and the early risk retirement of the most critical technologies as for instance detectors and gratings. An extensive breadboard development has been initiated since the phase A/B1 study of the mission, and its refurbishment is currently in progress for an EBB (Elegant BreadBoard). The current status of the breadboard includes the optical bench, the common telescope and the HR spectrometer. Service detectors with a pixel size comparable to the flight ones are implemented in the breadboard (Figure 26). The breadboard has been recently tested confirming the good optical quality of the design. A new test campaign is currently in preparation in the cleanroom ISO 5 to perform a better straylight characterization. The breadboard will be then refurbished with the inclusion of the polarization
Figure 26: (a) Optical Ground Support Equipment (OGSE) used for the phase A-B1 tests of the FLORIS BreadBoard (BB), (b) FLORIS BB for telescope and HR spectrometer (image credit: FLEX collaboration)
In summary, the baseline design is a very compact, robust, stable versus temperature and easy to align optical design. Good performance is achieved in terms of optical quality, distortions (keystone and smile) and Signal to Noise Ratio in the whole ﬁeld of view and spectral range. The common fore-optics and the double slit assembly assure an optimum spatial coregistration stability by design between the spectral channels. Particular attention has been also devoted to the reduction of the straylight effects by using efficient coatings, holographic gratings, low roughness for each optical elements and particular attention to the instrument contamination level. The adopted Offner solutions for the spectrometers have a large heritage in Leonardo. The CCD detector design is not critical from a technological point of view and a breadboard activity is on-going. The same format has been chosen for all the three focal planes in order to reduce smearing and achieve an high pixel capacitance with low noise, no Peltier stage and pixels devoted to smearing and dark current corrections in flight. A simple mechanical layout in a monolithic aluminum optical bench allows the use of isostatic mounts and a good thermal stabilization by means of radiators looking at the cold space view and heaters.
The FLEX tandem mission concept with its highly innovative FLORIS payload will provide unprecedented information on the functioning of vegetation, actual photosynthetic efficiency and health status. The primary data products contain quality controlled, spectrally and geometrically characterized and radiometrically calibrated TOA radiances that will be made available to the users within 24 h of sensing. Higher-level scientific data products comprise TOA synergy data sets from OLCI, SLSTR and FLORIS as well as fluorescence emission in the oxygen absorption lines, the spectrally integrated total fluorescence emission and the peak values around 680 nm and 740 nm. The validation of these products follows the recommendations of CEOS (Committee on Earth Observation Satellites) in that ground truth measurements at the top of the canopy will be up-scaled to the FLEX spatial resolution. The corresponding infrastructure at ground will be developed, tested, and implemented during the coming years.
As an Earth Explorer mission, FLEX can serve as a vanguard for a future operational mission supporting a large variety of services. Potential application areas comprise forest health monitoring, early detection and warning of stress-induced strain in perennial food crops, evaluation of land-use management strategies, or phenotyping assessments of food and feed crops. In coastal areas, there is the potential for using the observations to detect and track toxic algal blooms.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (email@example.com).