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The FLuorescence EXplorer (FLEX) mission proposes to launch a satellite for the global monitoring of steady-state chlorophyll fluorescence in terrestrial vegetation. Fluorescence is a sensitive probe of photosynthetic function in both healthy and physiologically perturbed vegetation, and a powerful non-invasive tool to track the status, resilience, and recovery of photochemical processes and moreover provides important information on overall photosynthetic performance with implications for related carbon sequestration. The early responsiveness of fluorescence to atmospheric, soil and plant water balance, as well as to atmospheric chemistry and human intervention in land usage makes it an obvious biological indicator in improving our understanding of Earth system dynamics. The amenability of fluorescence to remote, even space-based observation qualifies it to join the emerging suite of space-based technologies for Earth observation. FLEX would encompass a three-instrument array for measurement of the interrelated features of fluorescence, hyperspectral reflectance, and canopy temperature. FLEX would involve a space and ground-truthing program of 3-years duration and would provide data formats for research and applied science. Michael E (2006). FLuorescence EXplorer (FLEX): an optimised payload to map vegetation photosynthesis from space. ABSTRACTThe FLuorescence EXplorer (FLEX) mission proposes to launch a satellite for the global monitoring of steady-state chlorophyll fluorescence in terrestrial vegetation. Fluorescence is a sensitive probe of photosynthetic function in both healthy and physiologically perturbed vegetation, and a powerful non-invasive tool to track the status, resilience, and recovery of photochemical processes and moreover provides important information on overall photosynthetic performance with implications for related carbon sequestration. The early responsiveness of fluorescence to atmospheric, soil and plant water balance, as well as to atmospheric chemistry and human intervention in land usage makes it an obvious biological indicator in improving our understanding of Earth system dynamics. The amenability of fluorescence to remote, even space-based observation qualifies it to join the emerging suite of space-based technologies for Earth observation. FLEX would encompass a three-instrument array for measurement of the interrelated features of fluorescence, hyperspectral reflectance, and canopy temperature. FLEX would involve a space and ground-truthing program of 3-years duration and would provide data formats for research and applied science.

Detecting vegetation fluorescence from space would provide new insight on terrestrial biosphere response to climate variability, and help quantifying atmospheric carbon sequestration. This paper outlines technical and scientific studies undertaken for the preparation of the FLEX-Fluorescence Explorer space mission proposed to ESA in the framework of the Earth Explorer Opportunity Missions program.

FLEX is a scientifically driven space mission to provide demonstration/validation of the instrumentation and technique for measuring the natural fluorescence of vegetation in the Fraunhofer lines. The payload consists of high spectral resolution (0.1-0.3 nm) CCD imaging grating spectrometer with two channels: one in the red (648-664 nm)and one in the blue (39 1-438 nm) for working with several Fraunhofer lines. The across track FOV is 8.4°; ground spatial resolution is better than O.5x0.5km2. To increase the SIN ratio a steering mirror will be used, if necessary, to "freeze" the image and also to provide 4° across track depointing. Calibration is made by viewing the sun via a diffuser plate switched into the telescope field of view. A separate CCD camera will allow cloud detection and scene identification. A TIR radiometer will provide simultaneous surface temperature measurements.The spacecraft, overall mass estimated at 200kg, is derived from the ASI-MITA bus which provides all the necessary subsystems and stabilized platform. By use of on-board storage, ground requirements for satellite control and data link are minimized; the possibility of local stations for real time reception/distribution is also envisaged. Provisional orbit characteristics are: LEO sun synchronous, 500-900km altitude. Priority will be given to highest revisit frequency on a sufficient number of selected test sites.Keywords: Fraunhofer line detection, imaging spectrometer, fluorescence. -INTRODUCTIONFluorescence of vegetation is directly linked to photosynthesis and thus to light transformation and utilization for biomass production. Since many factors act on photosynthesis efficiency and photosynthetic capacity, at short term (stress factors: temperature, water, nutrients, ..) and long term (environmental factors: global air temperature, available PAR radiation, atmospheric composition, ..) fluorescence signal is thus highly specific of vegetation function, stress and vitality 2, 3, 4, Observation and measurement of fluorescence from space would significantly enhance our capability to monitor vegetation state and quantify biosphere processes and interactions. Although solar induced fluorescence is a very weak signal, its detection is possible using the Fraunhofer lines of the solar spectrum, a method that can be used from a satellite, as was already suggested some 25 years ago 6, 7, 8, 9 Since this time, development in optical imaging spectrometers and space technology makes this technique feasible. In a companion paper (this conference) the rationale and scientific motivations for a fluorescence space mission -FLEX -Fluorescence Explorer -recently proposed in response to ESA call for Earth Explorer Opportunity Missions, are discussed. This paper addresses the technical characteristics and mission elements: instrument concept I spacecraftl mission operations and implementation of the FLEX project, focussing on the Fraunhofer imaging spectrometer. -GENERAL MISSION CHARACTERISTICS2.1 Scientific requirements: the scientific payl...