THE EUROPEAN SPACE AGENCY (ESA) HAS RECENTLY SELECTED THE FLUORESCENCE EXPLORER (FLEX) MISSION AS THE NEXT SCIENTIFIC EARTH OBSERVATION MISSION OF ITS EARTH EXPLORER PROGRAM. THE MAIN OBJECTIVE OF FLEX, WHOSE PRINCIPAL INVESTIGATOR IS PROF. JOSE MORENO, FROM UNIVERSITY OF VALENCIA (SPAIN), IS THE RETRIEVAL OF THE VEGETATION FLUORESCENCE, WHICH IS A SUB-PRODUCT OF PHOTOSYNTHESIS. TO ACHIEVE THIS GOAL, FLEX IS EQUIPED WITH A VERY HIGH SPECTRAL RESOLUTION IMAGING SPECTROMETER, FLORIS, ON BOARD OF A POLAR-ORBITING SATELLITE. FLEX IS TO BE LAUNCHED IN 2022, AND WILL FLY IN TANDEM WITH ONE OF THE COPERNICUS/SENTINEL-3 (S3) SATELLITES. THE CO- REGISTRATION WITH S3 WILL PROVIDE THE NECESSARY ADDITIONAL INFORMATION FOR THE ATMOSPHERIC CORRECTION AND THE ESTIMATION OF VEGETATION BIOPHYSICAL PARAMETERS, WHICH WILL HELP IN THE INTERPRETATION OF THE FLUORESCENCE SIGNAL AS MEASURED BY FLORIS ESA WILL FINANCE THE MISSION DEVELOPMENT AND THE SCIENTIFIC STUDIES REQUIRED FOR THE DEFINITION OF THE MAIN LEVEL 2 PRODUCTS: VEGETATION FLUORESCENCE AND SURFACE (TOP-OF-CANOPY) REFLECTANCE. YET, THE SCIENTIFIC POTENTIAL OF FLEX GOES WELL BEYOND THOSE PRODUCTS. WITH NEW ALGORITHMS FOR BIOPHYSICAL PARAMETERS RETRIEVAL COMBINING FLEX/S3 AND SENTINEL-2 (S2), AND PROGRESS MADE IN THE INVERSION OF COUPLED VEGETATION-ATMOSPHERE RADIATIVE TRANSFER MODELS, FOR THE FIRST TIME EVER IT IS POSSIBLE TO OBTAIN ACTUAL PHOTOSYNTHESIS RETRIEVALS FROM SPACE AT A GLOBAL SCALE, AND AT A RESOLUTION PERMITTING THE FUNCTIONAL STUDY OF ECOSYSTEMS. MOREOVER, THE EXPLOITATION OF THE TEMPORAL DIMENSION OF FLEX COULD PROVIDE LEVEL 3 PRODUCTS THAT WERE SO FAR UNAVAILABLE, WHICH ARE OF GREAT SCIENTIFIC INTEREST FOR THE STUDY OF VEGETATION DYNAMICS. ADVANCED PRODUCTS INCLUDE: (1) IMPROVED PRECISION IN FLUORESCENCE ESTIMATION, (2) A PHOTOSYNTHESIS PRODUCT, E.G. GROSS PRIMARY PRODUCTION, (3) AN INDICATOR OF VEGETATION STRESS, (4) LEVEL 3 PRODUCTS BASED ON TEMPORAL DYNAMICS OF PHOTOSYNTHESIS, AND (5) IDENTIFICATION OF LEVEL 4 PRODUCTS THAT ARE BASED ON DATA FUSION WITH FLEX AND OTHER MISSIONS. THESE PRODUCTS WILL MAKE USE OF INFORMATION COMING FROM THE FLEX-S3 TANDEM MISSION AS WELL FROM AUXILIARY HIGH SPATIAL RESOLUTION S2 DATA. TO DO SO, PROGRESS IN DEVELOPMENT OF MULTI-SENSOR FUSION ALGORITHMS IS FORESEEN._x000D_ _x000D_ THESE ADVANCED PRODUCTS ARE NOT EXPECTED TO BE PART OF ESA-FINANCED FLEX GROUND SEGMENT. IT IS THE AMBITION OF THIS RESEARCH TEAM TO TAKE ADVANTAGE OF EXISTING EXPERIENCE AND THE AVAILABLE TOOLS THAT WERE DEVELOPED FOR MORE THAN A DECADE IN THE FRAMEWORK OF FLEX PREPARATORY ACTIVITIES. THE TEAM WANTS TO TAKE THE LEAD IN PROPOSING NOVEL ALGORITHMS, METHODS AND PROTOTYPES FOR FLEX SCIENTIFIC LEVEL 2, 3 AND 4 PRODUCTS._x000D_ _x000D_ FOR EACH OF THE PROPOSED ADVANCED PRODUCTS A DOCUMENT SHALL BE PREPARED THAT DESCRIBES THE THEORETICAL BASIS, CONSIDERED ALTERNATIVES, AND A DETAILED DESCRIPTION OF THE PROPOSED ALGORITHMS. BASED ON FLEX END-TO-END PERFORMANCE SIMULATOR (DEVELOPED AT UNIVERSITY OF VALENCIA) AND ON REAL S2 AND S3 IMAGES SIMULATED FLEX IMAGES SHALL BE GENERATED WHICH WILL BE USED TO DEVELOP TEST CASE MAPS FOR EACH OF THESE PRODUCTS. A CALIBRATION AND VALIDATION (CAL/VAL) PLAN SHALL BE DEVELOPED FOR EACH OF THE PROPOSED PRODUCT, AND POTENTIAL CAL/VAL TEST SITES WITHIN SPAIN SHALL BE EVALUATED. FINALLY, IN ORDER TO DETERMINE THE SUITABILITY OF THESE SITES FOR CAL/VAL ACTIVITIES AND TO SIMULATE MORE REALISTIC FLEX IMAGES, AN AIRBORNE REMOTE SENSING CAMPAIGN OVER THESE AREAS SHALL BE CARRIED OUT.