Sun-induced chlorophyll fluorescence (SIF) and vegetation-reflected radiance will be essential Level 2 operational products provided by the upcoming European Space Agency (ESA) FLuorescence EXplorer-Sentinel-3 (FLEX-S3) mission. Ensuring the accuracy of these products requires a validation process that compares satellite data with ground-reference (fiducial) measurements. To meet ESA's stringent uncertainty standards for Level 2 products, the SpaFLEX project, supported by the Spanish Ministry of Science and Innovation, aims to design and implement a robust calibration and validation (Cal/Val) strategy for the FLEX-S3 mission in Spain. This strategy will establish Cal/Val test sites; characterize ground-based, UAV, and airborne instruments; define reference measurement protocols and sampling methods; and develop uncertainty budgets for Level 2 product validation.

As a first step towards addressing these goals, in this work, we present the design and execution of a field campaign carried out in two Holm Oak forest areas of different tree densities in Sarrión - Manzanera (Teruel, Northeast Spain). In both areas, two CO2 flux towers and two fluorescence measurement systems (FloX) are installed. We analyzed the spatial heterogeneity of Sentinel-2 images in order to determine the minimum number of Environmental Sampling Units (ESUs) to carry out intensive spectroradiometric measurements with a Field-Portable ASD, quantify leaf fluorescence with a Fluowat device, and overfly with the Piccolo Fluorescence sensor onboard a UAV. For both areas, we also conducted a UAV flight campaign with the Cubert S185 hyperspectral camera for subsequent data scaling. These results are being used to define upscaling procedures towards the FLEX-S3 300 m nominal pixel.