# SpaFLEX: Field Campaign for Calibration and Validation of FLEX-S3 Mission Products +
## 2 Methodology
### Short Campaign  
The short campaign focused on intensive field sampling, which included performing regressions between biophysical parameters derived from Sentinel-2 imagery and analysis of the residue maps generated by these regressions. To ensure spatial representativeness and statistical robustness, spatial autocorrelation was analyzed using [PERSON]'s index and semivariograms to identify the most limiting variables for stratification. Based on the statistical distribution--mean and standard deviation--of these variables, the number of elementary sampling units (ESUs) was estimated (Equation (1)). Subsequent spatial interpolation using Support Vector Machine (SVM) methodology implemented in Smart-Map plugin [3] for QGIS (QGIS Development Team, 2025) allowed stratification and replication of ESUs across the study area. The biophysical parameters used were chlorophyll content (Cab), cover fraction (FCV), and fraction of photosynthetically active absorption (FAPAR), obtained with the SNAP program (ESA Sentinel Application Platform).  
\[n=((Z\cdot\sigma)/E)^{2} \tag{1}\]  
where n is the required sample size, Z is the value corresponding to a 95% confidence level, \(\sigma\) is the standard deviation and E defines the tolerable error.  
UAV measurements were made using the Cubert S186 hyperspectral sensor, (Cubert GmbH, Ulm, Germany) which captures 125 bands in the 450-950 nm range. Flights were conducted at an altitude of 120 m, resulting in a spatial resolution of approximately 4 cm and they were made on 30 May 2024. Key indices such as the Photochemical Reflectance Index (PRI) (Equation (2)) [4] for reflectance and the Red Edge Chlorophyll Index (ChlRE) (Equation (3)) [5] for fluorescence were targeted. The results of the hyperspectral flights over a representative 300 \(\times\) 300 m area provided essential insights for spatial scaling. The data collected allowed quantification of the number of field measurements required to ensure that uncertainties remain below the FLEX-S3 mission thresholds through Equation (1). Specifically, these thresholds were defined as \(<\)10% for solar induced fluorescence (SIF) and \(<\)1% for surface reflectance. ChlRE is used as a fluorescence proxy due to the strong correlation between chlorophyll content and fluorescence emission [6]. Conversely, the PRI serves as an indicator of reflectance errors in the spectral region where the non-photochemical quenching (NPQ) mechanism predominates [7]. NPQ is a critical high-level product that the FLEX mission also aims to retrieve, as it is, along with Photosynthetically Active Radiation (PAR), a primary driver of vegetation-emitted fluorescence [8].  
\[\text{PRI}=(531\text{ nm}-570\text{ nm})/(531\text{ nm}+570\text{ nm}) \tag{2}\]  
where PRI is the Photochemical Reflectance Index and 531 nm and 570 nm the reflectance values at wavelengths of 531 and 570 nanometers, respectively.  
\[\text{ChlRE}=(750\text{ nm}/710\text{ nm})-1 \tag{3}\]  
where ChlRE is the Red Edge Chlorophyll Index and 710 nm and 750 nm the reflectance values at wavelengths of 710 and 750 nanometers, respectively.