FROG: An unmanned aerial system for measuring solar-induced chlorophyll fluorescence and hyperspectral reflectance

The FROG (Full Range OperatinG system) is a modular spectroscopy system designed to measure solar-induced fluorescence (SIF) and hyperspectral reflectance from an unmanned aerial system (UAS). The system uses two point spectrometers to measure hyperspectral reflectance and SIF. The FROG is mounted on a commercial DJI Matrice 600 Pro UAS and has a dedicated graphical user interface (GUI) which enables customising the setup and data collection. The system was tested in the summer of 2023 in an agricultural area in Italy over five different crop fields. The drone was flown at 10 m above the ground, resulting in a sampling area of ∼ 4 m. The data was used to calculate reflectance and retrieve SIF at the O2A absorption band (i.e., 760 nm). SIF values varied among the crops, ranging from 0.2 mW m−2 sr−1 nm−1 in a dry grass field to 1.6 mW m−2 sr−1 nm−1 in a tomato field. The SIF measurements showed consistency with Normalised Difference Vegetation Index (NDVI) values measured by the UAS, which are a measure of the greenness of the fields. The FROG system can support the calibration and validation of FLEX satellite products, providing the ability to upscale SIF measurements.