Active and passive reference panels for the validation of solar-induced fluorescence

With the preparation of the FLEX satellite mission and other platforms providing quantitative measurements of solar-induced fluorescence (SIF), the calibration and validation (Cal/Val) of SIF products is becoming increasingly relevant. Owing to the dynamic nature of SIF, cross-validation requires synchronous data acquisition when performed on vegetated targets. This study investigates two new concepts for artificial SIF reference targets—active and passive—tested for airborne and proximal validation and designed for potential satellite applications. The active target uses two spectrally defined LEDs that produce a SIF-like emission spectrum. The comparably high signals were successfully retrieved using airborne and ground-based systems. Differential measurements enabled background correction for validating retrieval algorithms. The passive target uses pigments that emit SIF-like signals when induced by incoming radiation. Three designs were tested: the first proved suitable for airborne validation, the second enhanced temporal stability and the third added a near-infrared absorbing layer to fine-tune the spectral signature. While the use of these panels as Cal/Val tools for direct validation of spaceborne data (i.e. FLEX) seems challenging because of technical difficulties in producing and operating large reference panels, the findings of this study encourage the use of such panels for stepwise validation within nested Cal/Val frameworks.