## 1 Introduction
It has recently been shown that a number of satellite spectrometers can be used to retrieve SiF (particularly the far-red peak). These include the Greenhouse gases Observing SATellite (GOSAT) [32; 33; 34; 35], the Global Ozone Monitoring Experiment 2 (GOME-2) [21; 36], the SCanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) [33; 37], and the Orbiting Carbon Observatory 2 (OCO-2) [38]. Future instruments capable of measuring SiF are the TROPOspheric Monitoring Instrument (TROPOMI) [39] and the dedicated FLUorescence EXplorer mission (FLEX) [40]. From the current instruments, particularly the GOME-2 instruments profit from a good spatial coverage and the availability of long historical records: GOME-2 on Metop-A has been fully operational since 2007 and GOME-2 on Metop-B since 2013. Retrieving reliable SiF signals from spaceborne remote sensing instruments has been an active field of research in the past decennium. Early work on satellite remote sensing of vegetation fluorescence investigated the possibility of using the filling-in of strong Fraunhofer lines and specifically the filling-in of the dark and spectrally wide O\({}_{2}\) A (\(\sim\)760 nm) and O\({}_{2}\) B (\(\sim\)687 nm) line absorption bands (e.g., [28; 41; 42]). The proposedmethods typically require accurate modelling of atmospheric radiative transfer in the absorption bands. A subsequent series of studies showed that filling-in of deep solar Fraunhofer lines can also be used to detect vegetation fluorescence from space [32; 33; 35; 37; 43], which in some respects simplifies the retrieval problem. The first maps of global terrestrial fluorescence using filling-in of Fraunhofer lines were achieved with the high-spectral-resolution interferometer aboard GOSAT [32; 34; 35]. GOSAT's coarse temporal and spatial resolution, however, requires considerable averaging of the individual fluorescence retrievals into spatiotemporal composites to produce global maps, which may introduce sampling or representativeness errors. It was shown by [PERSON] et al. [21; 33] that the retrieval approaches developed for GOSAT could also be applied to the imaging spectrometers SCIAMACHY and GOME-2. This improves the spatial representativeness because these instruments achieve global coverage within a few days. In addition, the statistical or data-driven retrieval approach developed by [PERSON] et al. [35] for micro-windows centred at isolated Fraunhofer lines has been extended by [PERSON] et al. [21] to much broader spectral windows covering also the O\({}_{2}\) A and water vapour absorption bands. In this method, the modelling of atmospheric transmittance is approximated using a Principal Component (PC) analysis of a set of reference spectra. A large sample of observations over fluorescence-free scenes is needed for generating a representative set of PCs. The selection of these fluorescence-free scenes, the statistical analyses of atmospheric transmittances and the choice of the fitting window can however strongly impact the retrieved fluorescence signal (e.g., [21; 36]). For example, optimization of PC selection for the fluorescence retrieval is addressed by [PERSON] et al. [36]. The physiological relationships between SiF and GPP have been described in the literature (e.g., [44; 45; 46; 47; 48; 49; 50; 51; 52]). SiF is one of many quenching mechanisms that plants use to release excess energy. The amount of excess energy depends on the efficiency with which absorbed light is used for photosynthesis, which in turn depends on rate-limiting factors such as the atmospheric CO\({}_{2}\) concentration, canopy temperature, and the availability of water and nutrients. In high light conditions, studies have shown that SiF positively correlates with photosynthesis and relates to both absorbed PAR and photosynthetic efficiency [52]. In addition, we mention that each of the two SiF peaks is an expression of two related but individual photosystems.