## 3 Discussion
In this context, the quality assessment of the HyPlant FLUO products is another important topic, which would support the quantitative comparison of the results provided by the different SIF-retrieval methods. While radiance and reflectance measurements of several homogeneous surfaces on the ground (collected with field spectrometers) can be used to validate the HyPlant DUAL TOC radiance and reflectance products, the quality assessment of the SIF maps derived from HyPlant FLUO data remains a considerable challenge. Due to the limited availability of SIF-measurement devices on the ground and the fast-changing SIF emission of vegetation within a short period of time, to date, only a few studies have been able to perform a validation using SIF measurements collected in parallel to a HyPlant overpass [46; 69]. To conduct a quality assessment of the HyPlant SIF maps without having ground truth data available, a set of quality criteria was defined within the framework of the ESA HYPER project. [70] have recently demonstrated the benefits of applying the different criteria solely calculated on the basis of SIF maps to get a first impression of data quality. In this regard, the most important criteria are the expected value ranges, which should be between 0 and 3 mW m\({}^{-2}\) nm\({}^{-1}\) sr\({}^{-1}\) for the SIF\({}_{760}\) products and between 0 and 2 mW m\({}^{-2}\) nm\({}^{-1}\) sr\({}^{-1}\) for the SIF\({}_{687}\) products. As shown in Figure 9b,c the iFLD and SFM method provide values in the expected range for SIF\({}_{760}\). However, while the values of the SFM map on average seem to be slightly too high, the iFLD map shows some negative values for soil pixels. This is much more pronounced in the SVD map (Figure 8a), where values distinctly lower than 0 mW m\({}^{-2}\) nm\({}^{-1}\) sr\({}^{-1}\) can be detected. Since the SVD SIF retrieval provides values at 740 nm (peak of the SIF emission), it is realistic that values higher than 3 mW m\({}^{-2}\) nm\({}^{-1}\) sr\({}^{-1}\) occur. After visual inspection, the SFM and the NA SIF maps at 760 nm (Figure 9c,d) provide the best results, because they show clear patterns of the different fields and breeding experiments, and less scattering in comparison to the SVD and iFLD maps (Figure 9a,b). Although the SFM and NA map are visually comparable and show similar distributions of soil and vegetation pixels, the NA values are too high. Since the NA method applied to HyPlant data is currently under development, improved SIF\({}_{760}\) and first SIF\({}_{687}\) results will be available soon. The result maps for the O\({}_{2}\)-B absorption feature (Figure 10) are clearly more affected by noise because of the distinctly lower radiance signal of green vegetation in this spectral range (cf. Figure 6a). The iFLD and SFM result maps (Figure 10b and 10c) provide mainly positive values for vegetation pixels in the expected range between 0 and 2 mW m\({}^{-2}\) nm\({}^{-1}\) sr\({}^{-1}\), but also numerous negative values for soil pixels. Similar to the O\({}_{2}\)-A SIF maps, the SVD result for the O\({}_{2}\)-B absorption feature (Figure 10a) is more noisy in comparison to the other results. (v) The combined data acquisition of HyPlant, together with hyperspectral thermal and LiDAR data in previous years, enabled a better interpretation and understanding of the SIF signal, and now facilitates the calculation of higher-level products. Besides the O\({}_{2}\)-A and O\({}_{2}\)-B SIF emission, other products, such as total SIF emission and SIF peak values at 685 and 740 nm, are currently under development. Furthermore, it is planned to generate a product related to non-photochemical energy dissipation and different products incorporating SIF as an indicator of plant stress (e.g., drought stress,heat stress, nutrient deficiency) in the future. This is important in the preparation of the FLEX satellite mission, since the aforementioned products are partly included in the list of products, which should be delivered by the FLEX satellite [17].