## 3 Discussion  
The HyPlant imaging spectrometer is the first airborne system combining a conventional hyperspectral VNIR/SWIR module (DUAL) with an additional spectrally ultra-highly resolved fluorescence module (FLUO) that enables the retrieval of SIF. In order to process the raw images from both modules into final products, a customized processing chain has been developed in recent years. Due to the increasing availability of pushbroom airborne imaging systems covering the spectral range from 400-2500 nm with an FWHM of a few nanometers, processing using the DUAL module has now become an almost standard procedure, mainly based on commercial software. The entire workflow comprises three main processing steps: radiometric, atmospheric and geometric correction. These are established procedures and have been reported several times for comparable sensors (e.g., [31, 62]). The strength of the HyPlant DUAL imaging spectrometer is the automated connection of the different processing steps, which facilitates the processing of a high number of data cubes in a short space of time. The HyPlant DUAL processing has already led to high-quality TOC radiance and TOC reflectance maps for flat terrain areas (cf. Figure 1e,f; Figure 8) and is currently being expanded by the integration of a digital elevation model (DEM). This is useful to further improve the aforementioned products, particularly in mountainous regions. The implementation of a newer version of the ATCOR-4 software is, therefore, planned, which makes it easier to consider the relief of an area within the bidirectional reflectance distribution function (BRDF) correction as part of the atmospheric correction of HyPlant  
Figure 10: SIF maps derived for the O\({}_{2}\)–B absorption feature, showing a part of the agricultural research station at Campus Klein-Altendorf, acquired on 29 June 2018 and corresponding pixel value distributions for vegetation and soil pixels: (**a**) results of the SVD method; (**b**) results of the iFLD method; (**c**) results of the SFM method. DUAL data [34]. Since a DEM or DSM is already used for the boresight correction and generation of GLT files for both sensor modules within the CaliGeoPro software, this information is available and only needs to be transferred to ATCOR-4. Alongside the main products of HyPlant DUAL processing, ATCOR-4 generates some by-products during the atmospheric correction of an image cube, which contain valuable information that can be useful in other parts of the DUAL and FLUO processing. Differentiated spatial information on AOT and WVC of the atmosphere are, for example, beneficial input data for the SIF retrieval from the FLUO module using the SFM and NA methods. ATCOR-4 provides this information in the form of an AOT map derived at 550 nm and a map showing the spatial WVC distribution within the recorded scene. The use of this information for the SIF retrieval provides an insight into current atmospheric conditions on pixel level, whereas standard atmospheric measurement devices normally record the same information only at stationary locations, with a lower spatial resolution. Further by-products of ATCOR-4 are the so-called value-added products, which give information about the LAI, the fraction of absorbed photosynthetically active radiation (FPAR) and an albedo value for every pixel in a HyPlant dataset. LAI and FPAR are particularly important for studies related to plant science. In addition, the LAI product delivered by ATCOR-4 can be integrated into the calculation of the canopy photochemical reflectance index (cPRI), which is already one of the indices calculated from HyPlant DUAL TOC reflectance data (cf. Figure 8c). Table 10 shows the equation to calculate the cPRI, where the simple ratio (SR) index is currently used as an approximation of LAI.