## 2 Materials and Methods
### HyPlant Processing Chain
#### 2.2.2 HyPlant DUAL: From Raw Signals to Vegetation Products--Processing Cluster II
ATCOR-4 is based on MODTRAN5 (MODerate resolution atmospheric TRANsmission) [32] and uses LUTs (lookup tables) in order to accelerate the atmospheric correction. For the atmospheric correction of an image cube, the non-georectified at-sensor radiance image file must be available in 'band sequential' (BSQ) format, and an input file has to be generated, containing flight altitude, mean ground elevation and the heading of the aircraft. Further, the coordinates, acquisition date and time of the image cube are required to calculate the solar geometry (zenith and azimuth angle). All this information is automatically extracted from the navigation and header file of the image cube and added to the ATCOR-4 input file. Furthermore, additional settings like the aerosol type and spectral region  
Figure 3: Flow chart of the second cluster of the HyPlant processing chain, presenting the entire workflow involved in processing HyPlant DUAL data, from raw images to final products. used for the water vapor algorithm need to be selected before ATCOR-4 can perform the atmospheric correction. Since several flight lines acquired from one test site during an airborne campaign have similar atmospheric conditions, the same aerosol model and spectral region for water vapor retrieval can be chosen within the batch process. ATCOR-4 also enables detection of the spectral smile effect. This is an artifact in the data of pushbroom sensors like HyPlant, which is caused by the sensor entrance slit, and leads to a shift in the center wavelength of the spectral bands in the across-track direction [33]. A further option in ATCOR-4 allows the running of an inflight radiometric calibration to be performed. During this process, ground reflectance spectra of bright and dark targets are compared to the corresponding image pixels to determine radiometric calibration coefficients, which can be used to improve the results of the atmospheric correction results [34]. Finally, the determined smile effect for each spectral band, the generated input file and the inflight calibration coefficients are used to run the atmospheric correction of the image cube. As two of the main results, the user receives TOC radiance and TOC reflectance. Afterwards, spectral polishing can be applied to the data, to remove spectral artifacts before the spectral smile interpolation is conducted to correct the data for this spectral misregistration effect. Figure 1c,d illustrate the TOC radiance and TOC reflectance of selected HyPlant DUAL pixels. Based on the TOC reflectance product, several reflectance indices are additionally calculated, using another IDL sub-tool that can be run in batch mode. The indices are related to leaf area index (LAI) and chlorophyll content (e.g., NDVI, EVI, MTCI and TCARI), photosynthesis and non-photochemical quenching (PRI, CPRI), and canopy water content (WBI). A comprehensive list of all indices and their calculation can be found in Table 1.  
As the last step in the second processing cluster, the GLT file generated in CaliGeoPro is used to produce georectified and georeferenced maps of TOC radiance, TOC reflectance and indices. Figure 3 shows some result maps of the HyPlant DUAL processing.