## 2 Materials and Methods
### HyPlant Processing Chain
#### 2.2.2 HyPlant DUAL: From Raw Signals to Vegetation Products--Processing Cluster II  
The second processing cluster consists of different steps that describe the processing from HyPlant DUAL raw data to the final products, namely, TOC radiance, TOC reflectance and reflectance index maps (Figure 3). The first processing step in this cluster involves CaliGeoPro software, developed by the sensor manufacturer Specim. To enable automized processing of numerous DUAL image cubes, a software tool based on IDL (Harris Geospatial Solutions Ltd., USA) was developed to carry out batch processing. This software generates an option file containing all the necessary information relating to a data cube (e.g., in- and output directory, location of the radiometric calibration file and the sensor's field of view). This file, together with the raw data, header information and navigation file, is subsequently transferred to CaliGeoPro. CaliGeoPro is then used to perform the radiometric correction and to generate a geographic lookup table (GLT) file, which allows the geometric correction of the image cube. The radiometric correction consists of two steps. Firstly, dark frame correction is conducted. On completion of data acquisition for an image cube, the shutter of the sensor is closed for five seconds to record some image lines containing only the dark current noise of the sensor. This noise is then linearly subtracted from each image line, and the dark frame is truncated. Secondly, the radiometric correction file (containing the central wavelength positions and associated radiometric correction coefficients for each pixel on the sensor array) is used to convert the digital numbers (DNs) to at-sensor radiance (mW m\({}^{-2}\) sr\({}^{-1}\) nm\({}^{-1}\)). In parallel, a CaliGeoPro built-in tool is used to calculate the boresight angles. The boresight effect is caused by slight differences in the locations of imaging sensors and the GPS/INS unit in the aircraft. To correct for this effect, the displacement vectors of different devices and associated boresight angles must be determined [27]. To assess the angles, a boresight calibration flight must be conducted once during a campaign, consisting of three flight lines in a triangular pattern with three overlapping regions over an area with ground control points (GCPs). Using this information and an additional digital surface model (DSM) of the area, the boresight tool within CaliGeoPro can estimate the boresight angles by minimizing the difference between the coordinates of the GCPs and the projected coordinates of the same points in the image cube [28; 29]. Afterwards, the boresight angles, together with the navigation file containing information on sensor position and orientation for each line of an image cube, are used to generate the GLT file. Applying the GLT file to the image cube removes the aircraft waving  
Figure 2: Flow chart of the first cluster of the HyPlant processing chain, providing information on the data streams of both modules and associated calibration files for the subsequent processing clusters. effects and calculates ground coordinates for each image pixel [30]. Thus, it is possible to compute a georectified and georecferenced HyPlant DUAL dataset providing at-sensor radiance information. The at-sensor radiance spectra of selected HyPlant DUAL pixels are illustrated in Figure 1b. Besides this first intermediate result, the non-georectified at-sensor radiance, together with the GLT, header and navigation file, is transferred to the second part of the HyPlant DUAL-processing cluster, which describes the atmospheric correction and subsequent generation of the final products. Like the batch-processing within CaliGeoPro in the previous step, here, a software tool (automatic HyPlant processing tool) was developed in IDL, which enables the processing of numerous data cubes at once. The commercial software ATCOR-4 (Atmospheric and Topographic CORrection algorithm, ReSe Applications GmbH, Switzerland) [31] is used for the atmospheric correction of the DUAL data.