## 3 Currently Operational UAS Systems for SIF Retrieval
### \"AirSIF\"--A UAS-Based Spectroradiometer
This is achieved by a steep screw-like flight pattern, incorporating a stabilizing gimbal, which ensures accurate pointing of the sensor to the target. A co-aligned camera provides proof of the gimbal pointing accuracy and allows structure from motion processing for highly accurate positioning of the sensor in 3D space. Using a fixed wing aircraft turned out to be mandatory  
Figure 5: (**a**) Linear regression and (**b**) comparison between mean values of Flox instrument and UAS SIF measurements. Flox footprints were always located at the center of the plots, and UAS footprints were at the middle and edge of the plots. for this mission, since it can provide enough range to reach 4 km and will sail down to ground level. This concept ensures the necessary safety and fallback scenarios to achieve legal clearance into public airspace. At the time of writing, the complex legislation process allowing the experimental UAS flights in the controlled airspace has been successful. However, in this manuscript the results of previous tests flying to a maximum altitude of 600 m AGL are shown. Further experiments with the AirFloX system onboard a manned aircraft, a manned gas balloon, and a rotary wing DJI Matrice 600 have been performed in 2018 and 2019. A focus of the medium-altitude vertical flights is the determination of the change in the O\({}_{2}\)-A and O\({}_{2}\)-B absorption versus the distance from the target (i.e., flight altitude). In fact, the change of absorption in these two bands is directly affecting the SIF estimates, therefore, an accurate parametrization is a key factor for accurate SIF retrieval using flying platforms. Due to the changing light conditions during flights, the absolute oxygen band depth value could lead to erroneous results. Therefore, the relative band depth was calculated as reported in the following formula:  
\[Band\ depth=\frac{L\lambda out-L\lambda in}{L\lambda out} \tag{1}\]  
where \(L\lambda out\) and \(L\lambda in\) are the wavelength of the shoulder and the deepest point of the oxygen absorption band, respectively. Preliminary results reported in Figure 7 are promising. The O\({}_{2}\)-B and O\({}_{2}\)-A at sensor radiance spectra (Figure 7a,b) show a stable spectral and radiometric behavior i.e., no spectral shift caused by grating distortion due to air pressure is found. Figure 6: The FloXPlane fixed wing UAS (**a**), equipped with AirFloX system (**b**,**c**). As shown in Figure 8, the two flights performed were made on a 600 m vertical column. A pseudo normalized difference vegetation index (NDVI; at sensor radiance based) was calculated showing that the target was remaining the same during the whole ascending flight duration (bare soil). The invariant target selection is a key point, since no confounding SIF emission is expected to alter the oxygen band depth. Looking at the relative band depth change, an increase of 1% was found in the O\({}_{2}\)-B band, whilst for O\({}_{2}\)-A results are less clear. Although the preliminary results are aligned with theory, a stronger absorption takes place under a larger air column leading to SIF underestimations [57]. Thus, the encouraging results obtained hitherto support the needs of more and repeated flights, covering a wider range of atmospheric conditions (e.g., different solar zenith angle ranges, aerosol concentrations, and air pressure conditions).