## 3 Currently Operational UAS Systems for SIF Retrieval
### \"AirSIF\"--A UAS-Based Spectroradiometer
The results showed a link between the SIF retrieved from both platforms, which was stronger but negative for UAS footprints located at the plot edge (Figure 5a). A paired \(t\)-test showed no significant differences between the mean SIF retrieved from UAS and Flox. Moreover, there seems to be an underestimation of \(\approx\)13% with UAS-based measurements (Figure 5b). The implementation of a dual GNSS antenna system [50], the use of an IMU in the correct position, and the appropriate flight and sensor configurations [51] were found crucial to acquire the highest accuracy in the platform pose characterization and footprint localization for the improvement of SIF estimates, especially over small experimental plots like the ones presented here. Consequently,  
Figure 4: (**a**) AirSIF system components, (**b**) field bike used for ground Flox measurements, and (**c**) footprints of the flight at 12:00 local time with their respective O\({}_{2}\)-A SIF at edge (blue) and middle (red) sampled plots over the orthophoto derived from UAS photogrammetry [49]. the authors of [52] published the results of research concerning boresight (IMU-spectroradiometer-camera misalignment) and lever arm (GNSS antenna-spectroradiometer offset) correction methods. AirFloX\" on Board of the \"FloXPlane\"--Atmospheric Interference on SIF Retrieval Across a Multiple Klimeter Air Column  
The AirFloX is the lighter and simplified version of the Flox system manufactured by JB Hypersectral Devices [53], and up to date the only existing commercially available instrument for the continuous long-term top of canopy SIF measurements. Whilst FloX design, reported in [54], relies on the use of two channels to collect nearly simultaneously upwelling and downwelling at sensor radiance, AirFloX is designed to only acquire data collected by one channel (downward looking fiber), making it more similar, in a conceptual approach, to an image sensor. Due to the distance from the canopy at which the sensor is expected to fly, up to 4 km, the upwelling at sensor radiance is expected to be biased by atmospheric absorption, thus not easily used for SIF retrieval. The fluorescence retrieval will be performed by applying a similar processing scheme as the one used in imaging sensors data processing, e.g., HyPlant [20]. The investigations described here, alongside airborne (HyPlant) studies for SIF retrieval, contribute to integrate into the bigger picture of the FLEX satellite mission expected to be launched in 2022 [55]. Within this context, one of the major challenges is to precisely understand the impact of large atmosphere columns on a kilometer scale and their impact on SIF retrieval. Background studies started elucidating this topic, as presented by [56], who showed that SIF is strongly influenced by the atmospheric scattering and absorption, which in turn are defined by factors such as the surface pressure and concentration of aerosols. In practice, the higher the sensor-surface distance, the larger the air column above the targets [19], the stronger the scattering and the extinction of the signal in the atmosphere due to the absorption by aerosols and oxygen molecules, respectively. Hence, at this multiple km vertical scale, retrieving SIF becomes more challenging due to the need of increasing the accuracy of the atmospheric correction. Recently, researchers from JB Hypersectral Devices developed a high altitude and light fixed wing UAS, called 'FloXPlane' (Figure 6a), equipped with the AirFloX system (Figure 6b,c). The FloXPlane was manufactured with two purposes: (i) sampling large areas suitable for satellite calibration/validation (cal/val), and (ii) characterizing the surface radiance of one fixed spot at ground level across a multiple kilometer (up to 4000 m) air column. Their goal is to measure the atmospheric absorption of the upwelling SIF emission, considering that the majority of oxygen and aerosols are present in the innermost atmosphere layers.