## 3 Currently Operational UAS Systems for SIF Retrieval
### \"AirSIF\"--A UAS-Based Spectroradiometer  
The AirSIF platform (Figure 4a) shares the Piccolo-Doppio concept of splitting the optical path but using a single spectroradiometer (500-870 nm spectral range at an interval of 0.37 nm, with a FWHM of 0.8 nm), for collecting real time downwelling and upwelling radiance. AirSIF includes a dual global navigation satellite system (GNSS) antenna. Furthermore, in [36] the authors implemented the etaloning effect correction suggested by [34]. Additionally, they retrieved the corrected zenith angle between sun and cosine corrector (Ocean Optics CC-3), and the fore optic measuring downwelling radiance, depending on sun position and inclination of the cosine corrector during the UAS flight [28]. This information was computed from the platform pitch, roll and yaw obtained from an inertial measurement unit (IMU) mounted next to the cosine corrector. Cosine corrector measurements were not influenced by the azimuth angle, but sun zenith angles of \(>\)10\({}^{\circ}\) resulted in a significant deviation from the expected cosine response of the cosine corrector. Common flight conditions led to platform tilt angles of 6\({}^{\circ}\) maximum. In order to correct the uncertainty caused by the irradiance underestimation of the cosine corrector, the authors of [28] proposed to estimate a corrected zenith angle based on the dot product principle accounting for the two vectors involved: the vectors between cosine sensor and sun and the cosine corrector pointing direction. With the implementation of this function mean and maximum differences of 1.7% and 3.2% from the original radiance measurements were obtained. The latest results presented in March 2019 by [48] at the International Network on Remote Sensing of Terrestrial and Aquatic Fluorescence conference, reported the results of a comparison between AirSIF-derived spectra measured at 8 m AGL over barley experiment plots versus ground-based SIF retrieved with a high-resolution references system, the Fluorescence Box (Fox, JB Hyperspectral Devices, Dusseldorf, Germany) [37] mounted on a field-bike (Figure 4b). The results are visualized in a map of spatially explicit and geolocated AirSIF footprints with their respective O\({}_{2}\)-A SIF ranges at edge and middle sampled plots overlaid over an orthophoto derived from UAS imagery (Figure 4c). Flox footprints were estimated as round shapes, where the size was estimated from sensor height above canopy according the 25\({}^{\circ}\) FOV of the Flox instrument. The study had four main shortcomings: (i) the spatial misalignment of the footprints between repeated UAS flights, (ii) the lack of accurate geolocation of Flox ground measurements, (iii) the difference in footprint size between AirSIF and Flox (0.5-2.5 m\({}^{2}\)), and (iv) the resulting significant difference between UAS- and Flox-based SIF measurements. The accuracy of the footprint determination of the AirSIF platform was analyzed in a recent study being \(\pm\)15 cm for 10 m AGL height [49]. Figure 3: **(a)** Instrument presented by [33] and used by [31,44], highlighting the position of the solar induced ChIP (SIF) QEPro, downwelling and upwelling fore optics and VNIR QEPro (source: Dr. [PERSON], BBSRC/NERC ATEC project at University of Edinburgh-GeoSciences); and (**b**) the relationship between incoming radiation and SIF under ambient and elevated [CO\({}_{2}\)] in a mature oak forest, measured at the BIFoR FACE site in the UK. An initial test was carried out by comparing a single AirSIF flight with Flox observations from the field bike. The measurements were collected at the Campus Klein-Altendorf, an agricultural research station affiliated to the Agricultural Faculty of the University of Bonn (Germany). In total, 55 observations over nine \(3\times 5\) m barley plots were collected, three per plot by the Flox instrument, and between one and four per plot by AirSIF at 10:45-12:00 (Fox) and 12:00-12:15 (AirSIF), respectively.