## Materials and Methods
### UAV Mapping Experiment  
For the UAV mapping flight test of this study, the AIRS was integrated to a Gryphon Dynamics QX1400V quadcopter (GD1400) with a FPI2012b hyperspectral camera (VTT Technical Research Centre of Finland, Espoo, Finland) [28], and a Sony A7R (Sony Corporation, Minato, Tokyo, Japan) RGB consumer camera (Figure 3). GD1400 is a mid-size, multirotor UAV capable of carrying a 4.5 kg payload for a flight time up to 24 min. The FPI2012b is a hyperspectral frame camera based on a piezo-adjusted Fabry-Perot filter, allowing sequential acquisition of 1024x648 pixel snapshot images on selected spectral bands in a range of 500-900 nm. The Sony A7R is a consumer RGB camera with a 36.4-megapixel, full-frame CMOS sensor. In the installation, exposures of both cameras were synchronized to the AIRS with trigger cables. The AIRS was attached directly to the top of the UAV frame. The irradiance sensors had mostly clear hemispherical fields-of-view to the sky, with only minor obstructions from motors and propellers at the extreme low angles. The camera system was mounted under the UAV on a vibration-dampened rig. Figure 2: Flowchart of the real-time processing workflow. Experimental testing of the AIRS in realistic usage conditions was carried out at a grass experiment site located in Jokoiinen, in southern Finland, (60.82\({}^{\circ}\)N, 23.50\({}^{\circ}\)E) on 5 September 2017 around 11:30 at local time (Figure 4). The experimental field was set up to study variations and detectability of grass growth, but in this article, we did not do analysis the actual grass data. The sun had a zenith angle of 58\({}^{\circ}\) and an azimuth angle of 145\({}^{\circ}\). The area was mapped using a camera system described in Section 2.4. Figure 4: The experimental area, flight path and tilting of the UAV during the mapping experiment. (**a**) RGB orthomosaic of the Jokoiinen mapping test area created from the on-board Sony A7R images showing flight path and GCP positions. (**b**) UAV roll, pitch, yaw, and above-ground-level altitude during the flight (as recorded by GPS-INS and Kinematic GNSS receiver of AIRS). Figure 3: The unmanned aerial vehicle (UAV) set-up used in the mapping experiment. The FGI AIRS is installed on top of the UAV, while the FPI2012b hyperspectral camera and the Sony A7R consumer camera are installed under it. The size of the mapped area was approximately 250 m by 300 m. The flight consisted of 14 flight lines in approximately SE-NW directions with the nose of the UAV always pointing towards the direction of propagation. The flight produced 720 RGB images and 382 hyperspectral datacubes. In this analysis, hyperspectral images only on the 844-nm spectral band were used. The image overlaps were 80% along the path and 75% between flight lines. The images were acquired from altitudes between 50 m and 57 m above ground level, resulting in an average ground sampling distance of 5.7 cm for the hyperspectral data and better than 1 cm for the RGB data. In addition, additional on-ground measurements were performed. Firstly, an ASD FieldSpec Pro spectrometer (Analytical Spectral Devices Inc., Boulder, CO, USA) with cosine corrector was used to measure the irradiance continuously during the flight. The spectrometer was placed on the ground inside the mapped area with a distance of approximately 200 m from the furthest corner. Secondly, ten GCPs were placed around the target area, and their positions were measured with Trimble R10 RTK DGNSS (Trimble, Sunnyvale, CA, USA) with 0.03 m horizontal and 0.04 m vertical accuracy. During the flight, the atmospheric and illumination conditions represented typical suboptimal UAV mapping conditions. The atmospheric conditions were mostly blue sky with thin upper clouds and with some fast-moving broken Cumulus clouds at the lower altitude. The upper clouds caused continuous slow variations in illumination, while the Cumulus clouds passing over the area casted strong shadows over the imaged area on at least four clearly detectable occasions over the 27 min of the data acquisition.