## 2 Materials and Methods
### Image Data Collection and Processing
#### 2.2.2 UAV Imagery Post-Processing  
Image processing, including radial and radiometric calibration, was applied to the raw MAIA/Sentinel-2 imagery using the MultiCam Stitcher Pro software (v.1.1.8). The multiband images were first co-registered in order to correct the offsets between the nine sensors on the camera system and, thus, to provide a series of single images with multiple spectral bands. The radial calibration then involved a per-pixel correction for vignetting (i.e., the effect of a reduction in illumination from the centre to the edge of the image).  
To calculate ground-leaving reflectance, two identical white ground targets were placed at opposite ends of the UAV flight extents. Each target comprised a 1.0-m\({}^{2}\) panel that was made of a Lambertian-reflectant polyvinyl chloride (PVC)-coated material ('Odyssey' trademark material, Kayospruce Ltd.), which has previously been used in ESA remote-sensing fieldwork campaigns (see [46]). At the beginning of each UAV flight, the spectral reflectance of these targets was measured using an Analytical Spectral Devices (ASD) FieldSpec Pro. This target reflectance was converted to absolute reflectance, following the guidelines outlined by the National Environment Research Council (NERC) Field Spectroscopy Facility [47]. These data were then convolved with the spectral response of each MAIA/Sentinel-2 band to correct the reflectance digital number (DN) recorded at pixels for each of the MAIA/Sentinel-2 spectral bands to absolute reflectance [45]:  
\[DN_{i}^{\prime}=\frac{Rt_{i}}{Pt_{i}}\times DN_{i} \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Farm**} & **UAV/Ground** & \multirow{2}{*}{**Growth Stage (Description)**} & **Sentinel-2 Observation** \\  & **Observation Date (2018)** & & **Date (+/– UAV Day)** \\ \hline Farm 1 & 24 th May & 39 (Stem elongation—late) & 25 th May (+1) \\ Farm 1 & 5 th June & 51 (Ear emergence) & 7 th June (+2) \\ Farm 1 & 28 th June & 69 (Flowering completed) & 29 th June (+1) \\ Farm 2 & 4 th May & 31 (Stem elongation—early) & 5 th May (+1) \\ Farm 2 & 23 rd May & 39 (Stem elongation—late) & 28 th May (+5) \\ Farm 2 & 3 th July & 77 (Milk development) & 4 th July (+1) \\ \hline \hline \end{tabular}
\end{table}
Table 1: Dates of the UAV data acquisition and ground measurements across the within-field regions of interest at Farms 1 and 2, along with the corresponding average winter wheat growth stage (Zadoks scale) and nearest Sentinel-2 imagery date (within +/– 5 days of the Sentinel-2 overpass).  
where, for each MAIA/Sentinel-2 band, the corrected DN, \(DN_{i}^{\prime}\), is calculated based on the ratio of the reference target spectra, \(R_{i}\), to the same value measured by the camera, \(Pt_{i}\). We applied corrections to the recorded position of each of the MAIA/Sentinel-2 image by matching the GNSS timestamp to that of the UAV platform. Consequently, with the timestamps matched, we were able to use the more precise real-time kinematic GNSS position recorded in the UAV flight log. The collected images were then loaded into Agisoft PhotoScan Professional (v.1.3.3), where a photogrammetric workflow was applied to align and produce an othomosaic of the multiband images covering each of the field sites and was then spatially resampled to a ground sampling distance of 5 cm.