# The Value of Sentinel-2 Spectral Bands for the Assessment of Winter Wheat Growth and Development
## Materials and Methods
### UAV Platform and Data
#### 2.2.2 Data 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. 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 [39]. 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 [40]. 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 [37]:  
\[DN_{i}^{\prime}=\frac{Rt_{i}}{Pt_{i}}\times DN_{i}, \tag{2}\]  
where for each MAIA/Sentinel-2 band the corrected DN, \(DN_{i}^{\prime}\), is calculated based on the ratio of the reference target spectra, \(Rt_{i}\), to the same value measured by the camera, \(Pt_{i}\).  
We applied corrections to the recorded position of each of the MAIA/Sentinel-2 images by matching the GNSS time-stamp to that of the UAV platform. Consequently, with the time-stamps 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  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**MAIA/Sentinel-2**} & \multicolumn{3}{c}{**Sentinel-2 MSI**} \\ \hline \multirow{3}{*}{**Band Number**} & **Band** & **Central** & \multirow{3}{*}{**Band Width**} & \multirow{3}{*}{**Band Number**} & **Spatial** \\  & **Description** & **Wavelength** & & **(nm)** & & **Resolution (m)** \\ \hline
1 & Violet & 443 & 20 & 1 & 60 \\
2 & Blue & 490 & 65 & 2 & 10 \\
3 & Green & 560 & 50 & 3 & 10 \\
4 & Red & 665 & 30 & 4 & 10 \\
5 & Red Edge1 & 705 & 15 & 5 & 20 \\
6 & Red Edge2 & 740 & 15 & 6 & 20 \\
7 & NIR 1 & 783 & 20 & 7 & 20 \\
8 & NIR 2 & 842 & 115 & 8 & 10 \\
9 & NIR 3 & 865 & 20 & 8A & 20 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Description of the MAIA/Sentinel-2 UAV multispectral camera bands and the corresponding Sentinel-2 multispectral instrument (MSI) bands, including the near-infrared (NIR) bands, and spatial resolution. Note: Band 1 (violet) was not used in the MAIA/Sentinel-2 band analysis.  
a photogrammetric workflow was applied to align and produce an othomosaic of the 18 multiband images covering the trail plots with a ground sampling resolution of 0.04 m.  
The multiband MAIA/Sentinel-2 orthomosaics were overlaid with vector polygons for each of the 50 winter wheat trials plots. A buffer of \(-0.5\) m was applied to the polygon edges in order to ensure a representative coverage of the trail plots. For each plot, the vector dataset was then used to extract mean pixel values recorded in the MAIA/Sentinel-2 dataset. Since band 1 of the MAIA/Sentinel-2 data is measured by the Sentinel-2 multispectral instrument at a 60 m spatial resolution (Table 2), we considered this resolution to be too coarse for precision agricultural applications and, therefore, we omitted this band and reduced the analysis to the eight remaining bands.