## 2 Methods
### Mounted Sensors  
Sensors mounted on UAVs are limited in their dimensions and weight. Despite the use of lightweight materials, a reduction in manufacturing quality, data storage capacity and on-board processing features is inevitable [6]. However, the development of miniaturized electronic components in the last decades enables scientist to mount a variety of sensors on UAVs for small-scale remote sensing applications. The Carolo P360 is equipped with two sensors, a multispectral and a thermal sensor. However, thermal sensor data were not used in this study.  
The core of the sensor equipment is a 12-band miniature multi-camera array Mini-MCA 12 (MCA hereafter) (Tetracam Inc., Chatsworth, CA, USA). The compact modular construction of the MCA integrates two basic modules into one rugged chassis. Each module consists of an array of six individual CMOS sensors (1280 \(\times\) 1024 pixels; pixel size 5.2 \(\upmu\)m), lenses (focal length 8.5 mm) and mountings for user definable band-pass filters (Figure 3). Images can be stored on 2 GB CF cards for every sensor either in DCM or RAW format. The PixelWrench2 software (Tetracam Inc., Chatsworth,  
Figure 2: The Carolo P360 unmanned aerial vehicle (UAV) during a mission with open landing gear doors.  
CA, USA), shipped with the camera, enables the conversion of those images into single/multiband TIF or JPG format. Camera settings can be modified and enable the user to select camera exposure times between 1 ms and 20 ms (1 ms increment) and store images at a dynamic range of either 8 or 10 bit. In order to find an adequate exposure time, several ground-based tests prior to this study were performed at blue sky conditions over vegetation plots. A high dynamic range for most bands and no saturation at the same time was found for a fixed exposure time of 4 ms, which was then used in this study. The dynamic range was set to 10 bit.  
The interchangeable band-pass filters, manufactured by Andover (Andover Corp., Salem, NH, USA), were selected prior to delivery, based on their center wavelengths and bandwidths. The 12 narrow-band filters cover, almost equally distributed, the spectral range from visible to near infrared wavelengths with focus on the characteristic reflectance features of healthy vegetation including the chlorophyll absorption band around 650 nm, the red-edge region between 680 nm and 730 nm and one of the water absorption bands around 950 nm. The bandwidth (full width at half maximum FWHM) varies from 9.1 nm to 40.8 nm with increasing bandwidths towards larger wavelengths. The filter configuration and characteristics are given in Table 1.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Band**} & **Center** & **FWHM \({}^{*}\) Coordinates** & **Bandwidth** & **Peak** \\  & **Wavelength (nm)** & **(Bandwidth) (nm)** & **(10\%) (nm)** & **Transmission (\%)** \\ \hline b\({}_{471}\) & 471 & 466.0–475.1 (9.1) & 12.8 & 68.3 \\ b\({}_{515}\) & 515 & N/A (\(\approx\)10.0) & N/A & N/A \\ b\({}_{551}\) & 551 & 545.5–555.6 (10.1) & 14.8 & 56.4 \\ b\({}_{613}\) & 613 & 607.7–617.8 (10.2) & 14.2 & 67.6 \\ b\({}_{658}\) & 658 & 653.4–662.9 (9.5) & 13.6 & 69.2 \\ b\({}_{713}\) & 713 & 708.1–717.7 (9.6) & 13.4 & 63.0 \\ b\({}_{761}\) & 761 & 756.2–766.7 (10.5) & 14.7 & 71.9 \\ b\({}_{802}\) & 802 & 797.3–807.3 (10.1) & 14.5 & 56.3 \\ b\({}_{831}\) & 831 & 826.3–835.8 (9.5) & 13.1 & 55.3 \\ b\({}_{861}\) & 861 & 856.4–866.4 (10.1) & 14.0 & 64.2 \\ b\({}_{899}\) & 899 & 891.3–907.7 (16.4) & 22.9 & 63.6 \\ b\({}_{953}\) & 953 & 933.0–973.8 (40.8) & 58.2 & 69.6 \\ \hline \hline \end{tabular} \({}^{*}\) FWHM = Full width at half maximum.  
\end{table}
Table 1: Filter configuration of the Mini-MCA 12 and optical properties of the mounted filters. For band 2 (b), no fact sheet has been provided (N/A).  
Figure 3: Tetracam Inc. miniature multi-camera array Mini-MCA 12 with mounted narrow-band (10–40 nm) filters that cover the spectral range between the visible and the near-infrared light (470–953 nm; both center wavelengths).