## 3 Prototype Design
### Hardware Assembly  
After the definition of the functional architecture, the system components were assembled inside a proper case. The authors already showed a preliminary assembled prototype in [27], but such a configuration implied a parallelepiped-shaped case, which was not the best in terms of aerodynamic  
\begin{table}
\begin{tabular}{l l} \hline
**Hardware Component** & **Selected Device** \\ \hline Antenna (towards zenith): & Aircraft’s hemispherical L1 patch \\ Antenna (toward nadir): & Antcom dual-polarization L1/L2 1G1215 RL-PP-XS-X RevA \\ RF front-end: & NSL Stereo (2 boards mutually synchronized) \\ DSP (\(\mu\)-processor board): & ODROID-X2, 1.7 GHz ARM Cortex-A9 Quad Core platform, 2 GB RAM \\ Memory: & 64 GB eMMC \\ \hline \end{tabular}
\end{table}
Table 1: Summary of the selected principal hardware components. eMMC, embedded multimedia card.  
performance. For this reason, the final sensor case was designed _ad hoc_ in carbon fiber with a neutral wing profile, and the internal components were mounted accordingly. Some computer-aided design (CAD) views are reported in Figure 3. In particular, Figure 3A depicts the carbon fiber case in light violet, double-ended with two aluminum plates in grey. The bottom plate serves as the support for the nadir-oriented antenna, depicted by the dark violet cylindrical disk, while the top lid is designed to be screwed to a rectangular plate, called the trolley unit, which connects the sensor mechanically and electrically to the aircraft body. The trolley is specifically designed to be hosted in a structure (bay) composed of two rails and fastened externally to the lower part of the aircraft, for rapid boarding of the sensors. The bay can host 3-4 sensor carts boarded with a \"plug-and-play\" mechanism. The yellow object, which stands out from the case, acts as heat sink of the internal parts: in fact, it terminates with a cylindrical part directly in contact with the microprocessor of the ODROID-X2 board, depicted in orange. The two FEs in green are just behind the ODROID-X2. All of the boards are attached to the internal faces of the case by means of small resin supports. The chosen airfoil is better visualized in Figure 3B, where it is possible to observe how the internal components are placed, in order to optimize the available space.  
Once the carbon fiber case has been manufactured, the assembly of the sensor was completed. Figure 4 shows some pictures of the final version of the prototype. In particular, the carbon fiber case is well visible in Figure 4A,B: it is screwed to the nadir-oriented antenna at the bottom side, by means of a circular aluminum plate with the function of both support and ground plane, and to the trolley unit at the top side. Figure 4B shows the sensor under lab testing connected to an HDMI monitor, mouse, keyboard and external power supply. The case has been specifically designed in such a way as to be easily opened, for fast checks and maintenance service, as illustrated in Figure 4C,D, where internal components and connections are visible.  
Figure 3: Three-quarter (**A**) and top (**B**) views of the sensor case 3D and 2D CADs, with the main internal components visible.  
Designed and assembled in such a way, the sensor prototype dimensions resulted in being \(200\) mm \(\times\)\(200\) mm \(\times\)\(250\) mm (length \(\times\) width \(\times\) height) with a weight of less than \(3\) kg, including the nadir-pointing antenna with its ground plane, so as to be sufficiently light and compact to be mounted on-board UAVs and small aircraft. In particular, the target UAV belongs to the civilian category of remotely-controlled (RC) light UAVs, or more in general, light unmanned aircraft systems (UASs), with a maximum take-off mass (MTOM), including the fuel, of less than 150 kg and an autonomy of 1-2 h. Additionally, the manned airplane is a two-seat ultralight aircraft with fixed wings.