## 2 Materials and Methods
### Data Link  
For the intended type of UAV application, there are several possible communication solutions [35]. The IEEE 802.11 standard is a low-cost solution operating in the 2.4 to 5 GHz range. Another solution is cellular 3/4G communication; however, since SAR missions target campaigns on non-urban and possibly mountainous terrain, it is likely that a wireless signal will not be available. Solutions based on analogue signal communications, however, have problems with poor image quality and low resolution. Satellite links are popular solutions for larger fixed-wing aircraft and UAVs but are not practical for smaller UAVs due to their equipment size [36], antenna design and energy efficiency [37]. Moreover, the price of satellite link systems is an issue. There are a few challenges in transmitting data through an open-air interface [38; 39]. Interfering signals that occur in the same frequency band as the wireless video link will act as in-band noise (for example, another UAV in the area, a nearby Wi-Fi hotspot or a mobile phone). Reflection, scattering, diffraction and shadowing effects together with a direct or visual line-of-sight (VLOS) path are known as multipath propagation in communications and result in fading of the received signal due to constructive and deconstructive superposition [40]. Multiple paths and reflections can exist, especially in cluttered or urban environments. Multipath signals cancel the direct propagation path. Cancelling occurs because of the phase shift associated with different propagation delays. Lastly, mobility is an imperative issue that needs to be addressed in SAR UAVs. Along with robustness, keeping the throughput intact while a UAV is moving is also important. Moreover, the mobility requirements of data links are high for fixed-wing aircrafts compared to rotary quadcopters. As already described in the previous section, the expected maximal distance of the UAV to the IPP (and the GS in its vicinity) is around 5 km. Therefore, long-range solutions for communications at distances over 50 km [41; 42] will not be discussed here. It should be mentioned that these kinds of solutions are generally much more expensive than others. Therefore, the important requirement of keeping the system costs within a reasonable price range would be very difficult or impossible to satisfy. Future 5G and beyond 5G systems are also not likely to be available in typical non-urban SAR environments [43]. Finally, after considering available solutions on the market such as those in [44; 45; 46; 47; 48], a Microhard pMDDL2450 data link [49] (Table 2) that has the required features while being in the acceptable price range was chosen for testing and inclusion as part of a UAV communication system for SAR missions. To determine the viability of the proposed data link, a simple link power budget calculation can be performed (Equation (5)). This calculation sums all the gains and losses from the UAV transmitter, cables and connectors, as well as the free space to the GS receiver [50]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Data Rate** & **MIMO** & **Rx Sensitivity** & **Frequency** & **Range** & **Weight** & **Channel Bandwidth** \\ \hline
20 Mbps & \(2\times 2\) & \(-98\) dBm & 2.402–2.478 GHz & 5 km+ & \
\begin{tabular}{c} OEM 7 g + \\ Motherboard 50 g \\ approximately \\ \end{tabular} &
\begin{tabular}{c} 8 MHz and \\ 4 MHz \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table} 
Table 2: pMDDL2450 data link specification. \[P_{RX}(dBm)\ =\ P_{TX}(dBm)+G_{ATX}(dBi)\ -\ L_{TX}(dB)\ -\ L_{FS}(dB)\ -\ L_{M}(dB)+G_{ARX}(dBi)\ -\ L_{RX}(dB) \tag{5}\]  
where \(P_{RX}\) is the received power, \(P_{TX}\) is the transmitter power output, \(G_{ATX}\) is the transmitter antenna gain, \(L_{TX}\) is the losses from the transmitter (cable, connectors, etc. ), \(L_{FS}\) is the free space path loss, \(L_{M}\) is the miscellaneous losses (fade margin, polarization misalignment, etc. ), \(G_{ARX}\) is the receiver antenna gain and \(L_{RX}\) is the losses from the receiver (cable, connectors, etc.).