# BVLOS Unmanned Aircraft Operations in Forest Environments
## 5 Taking a Risk-Based Approach to BVLOS Operations in Forest Environments
### Technological Considerations
#### 5.1.7 Command and Control (C2)  
Command and Control (C2) links provide necessary bi-directional communication to and from the unmanned aircraft, normally consisting of commands from the operator's Ground Control Station (GCS) and telemetry on the unmanned aircraft's operation back to the GCS. C2, by definition, is a separate data stream to payload data (such as imagery) returning from the aircraft. Depending on the technology implemented on the UA to meet the necessary DAA requirements, it may or may not be necessary to have this payload data returned to the GCS; however, C2 links are required for all UA that are not operating completely autonomously (which is beyond the scope of this paper). While radio communication is not strictly limited to VLOS, radio signals refrac and attenuate when they encounter obstacles, such as trees, terrain, or atmospheric factors [22; 23]. The amount of degradation experienced varies greatly, depending upon a large number of variables. Best practice is to consider radio transmission to also have a line of sight, even if the UA is not able to be visually seen by the operator due to distance. Operating BVLOS in a forest environment, therefore, introduces a number of challenges, with similarities to the example shown in Figure 1.  
Many Consumer Off The Shelf (COTS) unmanned aircraft operate within the Industrial Scientific Medical (ISM) bands: primarily on 5.8 and 2.4 GHz. ISM bands are used due to being gazetted under a General Use Radio License (GURL) for Short Range Devices (SRD) in most countries, thereby not requiring any other licensing or certification other than complying with ISM requirements (established by ITU Radio Regulations worldwide, the primary requirement being equipment operating within ISM bands must tolerate interference generated by other ISM applications, and users have no regulatory protection from ISM device operation in these bands.). Due to the relative ease of operating in the ISM bands, many consumer electronics beyond the scope of UA operate on these same bands, such as Wi-Fi and Bluetooth. The prolific use of the 5.8 GHz and 2.4 GHz bands can result in these bands being overpopulated with equipment operating within them, which has led to a number of incidents involving unmanned aircraft [75]. However, in the comparatively remote locations of forest environments, the flooding of these bands is unlikely. Although the remote locations of many forest operations may result in relatively empty ISM bands, there are other possible radio frequency hazards of remote area operations such as backhaul internet installations and Peer to Peer (P2P) Wi-Fi (for instance to and from milking sheds). Both of these examples use highly directional radiation patterns resulting from using high gain antennas. P2P Wi-Fi use is largely undocumented, and due to the relatively low levels of regulation of the ISM bands, it often operates at power levels beyond legal specification. Backhaul internet and other radio frequency installations may be mapped by commercial service providers [76]. Mitigations for these installations can include conducting RF surveys of the area using RF spectrum analysers. Using backhaul maps and RF surveys in operational planning, one must also consider possible interference of harmonics; for instance, a 1.2 GHz transmitter will make a 1 st harmonic at 2.4 GHz, and a 2 nd harmonic at 3.6 GHz. The relatively high frequencies of the 2.4 and 5.8 GHz bands allow for a higher data rate, providing necessary bandwidth for digital video payload links, as well as the comparatively low bandwidth requirements for C2. Conversely, high frequencies are more easily attenuated by the environment and obstacles. Due to this many modern COTS unmanned aircraft operate on both bands: 5.8 GHz when received signal strength is high, and 2.4 GHz when attenuation happens on 5.8 GHz. This fall back to lower frequencies is associated with a lower data bandwidth and lower bitrate video stream.