# 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)
The amount of radio attenuation experienced by a transmitter varies drastically depending on numerous factors. For forest environments, the density and water content of foliage will have a dramatic effect on the performance of a payload or C2 link. Comparatively lower frequencies (such as the commonly used and gazetted 915 MHz band) are used for C2 links in commercial and military applications due to the lack of radio penetration of higher frequencies. This often results in the operations losing the payload data stream; however, maintaining command and control can allow the aircraft to reposition and resume payload data transmission. These lower frequencies do not support bandwidth capable of conveying data such as video in any meaningful resolution with current technology. The issue of higher frequencies experiencing greater attenuation and lower frequencies having lower bandwidth is one of the primary issues to solve for BVLOS, particularly in forest environments where the operator and GCS might be below the tree canopy. However, there are a number of available and feasible solutions. Traditional unmanned aircraft C2 links are a Point to Point (PTP) technology, meaning one GCS transmitter/receiver communicating to one aircraft. PTP architecture is comparatively simple, not requiring any further equipment to install (or batteries to charge), and is the most common form of C2 and payload datalink employed in UA operations. Moving beyond PTP architecture can solve many of the inherent problems of radio frequency line of sight, particularly on ISM bands. Mesh networks are one such radio architecture beyond the PTP links. Unlike PTP networks, mesh networks employ multiple receivers and transmitters, known as nodes, on a network. Nodes are dispersed strategically over the operating area. C2 and Payload data can be transmitted between these nodes over multiple short line of sight paths at higher frequencies, and finally, to and from the aircraft. This offers a number of benefits, such as high bandwidth, as well as the possibility for redundancy, depending on network configuration (data can be broadcast over multiple pathways to get to the aircraft). The downsides of the mesh network are the initial infrastructure cost and the relatively permanent nature of the network (particularly when the mesh is installed over a forestry block, mounted,for instance, in or above the forest canopy). Mesh networks could also offer further DAA options, such as stationary cameras (performing computer-based vision or electro optical traffic tracking) or ADS-B transceivers installed in each node. This concept introduces the possibility of operating relatively unsophisticated UA over a \"smart forest\". Another solution to radio frequency line of sight is the use of a single repeater station which has radio frequency line of sight to the operator's GCS and the aircraft, while being positioned between them. An example of a repeater would be an unmanned aircraft operating with a hill between the operational area and the operator, with the repeater positioned on top of the hill. This solution offers greater simplicity than a mesh network, and it is likely easier to install. However, it does not offer redundancy. The repeater concept can be taken further, with the repeater mounted on a second unmanned aircraft acting as a satellite and providing line of sight to both the unmanned aircraft and operator. This repeater unmanned aircraft would lend itself to being fixed-wing, lighter-than-air, or a tethered multi-rotor, capable of relatively long loiter times over a relatively fixed position. Cellular telephone networks, such as 3G/4G/5G have the required bandwidth to handle C2 and payload data streams, and they have been used to operate unmanned aircraft, offering RF ranges as far as the existing cellular network extends. Benefits of this architecture is the use of existing infrastructure rather than an ad hoc network. Cellular networks operate on multiple frequencies and can have exceptional high data throughput capabilities.