Unmanned Systems Technology 014 | Quantum Tron | Radio links and telemetry | Unmanned Aerial Vehicles | Protonex fuel cell | Ancillary systems | AUVSI 2017 Show report

38 the 2.4 GHz band) and so have not been designed for mobile applications such as UAVs and UGVs. The ‘kinetic mesh’ approach however embeds intelligence into the nodes so that there is no need for a central controller or hub (which is part of the Zigbee specification), making it more suited to hybrid swarm applications that combine UAVs and UGVs. That gives a more robust network as there can be multiple paths, and the mobile nodes expand the network’s capability. In the past, this type of technology has been deployed in the mining industry, connecting vehicles together that are constantly moving, so they need a network that supports mobility with ‘always on’ connectivity. One way to do that is to support multiple radios to mitigate any interference. Most mesh implementations use a single radio and frequency, and when interference is detected then the frequency band is changed; that though can cause problems if there is interference across all the available bands and with moving nodes. Instead, a system that has two or more radios can use a wider range of bands and does not suffer from the bandwidth reduction, as one radio can be receiving while the other is transmitting. However, that requires more complex software for choosing which radio to use on a packet- by-packet basis. This ‘instamesh’ protocol includes information that is sent with every packet so that each node knows the intended destination, and that is used to determine which node to transmit to and which frequency to use. This level of information can only be implemented close to the hardware (that is, in the MAC hardware layer), which is not accessible in other radio technologies such as wi-fi and Bluetooth but allows a latency across each node of less than 1 ms. That means for example that ten nodes can be connected together with a total latency of 3 ms across them, as the software determines the path of lowest latency across the network. This low latency is a direct result of the dual radios, which ensure there is always a transmit path available. Using multiple radios has an impact on antenna design. This is critical to ensuring a good connection between nodes that may be above or below the UAV. A kinetic mesh is now being used to operate 20 UAVs together in a swarm, which is the largest currently approved by the US regulator. This has been possible as a result of using the different radios with a wide range of frequency bands and modulation used for the connections. To provide video feeds, the latest mesh nodes include H.265 video compression that provides 1:100 compression and allows the nodes to relay UHD video back to a ground station, with network bandwidth of 50-80 Mbit/s. These feeds could be coming from all the UAVs in the swarm to give the operator a wide choice of viewpoint. Mesh networks are also being used in the 900 MHz and 2.4 and 5 GHz bands, as well as the 4.9 GHz band for public safety in the US. This allows UAVs to connect to ground vehicles that could be patrolling a perimeter, for example. Software-defined radio One way around the challenge of all the different protocols is software-defined radio (SDR). This uses an RF front end that can handle a range of bands coupled with a programmable baseband modem, usually a field programmable gate array (FPGA). The hardware for the different protocols can be downloaded to the FPGA to change the function of the device, and the band can be altered separately to match an available suitable frequency. June/July 2017 | Unmanned Systems Technology Focus | Radio links and telemetry This kinetic mesh module uses two radio front ends to minimise the latency across the mesh (Courtesy of Rajant Technology) Using multiple radios has an impact on antenna design. This is critical to ensuring a good connection between nodes