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

34 Focus | Radio links and telemetry to serve multiple users simultaneously Silicon for 802.11ax is likely to emerge in 2018, but will take a couple of years to reach full production. Bluetooth 5 is also set to come to market in silicon, later this year. It will provide four times the range of the current versions at the same 2.4 GHz frequency with twice the bandwidth of existing Bluetooth 4 links for the control links as well as reducing the power consumption. Suppliers of blocks for chip designs have already engaged with multiple customers whose silicon will incorporate the features of version 5. The chips will initially be used in UAV designs with dedicated controllers, then with the next generation of mobile phones as the Bluetooth 5 technology rolls out to a wider market. Another way of boosting the range and performance is to use a mesh protocol on top of the radio link. This is being introduced with Bluetooth 5, where each Bluetooth node can connect to a neighbour and feed data back to a central hub, and various radio chip makers have their own implementation of mesh protocols. Using a mesh protocol can extend the range of the network and is useful for swarm-type applications with lots of systems, but has some drawbacks. The time taken to hop from one node to the next increases the latency, which can vary as the control software in the hub does not know how many hops the data will take. Similarly, each node has to transmit its own data and data from other nodes, which can reduce the bandwidth available by half for each additional node. That limits the use of such a mesh for payloads, although there have been other mesh designs that avoid this problem. There are various satellite systems available for connecting to unmanned systems in rural or hostile environments such as boats at sea. Geostationary satellites sit above a set point of the Earth’s surface at a height of 35,768 km, which requires large antennas and high- power links up to the satellite in the Ku band (12-18 GHz) but the unmanned system remains connected to one satellite. In recent years, satellites in low Earth orbit (LEO) with L-band links have offered lower power connectivity. These satellites orbit at 780 km and so need much less power for the radio link, but are only visible to a system on the surface for about ten minutes; that requires a more complex handover scheme. LEO satellites also have to be replaced every ten years or so, and the next-generation satellites are being launched this year. Ten satellites were launched in January, with eight now operational, and ten more at a time will launch over the next 18 months until there are 70 in orbit. The network will provide a full internet protocol network connection that will allow more data services at rates of up to 1.4 Mbit/s for basic telemetry and location services, albeit to a limited number of users. June/July 2017 | Unmanned Systems Technology Seventy satellites are being sent into low Earth orbit over the next 18 months to provide IP data links across the globe (Courtesy of Iridium) PARAMETER SPECIFICATION Iridium satellite orbit Low Earth Orbit altitude 780 km Iridium applications Voice and data Satellites in constellation 70 User satellite link band 1616-1626.5 MHz L band Gateway -> Satellite uplink 29.1-29.3 GHz Satellite -> Gateway downlink 19.1-19.6 GHz Inter-satellite link 22.55-23.55 GHz Satellite relative velocity 26,804 kph Minimum angle of elevation for acceptable operation 8.2° Approximate satellite view time 9-10 minutes Access scheme FDMA/TDMA Frequency re-use factor 12 Total system capacity 172,000 users A low Earth orbit satellite comms system (Courtesy of Iridium)