Aerospace and Electronic Systems Magazine August 2016 - 8

Cellular for the Skies: Exploiting Mobile Network Infrastructure for Low Altitude Air-to-Ground Communications functions. There are various protocols available for the communication between MAVs and GCSs, e.g., UAVTalk, used by the OpenPilot platform, Micro Air Vehicle Communication Protocol (MAVLink10), adopted by the popular APM, and Ivy, implemented in the Paparazzi platform. These protocols provide a specific format for the transmission of telemetry, e.g., GPS position, IMU measurements, speed, barometric readings, battery status, and commands. They define a set of telemetry messages, the encapsulation method, and state machine for transmission of information over a serial communication channel, wired or wireless. To give an idea of Figure 4. the communication requirements, Flexible UAS architecture with support for cellular radio and other access technologies, multiple operators, and multiple UAVs. the MAVLink protocol defines the telemetry messages as arrays of 8 to 263 bytes, transmitted with a frequency of up to 20 Hz. Therefore, NAT traversal techniques for establishing direct or relayed conthe telemetry streams will require bitrates up to 42 kbps. nections. As mentioned before, a video stream is often necessary when controlling a UAV in order to assess the environment where the FAIL-SAFE MECHANISMS vehicle is deployed. The bitrate required to receive video with acceptable quality may typically lay between 200 kbps and 800 When exploiting mobile radio infrastructure and a packet-switched kbps, depending on the codec used, frames per second (fps), and network such as the Internet, there are performance issues that are video bitrate. Many different protocols may be used for streamrelevant for real time communications, namely latency, packet ing video over IP networks. A protocol extensively employed is loss, and jitter, which can impact the UAVs' operation and make the real-time transport protocol (RTP), RFC 3550, encapsulated manual flight potentially unsafe. Therefore, in order to deal with typically over the user datagram protocol. However this may crenetwork delays and possible low reliability of the connection, the ate problems when traversing firewalls. For this and other reasons, operation of UAVs under these networks should be based on auHypertext Transfer Protocol (HTTP) live streaming (HLS)11 and tonomous and semi-autonomous flight modes. Furthermore, as multiple 3G/4G networks are often availother competing alternatives are becoming increasingly popular. able, the probability of losing radio coverage can be reduced by Not only because they facilitate better connectivity but also bedesigning a UAS that operates with several simultaneous comcause they adapt better to network conditions by enabling adapmunication links. However, as the communication can still fail, tive bitrate streaming over HTTP. However, RTP, HLS, or similar additional fail-safe mechanisms can be implemented. A possibilalternatives are only defined for encapsulating the encoded data, ity is to incorporate the UAV with the capability of performing which for video can be based on H.261, H.263, H.264, and Moving self-rescue by backtracking. While the vehicle is airborne, if a Picture Experts Group-4 standards. loss of connectivity is detected and the link is not re-established, a local command can be executed by the flight controller in order to FLEXIBLE UAS ARCHITECTURE 1) fly to the last geolocation with radio coverage using the same route or 2) decide on an emergency landing. Other solutions exist, In Figure 4 we present a flexible UAS architecture for multiple UAVs such as the one proposed in [13], where a path planning algorithm and GCSs where radio links can be established through cellular netbased on the A* algorithm is used so as to maintain a continuous works. The system comprises the following three main modules: communication link with the vehicle. This solution requires previC UAV Register and UAV Relay are applications located in a ous knowledge about the network's cells conditions, which is not ground located server. The UAV Register is used for regisalways possible. tering and exchanging network information from each UAV MONITORING AND CONTROLLING UAVS Remotely operating UAVs requires specific application level protocols for executing monitoring, control, and mission command 8 MAVLink, Micro Air Vehicle Communication Protocol. [Online]. http:// qgroundcontrol.org/mavlink/start. 11 HTTP Live Streaming, https://tools.ietf.org/html/draft-pantos-http-livestreaming-16. 10 IEEE A&E SYSTEMS MAGAZINE AUGUST 2016 http://qgroundcontrol.org/mavlink/start http://qgroundcontrol.org/mavlink/start https://tools.ietf.org/html/draft-pantos-http-live-streaming-16 https://tools.ietf.org/html/draft-pantos-http-live-streaming-16

Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2016