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support other airframe configurations such as fixed-wing, helicopters, or even ground rovers. UAV COMMUNICATION ASPECTS In order to control and monitor UAVs, telemetry and command links are mandatory as they provide crucial information for the ground operators. Additionally, the transmission of video may also be required, either due to the specific application where the vehicle is being used, or as an aid for its proper operation by providing the operator with an exocentric view of the environment. In this section, we address several relevant aspects regarding air-to-ground (A2G) communications with UAVs, supported over mobile networks. WIRELESS COMMUNICATION TECHNOLOGIES In military applications, a beyond line of sight connection between the GCS and the vehicle is often provided through satellite links. However, these links are very expensive, have high latencies, and the hardware is too heavy and complex for most civil applications. For this reason, systems for the control and monitoring of UAVs in a civil context often rely on radio-frequency links, with conventional radio remote control (RC) being the most common technology employed. These radio systems generally work at the RC reserved frequency bands or at the industrial, scientific, and medical 2.4 GHz band, with most flight operations being accomplished in line of sight. Due to the different flight controller platforms available, some open source RC projects (hardware and software) have emerged to support remote vehicle control with multiple customizable features and telemetry transmission, such as OpenTx5 and OpenLRS.6 The video transmission usually requires separate hardware, which consists of a dedicated transmitter working at a higher frequency, often at 5.8 GHz. Besides RC radios, ad-hoc connections are becoming common as they can be easily implemented and, in some cases, directly support video transmission. The most popular radio system implementations are based on the Institute of Electrical and Electronics Engineers (IEEE) 802.11a/b/g/n [5], [6], IEEE 802.15.4 [4], and Bluetooth standards [7]. Despite the implementation simplicity, these radio technologies were not developed at aiming the aerial environment, their operational range is limited by the transmitter power, and the supported bit rates can be too restrictive as in the case of IEEE 802.15.4 (inadequate for video streaming). Another solution, which has received some attention in recent literature, in the context of autonomous cooperation between vehicles, relies on the use of mesh networks of UAVs. In this case, a mobile adhoc network approach can be adopted which consists of the formation of an ad-hoc airborne connection layer intended to communicate with the GCS layer [1]. The network comprises multiple nodes which are in-flight UAVs that support connectivity to their pairs [8]. In this network topology, at least one vehicle is responsible for the communication with the GCS, often through the aforementioned ad-hoc connections, thus experiencing the same limitations discussed above. Furthermore, the benefits of expanding the coverage require a great investment in several UAVs and respective operations, making it an interesting solution only in some specific applications. In [8], the possibility of employing dedicated cellular networks was discussed but this solution requires the implementation of the entire infrastructure, which most often is not economically feasible. A more viable approach is to take advantage of the existing infrastructure of mobile wireless networks. Besides their wide coverage and mobility support, current cellular technologies have capabilities in terms of throughput and latencies (see Table 1) which make them potential solutions to support real-time A2G communications with UAVs. These capabilities can make mobile cellular technology suitable not only for telemetry and commands, but also for streaming video, a functionality which is not always present in civil UAVs due to the additional hardware required. Although thorough performance assessments of cellular networks for UAVs are still lacking, the possibility of resorting to these for A2G communications was previously discussed in [10] and has started to be incorporated into some recent commercial projects such as Skydrone,7 StreamBox Drone,8 and DroneDeploy.9 The use of existing mobile radio inSkyDrone. [Online]. http://www.skydrone.aero/index.php. StreamBox Drone. [Online]. http://www.streambox.com/products/ streambox-drone. 9 DroneDeploy. [Online]. https://www.dronedeploy.com/. 7 Welcome to OpenTX. [Online]. http://www.open-tx.org/. 6 Openlrs-Opensource RC System. [Online]. https://code.google.com/p/ openlrs/. 5 AUGUST 2016 8 IEEE A&E SYSTEMS MAGAZINE 5 http://www.skydrone.aero/index.php http://www.open-tx.org/ http://www.streambox.com/products/streambox-drone http://www.streambox.com/products/streambox-drone https://code.google.com/p/openlrs/ https://code.google.com/p/openlrs/ https://www.dronedeploy.com/

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