Development and Testing of a Two-UAV Communication Relay System

sensors

Article Development and Testing of a Two-UAV Communication Relay System

Boyang Li, Yifan Jiang, Jingxuan Sun, Lingfeng Cai and Chih-Yung Wen *

Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China; [email protected] (B.L.); [email protected] (Y.J.); [email protected] (J.S.); [email protected] (L.C.) * Correspondence: [email protected]; Tel.: +852-2766-6644

Academic Editors: Felipe Gonzalez Toro and Antonios Tsourdos Received: 22 July 2016; Accepted: 10 October 2016; Published: 13 October 2016

Abstract: In the development of beyond-line-of-sight (BLOS) Unmanned Aerial Vehicle (UAV) systems, communication between the UAVs and the ground control station (GCS) is of critical importance. The commonly used economical wireless modules are restricted by the short communication range and are easily blocked by obstacles. The use of a communication relay system provides a practical way to solve these problems, improving the performance of UAV communication in BLOS and cross-obstacle operations. In this study, a communication relay system, in which a quadrotor was used to relay radio communication for another quadrotor was developed and tested. First, the UAVs used as the airborne platform were constructed, and the hardware for the communication relay system was selected and built up. Second, a set of software programs and protocol for autonomous mission control, communication relay control, and ground control were developed. Finally, the system was fully integrated into the airborne platform and tested both indoor and in-ﬂight. The Received Signal Strength Indication (RSSI) and noise value in two typical application scenarios were recorded. The test results demonstrated the ability of this system to extend the communication range and build communication over obstacles. This system also shows the feasibility to coordinate multiple UAVs’ communication with the same relay structure.

Keywords: two-UAV system; communication relay; mission control software; Ground Control Station

1. Introduction In recent years, as a result of decreased weights and sizes, reduced costs, and increased functionalities of different sensors and components, Unmanned Aerial Vehicles (UAVs) are becoming increasingly popular in a wide range of domestic applications (photography, surveillance, environment monitoring, search-and-rescue, etc.) [1–3]. As the market develops and the variety of applications expands, more requirements are now being imposed on extending the ﬂight range and increasing the adaptability to the complex missions of UAV systems. In missions such are surveillance and search-and-rescue, UAV systems are usually required to operate in complicated environments while still being able to maintain uninterrupted communication with the ground control station for the purpose of reporting, monitoring, and control. For example, in the mountainous or high-density urban regions, the communication between UAVs and the ground control station can be easily interrupted, causing the ground control station to lose real-time data feedback from the UAVs, leading to mission failure. In addition, if the mission requires long ﬂight range, the communication might also be interrupted by the distance. Commercially available yet economical UAV communication solutions, however, have a number of limitations that potentially hinder their widespread application. First, the communication range of major civil UAV communication solutions is comparatively short. Telemetry modules, such as 3DR

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range of major civil UAV communication solutions is comparatively short. Telemetry modules, such Sik2,as 3DR XBee, Sik2, and XBee, other and Wi-Fi other modules, Wi-Fi which modules, are common which are options common for civil options UAVs, for have civil communication UAVs, have rangescommunication that are limited ranges tothat a feware limited kilometers. to a Second,few kilome it isters. difficult Second, to establishit is difficult a stable to establish data link a stable when obstaclesdata link suchwhen as obstacles trees, tall such buildings, as trees, or tall mountains buildings, separate or mountains the UAVs separate and GSC. the UAVs and GSC. In situationssituations discussed above, it can be difficult difficult to complete the mission with only one UAV deployed, while keeping cost andand systemsystem complexitycomplexity requirementsrequirements metmet [[4–7].4–7]. To harness the advantages of UAVs inin thesethese situationssituations andand minimizeminimize thethe drawbacksdrawbacks atat the same time, an alternative solution is to deploy a multi-UAV system,system, whichwhich couldcould utilize the inter-connectivity among multiple UAVs toto maintain uninterrupted communication betweenbetween every UAV and the ground control control station. station. In this study, aa UAVUAV communicationcommunication relayrelay solutionsolution waswas developed,developed, whichwhich usesuses relayrelay andand routingrouting toto extend the communication range and bypass obstaclesobstacles atat lowlow cost.cost. The objective was to develop and test a proof-of-concept two-UAV systemsystem that demonstrateddemonstrated the ability to re relaylay radio radio communication. communication. This system only consisted of two UAVs, butbut itsits designdesign couldcould accommodateaccommodate the addition of more UAVs. TheThe system uses one UAV as communication relay point and enables the other UAV to operate inin areasareas wherewhere directdirect communicationcommunication withwith groundground controlcontrol cannotcannot bebe established.established. This configurationconfiguration allows communication to be established across obstaclesobstacles or over a distance exceeding the range of the onboardonboard radioradio transceiver,transceiver, bothboth ofof whichwhich willwill bebe demonstrateddemonstrated inin fieldfield teststests later.later. The typicaltypical application scenario of thisthis systemsystem isis shownshown inin FigureFigure1 1 schematically. schematically.

Figure 1. Application scenarios of UAV communication relayrelay system.system.

The paper is organized as follows. Section 2 introduced the related works. Section 3 introduces The paper is organized as follows. Section2 introduced the related works. Section3 introduces the hardware configurations of the multiple UAVs communication relay system. Section 4 discusses the hardware conﬁgurations of the multiple UAVs communication relay system. Section4 discusses the software aspects of the system. In Section 5, both the indoor and outdoor tests methods and results the software aspects of the system. In Section5, both the indoor and outdoor tests methods and results are presented. A conclusion of the work is given in Section 6. are presented. A conclusion of the work is given in Section6.

2. Related Related Works Works Numerous researches havehave beenbeen performedperformed inin UAVUAV communicationcommunication relayrelay scheme.scheme. In the conceptual studystudy of of UAV UAV based based military military communication communication relays, relays, Pinkney Pinkney et al. et [al.8] described[8] described a multi-year a multi- airborneyear airborne communication communication range range extension extension program program plan plan which which was was primarily primarily used used to support to support the Battlefieldthe Battlefield Information Information Transmission Transmission System. System. The The main main objective objective of the of programthe program was was to provide to provide the beyondthe beyond line ofline sight of (BLOS)sight (BLOS) communications communications capabilities capabilities without without the help the of scarce help satelliteof scarce resources. satellite Theirresources. range Their of coverage range of can coverage vary from can 50 vary to 100 from miles 50 for to UAVs100 miles of interest. for UAVs Rangel of interest. et al. [9] Rangel developed et al. a multipurpose[9] developed aerial a multipurpose platform using aerial a UAV platform for tactical usin surveillanceg a UAV for of tactical maritime surveillance vessels. They of concludedmaritime thatvessels. the communicationThey concluded relaythat the using communication a UAV system rela isy a using low-cost a UAV application system whichis a low-cost could application provide air dominancewhich could with provide satisfied air dominance communication with rangesatisfied with communication lost cost resources. range In with Orfanus lost cost et al. resources. [10], the use In ofOrfanus the self-organizing et al. [10], the paradigm use of the to self-organizing design efficient paradigm UAV relay to networksdesign efficient to support UAV military relay networks operations to wassupport proposed. military operations was proposed. Abundant previous previous studies studies have have focused focused on on the the algorithms algorithms for forvehicle vehicle position position assignment assignment and andnetwork network configuration. configuration. Choi Choiet al. [11] et al. developed [11] developed guidance guidance laws to laws optimize to optimize the position the position of a Micro- of a UAV which relayed communication and video signals when the other UAV is out of radio contact range with the base. Zhan et al. [12] investigated a communication system in which UAVs were used

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Micro-UAV which relayed communication and video signals when the other UAV is out of radio contact range with the base. Zhan et al. [12] investigated a communication system in which UAVs were used as relays between ground terminals and a network base station. Their work mainly focused on developing the algorithm for optimizing the performance of the relay system. In [13,14], Cetin and coworkers proposed a novel dynamic approach to the relay chain concept to maintain communication between vehicles in the long-range communication relay infrastructure. Their path planning method was based on the artificial potential field. Many other researchers used simulation methods to propose and test the relay systems. Hauert et al. [15] conducted 2D simulations of the design on a swarm system which does not make use of positioning information or stigmergy for application in aerial communication relay. Jiang and Swindlehurst [16] simulated the dynamic UAV relay positioning for multiple mobile ground nodes and multi-antenna UAVs. In [17], a relay network composed of UAVs keeping the Wireless Sensor Network connected was proposed. They also used cooperative multiple input multiple output (MIMO) techniques to support communication. Ben Moussa et al. [18] provided a simulation-based comparison between two strategies for an autonomous mobile aerial node communication relay between two mobile BLOS ground nodes. Lee et al. [19] proposed an airborne relay-based regional positioning system to address pseudolite system limitations. They conducted simulations and demonstrated that the proposed system guarantees a higher accuracy than an airborne-based pseudolite system. For experimental studies, most of the previous works were based on IEEE 802.11b (Wi-Fi) standard. Brown et al. [20,21] implemented a wireless mobile ad hoc network with radio nodes mounted at fixed sites, ground vehicles, and UAVs. One of their scenarios was to extend small UAVs’ operational scope and range. They measured detailed data on network throughput, delay, range, and connectivity under different operating regimes and showed that the UAV has longer range and better connectivity with the ground nodes. Jimenez-Pacheco et al. [22] also built a mobile ad hoc network (MANET) of light UAVs. The system has implemented a line of sight dynamic routing in the network. Morgenthaler et al. [23] described the implementation and characterization of a mobile ad hoc network for light flying robots. Their UAVNet prototype was able to autonomously interconnect two end systems by the airborne relay. Asadpour et al. [24,25] conducted simulation and experimental study of image relay transmission for UAVs. Yanmaz et al. [26] proposed a simple antenna extension to 802.11 devices for aerial UAV nodes. They tested the performance of their system and showed that 12 Mbps communication can be achieved at around 300 m. Yanmaz et al. [27] also did a follow up work on a two-UAV network using 802.11a. In this work, they showed that high throughout can be achieved using two-hop communications. Johansen et al. [28] described the field experiment with a fixed wing UAV acted as a wireless relay for Underwater Vehicle. They only tested the download data rates. Notably, although the Wi-Fi network is convenient to build multiple access points, its communication range is highly restricted and a large number of nodes are needed for long range applications. One of the advantages of a Wi-Fi network is its high throughput. However, for UAV telemetry communication, the distance is a more important requirement than the transmission speed. For experimental works without the use of Wi-Fi network, Guo et al. [29] analyzed the use of small UAVs for 3G cellular network relay. They tested the upload and download throughputs and the ping time in both urban and rural environments. Deng et al. [30] developed the UAVs and the communication relay system for power line inspection. Their system consisted of multi-platform UAVs and multi-model communication system for image/video transmission. However, they did not test the communication range or quality of their system. The only one work used 433/915 MHz as the communication frequency was Willink et al. [31] who measured the low-altitude air-to-ground channel at 915 MHz. They mainly focused on studying the spatial diversity to and from the airborne node and also measured time delay and received power. For all above-mentioned works, The UAVs in their systems did not exchange messages with the GCS by the relayed communication, which means the UAVs were used as carriers to realize communication relay for other systems rather than acting as the active parts in the relay systems. Therefore, the applications would be constrained if UAVs lost connection with the GCS. The highlights Sensors 2016, 16, 1696 4 of 21 ofSensors our 2016 work, 16 are, 1696 the building of an interface and protocol with the flight controller and direct monitoring4 of 20 and control of the UAVs with relayed communication. This study provides a viable and economic and economic way, with detailed hardware and software methods described, to construct a prototype way, with detailed hardware and software methods described, to construct a prototype of the UAV of the UAV communication relay system. communication relay system.

3. Hardware Hardware Platform Platform In this this section, section, the the hardware hardware construction construction of of the the miniature miniature two-UAV two-UAV system system is introduced. is introduced. A Aminiature miniature UAV UAV relay relay system system with with the the ability ability to to relay relay radio radio communications communications was was built. built. The The hardware hardware mainly consisted of three communication nodes: a a GCS; GCS; a a UAV UAV that relays radio communication, which was named “Mom”; and a second UAV namednamed “Son”,“Son”, whosewhose communicationcommunication with the GCS was relayed through the UAV “Mom”.“Mom”. The schematic of the major hardware platform for the “Mom and Son” relay system is illustrated in Figure2 2 and and thethe detaileddetailed communicationcommunication subsystemsubsystem willwill bebe introduced in Section 3.33.3 later.later.

Figure 2. Whole architecture of the communication relay system.

3.1. System Architecture The relay system can be conceptualizedconceptualized as the following majormajor functional blocks, with the corresponding hardware architecture shown in FigureFigure3 .3. TheThe ﬁgurefigure mainlymainly describesdescribes thethe mainmain components in UAV “Mom”“Mom” whilewhile allall otherother UAVsUAVs havehave thethe same same hardware hardware architecture. architecture.

• Aircraft: ThisThis isis the the body body of of the the UAV, UAV, usually usually consisting consisting of structural of structural components, components, power system,power andsystem, battery. and Inbattery. the test In system, the test the aircraftsystem, werethe constructedaircraft were using constructed commercially using available commercially frame kits.available Both frame of the kits. UAVs, Both i.e., of “Mom” the UAVs, and “Son”,i.e., “Mom were” selectedand “Son”, as quadrotorswere select fored easilyas quadrotors launch and for recoveryeasily launch ability and during recovery the development ability during period. the Communication development period. relay systems, Communication however, are relay not limitedsystems, to however, quadrotors are and not can limited be integrated to quadrotors into any and desired can be airborneintegrated platform into any such desired as fixed airborne wing UAVplatform for longsuch enduranceas fixed wing and UAV distance for long relay endurance service. and distance relay service.  Flight Controller: This includes main flight control boards as well as the necessary sensors, that • Flight Controller: This includes main flight control boards as well as the necessary sensors, together provide attitude and position control for the UAVs. In this test system, 3DR Pixhawk that together provide attitude and position control for the UAVs. In this test system, 3DR Pixhawk kit [32] was selected as the main flight controller. The controller has an internal IMU, barometer, kit [32] was selected as the main flight controller. The controller has an internal IMU, barometer, external GPS, and a compass for attitude and position measure. It also includes safety switch, external GPS, and a compass for attitude and position measure. It also includes safety switch, buzzer, LEDs, and different kinds of interfaces combined to provide flight control. buzzer, LEDs, and different kinds of interfaces combined to provide flight control.  Communication Module: This functional block directly handles wireless radio • Communication Module: This functional block directly handles wireless radio communications, communications, and consists of radio transceivers and antennas. The paired modules will and consists of radio transceivers and antennas. The paired modules will connect automatically connect automatically and transmit data instructed by the mission controller. and transmit data instructed by the mission controller.  Mission Controller: This functional block is a microcomputer running Linux operation system • Mission Controller: This functional block is a microcomputer running Linux operation system and mainly responsible for the following functions: (1) conducting communication flow control, and mainly responsible for the following functions: (1) conducting communication flow control, message routing, and message error checking; (2) monitoring UAV flight conditions and message routing, and message error checking; (2) monitoring UAV flight conditions and reporting reporting to GCS; and (3) translating mission commands from GCS to low-level instructions, to GCS; and (3) translating mission commands from GCS to low-level instructions, which are which are directly supplied to the flight controller. directly supplied to the flight controller.  Ground Control Station: This functional block refers to a Windows-based laptop running the • Ground Control Station GCS software to send commands: This functional to all the block UAV refersnodes to and a Windows-basedmonitor the status laptop of all running UAVs. the  GCSEmergency software Remote to send Controller commands: The toall remote the UAV controller nodes andis connecte monitord theto statusthe flight of all controller UAVs. via • Emergency2.4 GHz wireless Remote communication. Controller: TheIn normal remote operation, controller all is the connected commands to and the status flight can controller be set viaby the 2.4 GHzGSC wirelesspoint. This communication. device has switch In normal to directly operation, takeover all the the commands control of andUAVs status in case can beof emergency situation in the debugging process.

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set by the GSC point. This device has switch to directly takeover the control of UAVs in case of emergency situation in the debugging process. Sensors 2016, 16, 1696 5 of 20

Figure 3. System hardware architecture.

3.2. Communication Subsystem To constructconstruct the the communication communication subsystem, subsyste a numberm, a ofnumber commonly of usedcommonly wireless communicationused wireless technologiescommunication were technologies considered were as considered the possible as candidate:the possible IEEEcandidate: 802.11n IEEE Wi-Fi, 802.11n 3G/4G-LTE Wi-Fi, 3G/4G-LTE mobile network,mobile network, and 433 and MHz/915 433 MHz/915 MHz radio.MHz radio. Wi-Fi Wi-F cani provide can provide a reliable, a reliable, high-speed high-speed connection, connection, but hasbut has a comparatively a comparatively small small range. range. Typical Typical Wi-Fi Wi-Fi routers routers have have an an effective effective range range of of fewerfewer thanthan 100 m, rendering themthem unsuitableunsuitable forfor long-rangelong-range outdooroutdoor missions.missions. TheThe applicabilityapplicability ofof thethe 3G/4G-LTE3G/4G-LTE mobile network relies heavily on the strength of mobile phone signal, which varies greatly in different locations,locations, and is not alwaysalways available in remoteremote areasareas wherewhere surveillancesurveillance andand rescuerescue missionsmissions areare likely to take place. A 433 MHz/915 MHz radio can be operated without a license in many parts of likely to take place. A 433 MHz/915 MHz radio can be operated without a license in many parts of the world, and can be implemented with easily accessible, low-cost transceivers (for example, 3DR the world, and can be implemented with easily accessible, low-cost transceivers (for example, 3DR Radio Telemetry as discussed below). A range of a few kilometers could be achieved. The radio can Radio Telemetry as discussed below). A range of a few kilometers could be achieved. The radio can also be deployed at any desirable site without the need for supporting infrastructure. These features also be deployed at any desirable site without the need for supporting infrastructure. These features give 433 MHz/915 MHz radio advantages in various outdoor missions. The frequency options, 433 MHz give 433 MHz/915 MHz radio advantages in various outdoor missions. The frequency options, or 915 MHz, can be selected to ensure compliance with local radio regulations. 433 MHz or 915 MHz, can be selected to ensure compliance with local radio regulations. Based on the discussion above, to establish communication between the UAVs and GCS and to Based on the discussion above, to establish communication between the UAVs and GCS and facilitate the communication relay system, 3DR 915 MHz Radio Telemetry (3DR Radio) [33] was to facilitate the communication relay system, 3DR 915 MHz Radio Telemetry (3DR Radio) [33] was selected as our radio transceiver hardware. 3DR Radio, the built-in processor of which runs open- selected as our radio transceiver hardware. 3DR Radio, the built-in processor of which runs open-source source SiK firmware [34], has the following useful features that enable communication relay: SiK firmware [34], has the following useful features that enable communication relay:  Built-in time division multiplexing (TDM) support. •  Built-inSupports time frequency division hopping multiplexing spread (TDM) spectrum support. (FHSS) among a configurable number of channels • Supports(configured frequency to 50 channels hopping inspread this study). spectrum Its frequency (FHSS) among hopping a configurable sequence is numberuser-configurable. of channels (configured to 50 channels in this study). Its frequency hopping sequence is user-configurable. Figure 4 illustrates the radio communication relay setup. Four 3DR Radios were used, numbered T1-1,Figure T1-2, 4T2-1, illustrates and T2-2. the radio Table communication 1 gives the details relay setup. of the Four configuration 3DR Radios and were installation used, numbered of the T1-1,telemetries. T1-2, T2-1,T1-1 andand T2-2.T1-2 carried Table1 givescommunication the details between of the configuration GCS and UAV and “Mom” installation exclusively, of the telemetries.whereas T2-1 T1-1 and and T2-2 T1-2 carried carried communication communication between between UAV GCS “Mom” and UAV and “Mom” UAV exclusively,“Son”. whereas T2-1 and T2-2 carried communication between UAV “Mom” and UAV “Son”.

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FigureFigure 4. 4.Radio Radio communicationcommunication relay relay setup. setup.

Table 1. Telemetry Installation and Settings. Table 1. Telemetry Installation and Settings. 3DR Radio Installed at Frequency Hopping Setting 3DR RadioT1-1 Figure Installed4. GCSRadio communication at hopping Frequencyrelay setup.sequence Hopping 1 Setting T1-1T1-2 UAV “Mom” GCS hopping hopping sequence sequence 1 1 T1-2T2-1 Table UAV UAV1. Telemetry “Mom” “Mom” Installation hopping and Settings. hopping sequence sequence 2 1 T2-2 UAV “Son” hopping sequence 2 T2-13DR Radio InstalledUAV “Mom” at Frequency Hopping hopping Setting sequence 2 T2-2 UAV “Son” hopping sequence 2 T1-1 GCS hopping sequence 1 A combination of TDM and FHSS techniques was used to ensure that the presence of multiple T1-2 UAV “Mom” hopping sequence 1 radio telemetries did not lead to radio-frequency interference. By default, telemetries Tn-1 and Tn-2 A combination of TDMT2-1 and FHSS UAV techniques “Mom” was hopping used to sequence ensure 2 that the presence of multiple (where n = 1, 2…) formedT2-2 one radio UAV link “Son”between each hopping another. sequence On power-up, 2 Tn-1 and Tn-2 first radiosearched telemetries for synchronization did not lead to signals radio-frequency from each an interference.other; these signals By default, were used telemetries to synchronize Tn-1 and the Tn-2 (where n = 1, 2 ... ) formed one radio link between each another. On power-up, Tn-1 and Tn-2 ﬁrst internalA clocks combination of both of telemetries. TDM and FHSS Once techniquesthe clocks werewas used synchronized, to ensure that the thetelemetries presence then of multiple started a searchedTDMradio forprocess, telemetries synchronization in whichdid not Tn-1 lead signals toand radio-frequency fromTn-2 alternately each another; interference. transmitted these Bysignals default, data over weretelemetries radio, used Tn-1and to synchronize listenedand Tn-2 to the internalincoming(where clocks ndata of= 1, both when2…) telemetries.formed not transmitting. one radio Once link The the between alternatio clocks wereeachn of another. a synchronized, transmission On power-up, window the telemetries Tn-1 occurred and Tn-2 thenat afirst fixed started a TDMfrequency, process,searched in andfor which synchronization the time Tn-1 between and Tn-2signals consecutive alternately from each altern transmittedanother;ations these was data signals referred over were radio,to asused the and to turnover synchronize listened time. to the incomingThe dataselection wheninternal not of clockstransmitting. the turnover of both telemetries. time The depends alternation Once on the the ofcloc time aks transmission wererequired synchronized, to transmit window the messages. occurredtelemetries On at then one a ﬁxed started hand, frequency, athe and theturnoverTDM time process, betweentime should in consecutivewhich be long Tn-1 enou and alternationsgh Tn-2 to allow alternately an was adequate referred transmitted number to as data theof messagesover turnover radio, to time. andbe transmitted listened The selection to in of incoming data when not transmitting. The alternation of a transmission window occurred at a fixed the turnovera single transmission time depends window. on the On time the required other hand to transmit, the turnover messages. time should On one not hand, be too the long, turnover otherwise,frequency, the and other the timetelemetry between would consecutive have to alternwait ationslonger was before referred it can to transmit as the turnover its own time. messages, The time should be long enough to allow an adequate number of messages to be transmitted in a single causingselection delays of the in turnover information time dependsupdate, onand the compromising time required tothe transmit synchronization messages. Onaccuracy one hand, between the transmissiontelemetries.turnover window. time The shouldvalue On of be thethe long turnover other enou hand,gh timeto allow thecould turnoveran beadequate tuned time based number should on of the messages notabove-mentioned be tooto be long, transmitted principles. otherwise, in the otherIn telemetry athis single research, transmission would the have turnover window. to wait time On longer wasthe beforeotherset to hand it100 can, ms,the transmit turnoverwhich itsensured time own should messages,stable not operation be causing too long,of delaysthe in informationcommunicationotherwise, update, the relayother and compromisingtelemetryfunction. Valuwouldes the haveranging synchronization to wait from longer 90 ms before to accuracy 140 it canms betweentransmithave been its telemetries. testedown messages, and could The value of theprovide turnovercausing similar delays time results. in could information This be process tuned update, is based illustrated and oncompromising the in Figures above-mentioned the5 and synchronization 6. principles. accuracy In between this research, telemetries. The value of the turnover time could be tuned based on the above-mentioned principles. the turnover time was set to 100 ms, which ensured stable operation of the communication relay In this research, the turnover time was set to 100 ms, which ensured stable operation of the function.communication Values ranging relay fromfunction. 90 msValu toes 140ranging ms havefrom 90 been ms testedto 140 ms and have could been provide tested and similar could results. This processprovide is similar illustrated results. in This Figures process5 and is illustrated6. in Figures 5 and 6.

Figure 5. Time division multiplexing (TDM) working principle.

Figure 5. Time division multiplexing (TDM) working principle. Figure 5. Time division multiplexing (TDM) working principle.

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Figure 6. TDM process flow chart. Figure 6. TDM process ﬂowflow chart. In addition to the TDM process, the transmission of telemetries Tn-1 and Tn-2 were frequency- In addition addition to to the the TDM TDM process, process, the transmission the transmission of telemetries of telemetries Tn-1 and Tn-1 Tn-2 andwere Tn-2frequency- were hopped. The telemetries transmitted in one of the 50 sub-channels that were obtained by dividing the frequency-hopped.hopped.915 MHz The band telemetries into The50 equal transmitted telemetries portions. in transmitted On one the of occurrence the 50 in sub-channels one of of the the alternation 50 that sub-channels were of a obtained transmission that by were dividing window, obtained the by915both dividingMHz Tn-1 band theand into 915Tn-2 50 MHz equalsimultaneously band portions. into On 50hopped equalthe occurrence to portions. a new ofsub-channel, On the the alternation occurrence which of awas oftransmission thedetermined alternation window, by ofa a transmissionbothrandom Tn-1 channeland window, Tn-2 hopping simultaneously both sequence Tn-1 and shared Tn-2hopped simultaneously between to a new Tn-1 sub-channel, and hopped Tn-2. to As a which newshown sub-channel, wasin Table determined 1, T1-1 which and by was a determinedrandomT1-2 were channel byconfigured a hopping random to usesequence channel a different hoppingshared hopping between sequence sequence Tn shared-1 andthan Tn-2. betweenT2-1 andAs shown Tn-1T2-2. andTherefore, in Table Tn-2. 1,although As T1-1 shown and inT1-2multiple Table were1, configureddevices T1-1 and might T1-2 to beuse weretransmitting a different conﬁgured radio hopping towaves use se quenceat a the different same than time, hoppingT2-1 the and sub-channels sequenceT2-2. Therefore, thanin which T2-1 although they and T2-2.multipleare transmitting Therefore, devices might althoughare different, be transmitting multiple as illustrated devices radio in waves might Figure at be7. the transmitting same time, radiothe sub-channels waves at the in which same time,they theare sub-channelstransmitting are in whichdifferent, they as are illustrated transmitting in Figure are different, 7. as illustrated in Figure7.

Figure 7. Frequency hopping of telemetries. 3.3. Mission Control SubsystemFigure 7. Frequency hopping of telemetries. The control of key onboard functions: communication relay, flight status monitoring, and 3.3. Mission Control Subsystem mission control, is centrally managed by the mission control subsystem, which UAV “Mom” and “Son”The controlbothcontrol carry of of key onboard.key onboard onboard The functions: onboardfunctions: communication mission communica controltion relay, subsystem relay, flight statusflight is mainly monitoring,status in monitoring,charge and of mission the and control,missionfollowing iscontrol, centrally functions: is centrally managed managed by the mission by the control mission subsystem, control subsystem, which UAV which “Mom” UAV and “Mom” “Son” bothand “Son” both carry onboard. The onboard mission control subsystem is mainly in charge of the carry onboard.Executing The radio onboard communication mission control frame subsystemprocessing and is mainly relay control. in charge of the following functions: following functions:  Decoding commands contained in frames, and translating the commands into low-level • Executing radio communication frame processing and relay control.  Executinginstructions radio to thecommunication UAV’s flight controller.frame processing and relay control. • Decoding commands contained in frames, and translating the commands into low-level   DecodingMonitoring commands UAV flight contained status, and in repo frames,rting to and the GCStranslating on a regular the basis.commands into low-level instructions to the UAV’s flight controller.  instructionsHandling adto thehoc UAV’s functions flight that controller. specific missions necessitate, for example, image processing, • Monitoring UAV flight status, and reporting to the GCS on a regular basis.  Monitoringdynamics flightUAV routeflight planning,status, and sm repoart targetrting identification,to the GCS on etc. a regular basis. • Handling ad hoc functions that specific missions necessitate, for example, image processing,  Handling ad hoc functions that specific missions necessitate, for example, image processing, dynamics flight route planning, smart target identification, etc. dynamics flight route planning, smart target identification, etc.

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TheThe above-mentioned above-mentioned functions functions requirerequire the UAVs to be be equipped equipped with with an an on-board on-board computer computer Sensors 2016, 16, 1696 8 of 20 withwith adequate adequate processing processing power. power. In In this this research, research, Raspberry Raspberry Pi Pi 2 2 Module Module B B (Rpi (Rpi 2B) 2B) was was chosen chosen as as the onboardthe onboardThe computer above-mentioned computer for both for both UAVs functions UAVs because require because of itsthe of relativelyUAVs its relatively to be small equipped small size, size, strongwith strong an processing on-board processing computer power, power, and abundantandwith abundant adequate IO port. IO processing Figureport. Figure8 shows power. 8 shows the In on-boardthis the research, on-board computer Raspberry computer module, Pi module, 2 Module with with Rpi B (RpiRpi 2B mounted 2B2B) mounted was chosen on on top. top.as the onboard computer for both UAVs because of its relatively small size, strong processing power, and abundant IO port. Figure 8 shows the on-board computer module, with Rpi 2B mounted on top.

Figure 8. Onboard computer module. Figure 8. Onboard computer module. The onboard computer needsFigure to frequently 8. Onboard exchange computer module.data with and acquire information from otherThe onboard onboard devices: computer radio needs telemetries, to frequently the flight exchange controller, data the withGPS module, and acquire and various information onboard from The onboard computer needs to frequently exchange data with and acquire information from othersensors. onboard The onboard devices: computer radio telemetries, communicates the flight with controller,the peripheral the devices GPS module, through and USB various ports. Rpi onboard 2B other onboard devices: radio telemetries, the flight controller, the GPS module, and various onboard sensors.readily The supports onboard USB computer protocol communicatesand has four USB with port thes available. peripheral Most devices peripheral through devices, USB ports. however, Rpi 2B sensors. The onboard computer communicates with the peripheral devices through USB ports. Rpi 2B readilyutilize supports a low-level USB communication protocol and method has four named USB portsUniversal available. Asynchronous Most peripheral Receiver and devices, Transmitter however, readily supports USB protocol and has four USB ports available. Most peripheral devices, however, utilize(UART). a low-level Therefore, communication a protocol conversion method namedmodule Universal based on AsynchronousCP2102 microchip Receiver was wired and Transmitterbetween utilize a low-level communication method named Universal Asynchronous Receiver and Transmitter (UART).the(UART). USB Therefore, ports Therefore, of Rpi a protocol 2Ba protocol and the conversion co peripheralnversion module moduledevices, based which on on handles CP2102 CP2102 the microchip microchip conversion was was between wired wired between the between USB theprotocol USBthe USB ports and ports ofthe Rpiof UART Rpi 2B 2B and protocol. and the the peripheral peripheralFigure 9 shows devices, devices, the which hardware handles handles conn the theection conversion conversion diagrams between betweenof the the onboard theUSB USB protocolcomputerprotocol and andmodule the the UART UARTwith protocol. other protocol. subsystems. Figure Figure9 9shows showsThe Rpi the the 2B hardwarehardware of “Son” conn connectionwasection connected diagrams diagrams to two of ofthe USB the onboard devices onboard computerwhichcomputer are module T2-2 module and with withthe other flight other subsystems.controller. subsystems. The TheThe Rpi Rpi 2B 2B 2Bof of“Mom” of “Son” “Son” waswas was connectedconnected connected to three totwo two USB USB USB devices devices devices whichincludingwhich are are T2-2 T1-2, T2-2 and T2-1, and the theand flight flight the controller.flight controller. controller. The The Rpi Rpi 2B2B of “Mom” was was connected connected to tothree three USB USB devices devices includingincluding T1-2, T1-2, T2-1, T2-1, and and the the flight flight controller. controller.

(a) (b) (a) (b) Figure 9. Hardware connection diagram of (a) UAV “Son” and (b) UAV “Mom”. Figure 9. Hardware connection diagram of (a) UAV “Son” and (b) UAV “Mom”. Figure 9. Hardware connection diagram of (a) UAV “Son” and (b) UAV “Mom”. The onboard computer module is powered by the UAV’s main battery and is switched on when The onboard computer module is powered by the UAV’s main battery and is switched on when the UAV is powered up. After power-up, the onboard computer module then automatically executes theThe UAV onboard is powered computer up. After module power-up, is powered the onboar by thed computer UAV’s main module battery then andautomatically is switched executes on when a self-initialization program, which configures necessary system parameters and starts up a set of the UAVa self-initialization is powered up. program, After power-up, which configures the onboard necessary computer system moduleparameters then and automatically starts up a set executes of

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software programs. The programs then run on the onboard computer module and execute functions a self-initialization program, which configures necessary system parameters and starts up a set of such as communication relay control and mission control. Details about software structure and software programs. The programs then run on the onboard computer module and execute functions realization of the communication relay subsystem will be discussed in Section 4. such as communication relay control and mission control. Details about software structure and realization of the communication relay subsystem will be discussed in Section4. 4. Software Development 4. SoftwareThe software Development developed in this project includes mission control software running on the mission controlThe board software and developedMini GCS running in this project on the includes GCS laptop. mission The control mission software control runningsoftwareon is in the charge mission of controlanalyzing board the andtarget Mini node GCS as runningwell as the on thecommand GCS laptop. contents The of mission each message control softwareit receives. is inIf chargethe ID of analyzingthe messages the target represents node as other well asUAV the command nodes, the contents software of each will message bypass it receives.this message If the ID to of the messagescorresponding represents targets. other If the UAV ID of nodes, the message the software corresponds will bypass to the this receiving message UAV to theitself, corresponding the software targets.will further If the resolve ID of the command message corresponds data in the message to the receiving and send UAV it to itself, the flight the software controller. will The further Mini resolveGCS was the developed command to databetter in coordinate the message the and multiple send itUAV to the system. flight The controller. condition The of Minieach UAV GCS waswill developedbe displayed to betteron the coordinate screen including the multiple the flight UAV system.mode, the The GPS condition position, of each the UAVattitude will etc. be displayedThe Mini onGCS the was screen also includingused to send the commands flight mode, to each the GPS UAV position, node by the the attitude user friendly etc. The Graphic Mini GCSUser wasInterface also used(GUI). to send commands to each UAV node by the user friendly Graphic User Interface (GUI).

4.1. Data Framing for Communication Relay To facilitate the communication relay, thethe messagesmessages andand data to be exchanged between the UAVs and GCS were grouped into frames, the format of which is shown in Figure 1010.. Each communication node (“Mom”, “Son”, and GCS) was given a unique ID. The header of the the frame frame contained contained two two fields, ﬁelds, “Source“Source ID”ID” andand “Target“Target ID”.ID”. “Source“Source ID”ID” indicatedindicated the node from which the frame was transmitted, and “Target ID”ID” indicated thethe target receiver ofof this frame. This mechanism enabled the identification identiﬁcation of the sender and receiver ofof aa datadata frame,frame, whichwhich waswas importantimportant toto thethe realizationrealization ofof frameframe routing.routing.

Figure 10. Format of frame.

4.2. Mission Control Software The mission control software, which runs on th thee UAV’s on-board computer, consists of the following seven modules,modules, andand thethe software’ssoftware’s blockblock diagramdiagram isis shownshown inin FigureFigure 1111..

Figure 11. Block diagram of mission control software.

1. FlightFlight Controller Controller Interface: manages manages direct direct commu communicationnication with Pixhawk flight ﬂight controller over USBUSB port. port.

2. UAVUAV Status Monitor: monitors monitors flight ﬂight and and miss missionion parameters parameters of of the the UAV, UAV, and reports them regularlyregularly to to the the GCS. GCS.

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3. Command Interface: translates user commands into low-level instructions to the ﬂight controller; records and reports command execution status. 4. Communication System Interface: manages low-level message trafﬁc between mission control subsystem and ration telemetries; executes message framing, frame reception, and error checking. 5. Communication Relay Control: controls the relay of radio communication. 6. Startup Control: bootstraps the whole mission control software system during UAV power-up process.

4.3. Software Realization of Relay Process The presence of a relay implies that the relaying node (UAV “Mom” in this case) needs to handle a frame routing. When processing received frames, the UAVs made use of the “Source ID” and “Target ID” fields to filter and/or route the frames in the following manner. • UAV “Son” is the terminal receiver, which only receives frames over the relay from “Mom.” Therefore, “Son” only saves for further processing frames whose “Target ID” is equal to the ID of “Son” itself. • UAV “Mom” functions as a communication relay node, but it also receives control commands from GCS. Therefore, “Mom” will save for further processing frames whose “Target ID” is equal to the ID of “Mom” itself, and will re-transmit frames targeted at “Son” over Radio T2-1. The above-mentioned frame routing logic is elaborated in the following pseudocode (Algorithms 1 and 2) and flow chart in Figure 12.

Algorithm 1: Frame Processing Logic, UAV Son For (true) do: if (frame receive from T2-2) do: if (frame_target_id == id_son) do: save frame for local processing. else do: discard frame. end end end

Algorithm 2: Frame Processing Logic, UAV Mom For (true) do: if (frame receive from T1-2) do: if (frame_target_id == id_mom) do: save frame for local processing. else if (frame_target_id == id_son) do: send frame out from T2-1. end else if (frame received from T2-1) do: if (frame_target_id == id_mom) do: save frame for local processing. else if (frame_target_id == id_GCS) do: send frame out from T1-1. end end end Sensors 2016, 16, 1696 11 of 21 Sensors 2016, 16, 1696 11 of 20

The last ﬁeldfield of the frame contained a four-bytefour-byte cyclic redundancy checking (CRC) code, which waswas computedcomputed usingusing thethe IEEEIEEE 802.3 802.3 32-bit 32-bit CRC CRC polynomial. polynomial. This This CRC CRC code code was was important important because because it helpsit helps verify verify data data frame frame integrity integrity and and protect protect the the system system from from the the inﬂuenceinfluence ofof error-corruptederror-corrupted datadata or messages.messages. Error-corrupted data or messages co coulduld be dangerous, as they might be mistakenlymistakenly understood by the UAV’s missionmission control subsystem as wrong commands, resulting in unpredictable and potentially dangerousdangerous behavior.behavior.

(a) (b)

Figure 12. FlowFlow chart chart of mission control software for ( a) UAV “Son”“Son” andand ((bb)) UAVUAV “Mom”.“Mom”.

4.4. Mini GCS A ground control software, named “Mini GCS”, was developed in-house. Mini GCS is an A ground control software, named “Mini GCS”, was developed in-house. Mini GCS is an integrated UAV control interface that is compatible with Windows 7, 8, and 10 systems. The functions integrated UAV control interface that is compatible with Windows 7, 8, and 10 systems. The functions of Mini GCS include handling data exchanges with UAVs, displaying information about the UAVs, of Mini GCS include handling data exchanges with UAVs, displaying information about the UAVs, and and providing an interface for users to send commands to the UAVs. It allows users to monitor the providing an interface for users to send commands to the UAVs. It allows users to monitor the status of status of both UAVs, and to control the mission of each UAV from a single, integrated interface, both UAVs, and to control the mission of each UAV from a single, integrated interface, which makes it which makes it a crucial communication node in the communication relay network. Mini GCS was a crucial communication node in the communication relay network. Mini GCS was implemented using implemented using C#, and the graphic user interface (GUI) was constructed using Microsoft C#, and the graphic user interface (GUI) was constructed using Microsoft Windows .NET framework. Windows .NET framework. Because the main focus of this work is to demonstrate a signal relay Because the main focus of this work is to demonstrate a signal relay system for the UAV applications, system for the UAV applications, details of this Mini GCS software are not emphasized in the details of this Mini GCS software are not emphasized in the manuscript and the source code is included manuscript and the source code is included in the section of Supplementary Materials for reference. in the section of Supplementary Materials for reference. Figure 13 shows a screenshot of Mini GCS. A number of features are available. Figure 13 shows a screenshot of Mini GCS. A number of features are available.  UAV Status Display: displays to users a selected set of important information about each UAV, • UAVincluding Status communication Display: displays status, to usersGPS stat a selectedus, flight set mode, of important altitude, information speed etc. about each UAV,  includingUAV Command communication Line: a command status, GPS line status, interface ﬂight that mode, allows altitude, users to speed send etc. specific commands to • UAVany UAV Command nodes in Line: the anetwork command such line as interfacetakeoff, land, that allowsreturn usersto launch, to send etc. speciﬁc commands to  anyUAV UAV Position nodes Display: in the network displays such the as real-time takeoff, land,position return and to heading launch, of etc. both UAVs in a map of • UAVthe mission Position area. Display: displays the real-time position and heading of both UAVs in a map of the mission area.

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Figure 13. Graphic user interface (GUI) of Mini GCS.

5. Performance Performance Tests Tests To verifyverify thethe performanceperformance of the system, a series of indoor and outdoor tests were carried out. First, the data loss rate and bit error raterate (BER)(BER) werewere measuredmeasured inin thethe laboratorylaboratory environmentenvironment to choose the suitablesuitable datadata transmissiontransmission raterate forfor thethe relayrelay system.system. Then, for outdooroutdoor tests,tests, ReceivedReceived Signal Strength Indicator (RSSI), a measurement of the power present in a received radio signal, was measured. The RSSI value scales as 1.91.9×× thethe dBm dBm si signalgnal strength, plus an offset, which means the RSSI is proportional to the dBm [[35].35]. We tested both UAVs inin twotwo typicaltypical applicationapplication sceneriosscenerios in Hong Kong: one was to cross obstacles and the the othe otherr was was to to extend extend the the communication communication range. range. Finally, Finally, a BLOSBLOS demonstrationdemonstration was was conducted conducted to showto show the rundownthe rundown and resultsand results of a whole of a applicationwhole application process inprocess Taiwan. in Notably,Taiwan. whenNotably, the transmissionwhen the transmission power of the power telemetry of modulethe telemetry is set to module the maximum is set to value, the whichmaximum is 20 value, dBm which (100 mW), is 20 dBm the typical (100 mW), range the achieved typical range using achieved standard using configuration standard configuration and antenna isand around antenna 500 is m.around Due 500 to them. Due stringent to the flightstringent regulation flight regulation and the areaand the constraint area constraint of the test of the site test in Hongsite in Kong,Hong Kong, it is almost it is almost impossible impossible for the for authors the authors to test to thetest beyond-line-of-sight the beyond-line-of-sight flight flight and and the maximumthe maximum power power relay relay which which would would exceed exceed the allowedthe allowed flight flight area. area. Therefore, Therefore, to demonstrate to demonstrate the relaythe relay function function in Hong in Hong Kong, Kong, the the authors authors have have adjusted adjusted the the transmission transmission power power of theof the radio radio to itsto minimumits minimum value value (1/10 (1/10 of of the the max.) max.) so so that that the the radios radios will willdisconnect disconnect atat aboutabout 100100 m.m. In this way, way, the relayrelay methods methods can can be be tested tested in ain relatively a relatively small small area, area, otherwise, otherwise, the telemetry the telemetry modules modules will always will bealways connected. be connected. In missions In missions requiring requiring long distance long distance communication, communication, the transmission the transmission power canpower be easilycan beturned easily turned to a maximum to a maximum value to value get the to get best the communication best communication range. range.

5.1. Indoor Tests To investigateinvestigate the the reliability reliability of of our our radio radio communication communication relay, relay, the datathe data loss rateloss andrate theand BER the wereBER firstwere tested first tested indoors. indoors. Verifying Verifying the data the loss data rate lo andss rate BER and in anBER indoor in an environment indoor environment under different under datadifferent rates data gave rates us knowledge gave us knowledge about the usableabout bandwidththe usable bandwidth of the system of andthe helpedsystem usand estimate helped theus maximumestimate the data maximum rate that data can berate used that in can the flightbe used tests. in Thethe flight GCS wastests. configured The GCS towas transfer configured data at to a fixedtransfer data data rate, at a and fixed the data statistics rate, and of the the received statistics data of the at received UAV “Son” data were at UAV measured. “Son” were Two measured. aspects of theTwo data aspects transfer of the were data studied: transfer the were data studied: loss rate, the which data loss is the rate, percentage which is of the data percentage that was of lost data during that transmissionwas lost during (data transmission that was transmitted (data that from was GCStransmitted but was from not received GCS but by was UAV not “Son”); received and by the UAV BER, which“Son”); was and the the percentage BER, which of was data the that percentage was received of da byta “Son” that was but received did not pass by “Son” the CRC but test. did not pass the CRCTests test. were conducted at four different data rates: 4 kilobits per second (kbps), 8 kbps, 12.8 kbps, and 16Tests kbps. were Figure conducted 14 shows at four the setup different of the data test. rates: The 4 percentages kilobits per of second data loss (kbps), and 8 BER kbps, with 12.8 respect kbps, toand the 16 data kbps. rate Figure are shown 14 shows in Figure the setu 15p. of the test. The percentages of data loss and BER with respect to the data rate are shown in Figure 15.

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Figure 14. Radio communication relay testing setup.

(a) (b) Figure 15. (a) Percentage of data loss and (b) bit error rate with respect to data rate. Figure 15. (a) Percentage of data loss and (b) bit error rate with respect toto datadata rate.rate.

The tests indicated that when the data rate was equal to or higher than 4 kbps, the data lost The tests indicated that when the data rate was equal to or higher than 4 kbps, the data lost percentage rose from 3% to 9% and the data error rate rose from 0.2% to 0.5%, approximately. A percentage rose from 3% to 9% and the data error rate rose from 0.2% to 0.5%, approximately. higher data rate resulted in both more data loss and a higher chance of data transmission errors. A higher data rate resulted in both more data loss and a higher chance of data transmission Andre et al. [7] estimated that the throughput of typical UAV mission commands is less than 10 kbps. errors. Andre et al. [7] estimated that the throughput of typical UAV mission commands is less In our tests, the current used data rate for a two-UAV system was about 8 kbps. Then, for than 10 kbps. In our tests, the current used data rate for a two-UAV system was about 8 kbps. Then, communication relay applications with this system and configuration, a data rate of 8 kbps is for communication relay applications with this system and configuration, a data rate of 8 kbps is recommended to ensure an acceptable compromised data lose and error rate. recommended to ensure an acceptable compromised data lose and error rate. 5.2. Cross-Obstacle Test 5.2. Cross-Obstacle Test The outdoor flight tests were first conducted in the International Model Aviation Center of the The outdoor flight tests were first conducted in the International Model Aviation Center of the Hong Kong Model Engineering Club (22°24′58.1″ N 114°02′35.4″ E). Figure 16 shows the Google map Hong Kong Model Engineering Club (22◦24058.100 N 114◦02035.400 E). Figure 16 shows the Google map of the test site and final positions of the UAVs in the test. In the simulated cross-obstacle of the test site and final positions of the UAVs in the test. In the simulated cross-obstacle communication communication relay test, the UAV “Son” was designed to conduct a mission that takes off from the relay test, the UAV “Son” was designed to conduct a mission that takes off from the “Home Position”, “Home Position”, flies over a part of the forest with tall trees, and then lands on the road across the flies over a part of the forest with tall trees, and then lands on the road across the forest (UAV “Son” forest (UAV “Son” in Figure). The tall trees would block the communication signal from GCS to UAV in Figure). The tall trees would block the communication signal from GCS to UAV “Son”. Then the “Son”. Then the UAV “Mom” took off and loitered at the UAV “Mom” position, which was above UAV “Mom” took off and loitered at the UAV “Mom” position, which was above the forest. The UAV the forest. The UAV “Son” would recover the communication with home GSC with the help of relay “Son” would recover the communication with home GSC with the help of relay point UAV “Mom”. point UAV “Mom”. The relative positions of all the communication nodes can be more clearly The relative positions of all the communication nodes can be more clearly conceived in the schematic conceived in the schematic shown in Figure 17. shown in Figure 17.

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FigureFigure 16. 16. Final Final positions positions of of nodes nodes in in the the Google Google mapmap in in the cross-obstacle tests tests test test in in Hong Hong Kong. Kong.

Figure 17. Schematic of cross-obstacle communication relay flight test. Figure 17. Schematic of cross-obstacle communication relay flightflight test. RSSI and noise of both UAV “Son” and UAV “Mom” were recorded and shown in Figure 18. RSSI and noise of both UAV “Son” and UAV “Mom” were recorded and shown in Figure 18. RSSIRSSI can and reflect noise the of strength both UAV of “Son”wireless and signal UAV received “Mom” by were the recorded UAV “Mom” andshown and UAV in Figure “Son”. 18 After. RSSI RSSI can reflect the strength of wireless signal received by the UAV “Mom” and UAV “Son”. After canseveral reflect tests, the strength we found of that wireless only signalwhen RSSI received value by is the20 scale UAV (about “Mom” 7 dBm) andUAV higher “Son”. than the After average several several tests, we found that only when RSSI value is 20 scale (about 7 dBm) higher than the average tests,noise we value found can that the onlywireless when modules RSSI valuekeep a is stable 20 scale connection. (about 7Therefore, dBm) higher a dashed than blue the line average is added noise noise value can the wireless modules keep a stable connection. Therefore, a dashed blue line is added valueto each can figure the wireless to indicator modules when keep the connection a stable connection. is lost or unstable. Therefore, The a test dashed started blue at time line is0 s, added when to to each figure to indicator when the connection is lost or unstable. The test started at time 0 s, when eachUAV figure “Son” to indicator took off and when flew the to connection the mission is area. lost orTh unstable.e duration The of the test whole started test at mission time 0 s, was when about UAV UAV10 “Son”min. Since took 120 off ands, the flew UAV to “Son”the mission reached area. the Th areae duration behind theof thetall wholetrees so test the mission RSSI decreased was about “Son” took off and flew to the mission area. The duration of the whole test mission was about 10 min. 10 significantlymin. Since 120 until s, itthe reached UAV “Son”the noise reached level theand areathe connectionbehind the withtall treesthe GCS so the was RSSI lost. decreased At this Since 120 s, the UAV “Son” reached the area behind the tall trees so the RSSI decreased significantly significantlymoment, the until UAV it “Mom”reached took the offnoise and levelflew toand the therelay connection point. Before with 120 the s, UAV GCS “Mom” was lost. stayed At atthis until it reached the noise level and the connection with the GCS was lost. At this moment, the UAV moment,home nearthe UAVthe GSC “Mom” so the took RSSI off remained and flew at to a thehigh re value.lay point. After Before take off, 120 the s, UAVRSSI of“Mom” UAV “Mom”stayed at “Mom” took off and flew to the relay point. Before 120 s, UAV “Mom” stayed at home near the GSC so homedecreased near the but GSC still soremained the RSSI much remained higher at than a high the noise,value. which After meanstake off, that the the RSSI UAV of “Mom”UAV “Mom” kept the RSSI remained at a high value. After take off, the RSSI of UAV “Mom” decreased but still remained decreasedconnecting but with still theremained GSC. During much the higher 180 to than 420 s,the the noise, UAV which“Son” flewmeans over that the the forest UAV and “Mom” landed onkept much higher than the noise, which means that the UAV “Mom” kept connecting with the GSC. During connectingthe road. withIn the the Figure GSC. 17a, During the RSSI the 180fell toto 420the sas, methe scaleUAV with “Son” noise flew value. over Inthe this forest period, and the landed UAV on the 180 to 420 s, the UAV “Son” flew over the forest and landed on the road. In the Figure 17a, the RSSI the“Son” road. remainedIn the Figure disconnected 17a, the RSSIwith fellGCS. to Afterthe sa theme timescale 420s, with the noise UAV value. “Mom” In this flew period, close to the UAV UAV fell“Son” to the and same relayed scale the with communication noise value. Into thishelp period, UAV “Son” the UAV reconnect “Son” with remained the GCS. disconnected Then the RSSI with “Son” remained disconnected with GCS. After the time 420s, the UAV “Mom” flew close to UAV GCS. After the time 420s, the UAV “Mom” flew close to UAV “Son” and relayed the communication to “Son” and relayed the communication to help UAV “Son” reconnect with the GCS. Then the RSSI

Sensors 2016, 16, 1696 15 of 21 Sensors 2016, 16, 1696 15 of 20 Sensors 2016, 16, 1696 15 of 20 helpvalue UAV of “Son” “Son” recovered. reconnect The with UAV the GCS.“Mom” Then continually the RSSI flied value to of UAV “Son” “Son” recovered. (away from The UAV the GCS) “Mom” for continuallyvaluea while of which“Son” ﬂied causedrecovered. to UAV further “Son” The increaseUAV (away “Mom” from of RSSI the continually GCS)of “Son” for aandflied while decrease to whichUAV “Son”of caused “Mon” (away further at the from increase end the period. GCS) of RSSI for ofa while “Son” which and decrease caused further of “Mon” increase at the of end RSSI period. of “Son” and decrease of “Mon” at the end period.

(a) (b) (a) (b) Figure 18. Received Signal Strength Indicator (RSSI), noise and critical curve of both (a) UAV “Son” Figureand (b ) 18. UAV Received “Mom”. Signal Strength Indicator (RSSI), noise noise and and critical critical curve curve of of both both ( (aa)) UAV UAV “Son” “Son” and (b)) UAVUAV “Mom”.“Mom”. 5.3. Extend Distance Test 5.3. Extend Distance Test 5.3. ExtendIn the Distancecommunication Test distance extension tests in Hong Kong, the UAV “Son” was firstly given In the communication distance extension tests in Hong Kong, the UAV “Son” was firstly given a missionIn the to communication fly away as far distanceas possible extension until the tests connection in Hong was Kong, lost. the After UAV recording “Son” was the firstly lost position, given a a mission to fly away as far as possible until the connection was lost. After recording the lost position, missionwhich is to about fly away 100 m, as the far UAV as possible “Son” untilcontinued the connection to fly away was for lost.about After 60 m recording and stayed the at lost the position, which is about 100 m, the UAV “Son” continued to fly away for about 60 m and stayed at the position, whichwaiting is for about the 100 UAV m, “Mom” the UAV to “Son” relay continued the communication. to fly away Thereafter, for about 60 the m UAV and stayed “Mom” at thetook position, off and waiting for the UAV “Mom” to relay the communication. Thereafter, the UAV “Mom” took off and waitingflew to forthe thedirection UAV “Mom” of UAV to “Son” relay theuntil communication. the connection Thereafter,was rebuild. the The UAV final “Mom” positions took of off UAV and flew to the direction of UAV “Son” until the connection was rebuild. The final positions of UAV flewnodes to in the this direction test is ofshown UAV in “Son” Figure until 19. the With connection the help wasof a rebuild.relay node, The a final 160 positionsm total connection of UAV nodes was nodes in this test is shown in Figure 19. With the help of a relay node, a 160 m total connection was inachieved. this test is shown in Figure 19. With the help of a relay node, a 160 m total connection was achieved. achieved.

FigureFigure 19.19. FinalFinal positionspositions ofof nodesnodes inin thethe GoogleGoogle mapmap inin thethe extendedextended distancedistance testtest inin HongHong Kong.Kong. Figure 19. Final positions of nodes in the Google map in the extended distance test in Hong Kong.

Sensors 2016, 16, 1696 16 of 21 Sensors 2016, 16, 1696 16 of 20 Sensors 2016, 16, 1696 16 of 20 The RSSI, noise and critical curves during the full test are shown in Figure 2020.. As is similar with The RSSI, noise and critical curves during the full test are shown in Figure 20. As is similar with the cross obstacle test, the communication between UAV “Son”“Son” andand GCSGCS was firstfirst lost at about 180 s the cross obstacle test, the communication between UAV “Son” and GCS was first lost at about 180 s and finallyfinally reconnected at about 300 s s when when the the UAV UAV “Mom” takeoff takeoff and and flew flew near near to to UAV UAV “Son”. “Son”. and finally reconnected at about 300 s when the UAV “Mom” takeoff and flew near to UAV “Son”. The drop of RSSIRSSI ofof UAVUAV “Mom”“Mom” waswas duedue toto itsits takeofftakeoff andand flyingflyingaway awayfrom from home home position. position. The drop of RSSI of UAV “Mom” was due to its takeoff and flying away from home position.

(a) (b) (a) (b) FigureFigure 20. RSSI,RSSI, noise, noise, and and critical critical curve curve of both ( a)) UAV UAV “Son” and (b) UAV “Mom”.“Mom”. Figure 20. RSSI, noise, and critical curve of both (a) UAV “Son” and (b) UAV “Mom”. 5.4. Beyond-Line-of-Sight Tests 5.4. Beyond-Line-of-Sight Tests To furtherfurther verifyverify thethe reliabilityreliability ofof thethe communicationcommunication relay system, including mini GCS, in a beyond-line-of-sightTo further verify missions, the reliability outdoor of flightthe communication tests were conducted relay system, in Xigang including flight site, mini Tainan GCS, City, in a beyond-line-of-sight missions, outdoor outdoor flight flight tests tests were were conducted conducted in in Xigang Xigang flight flight site, site, Tainan Tainan City, City, Taiwan (23°6◦ 0′21.5″00 N, 120°12◦ ′036.0″00 E). In this test, the transmission power of the telemetry modules Taiwan (23(23°66′21.5″ N, 120°12 120 12′36.036.0″ E).E). In In this this test, test, the the transmissi transmissionon power of the telemetry modules were turned to the maximum value to get thethe longestlongest communicationcommunication range. The Google map of the weretest site turned and finalto the positions maximum of thevalue UAVs to get in thethe longtest areest communicationshown in Figure range. 21. The Google map of the test site and finalfinal positions of the UAVs inin thethe testtest areare shownshown inin FigureFigure 21 21..

Figure 21. Final positions of nodes in the Google map in the extended distance test in Taiwan. FigureFigure 21. FinalFinal positions positions of of nodes in the Google map in the extended distance test in Taiwan. The rundown of the typical flight mission is given below: The rundown of the typical flight mission is given below: 1. TheAll UAVs rundown (“Mom” of the and typical “Son” ﬂight in this mission case) isare given powered below: up.

1.2. AllStart UAVs GCS (“Mom”software and“Mini “Son” GCS”. in this case) are powered up. 1. All UAVs (“Mom” and “Son” in this case) are powered up. 2.3. StartWait GCSfor communication software “Mini relay GCS”. network to start. The startup process is automatic and normally 2. Start GCS software “Mini GCS”. 3. Waittakes forless communication than three minutes. relay network to start. The startup process is automatic and normally takes less than three minutes.

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3. Wait for communication relay network to start. The startup process is automatic and normally takes less than three minutes. Sensors 2016, 16, 1696 17 of 20 4. Using “Mini GCS”, command “Son” to take off and start the execution of a preprogrammed 4. mission,Using “Mini which GCS”, brings command “Son” to “Son” a destination to take faroff enoughand start for the the execution radio communication of a preprogrammed between “Son”mission, and which “Mini brings GCS” to“Son” break. to a destination far enough for the radio communication between 5. Wait“Son” until and the “Mini communication GCS” to break. between “Son” and “Mini GCS” breaks due to distance. 6. 5. UsingWait “Miniuntil the GCS”, communication command “Mom” between to “Son” take offand and “Mini move GCS” into breaks the proximity due to distance. of the destination 6. areaUsing of “Son”.“Mini GCS”, “Mom” command automatically “Mom” performsto take off aand communication move into the proximity relay when of itthe moves destination toward “Son”area andof “Son”. reconnects. “Mom” automatically performs a communication relay when it moves toward “Son” and reconnects. The rundown of the test is shown as a series of pictures in Figure 22. A video clip demonstrating The rundown of the test is shown as a series of pictures in Figure 22. A video clip demonstrating the complete test procedure is given in the Supplementary Material. In the video demonstration, the complete test procedure is given in the Supplementary Material. In the video demonstration, UAV “Son” started a pre-deﬁned mission. The mission is to ﬂy away for about 600 m until the UAV UAV “Son” started a pre-defined mission. The mission is to fly away for about 600 m until the UAV waswas BLOS BLOS in in Figure Figure 22 22c.c. When When thethe connectionconnection betweenbetween “Son” “Son” and and GCS GCS was was lost lost (indicated (indicated byby the the greengreen “Connected” “Connected” icon icon changing changing to to thethe redred “Disconnected”),“Disconnected”), “Mom” “Mom” was was deployed deployed toto a designated a designated relayingrelaying position position (about (about 400 400 m awaym away from from home home position). position). Once Once “Mom” “Mom” reached reached the relaying the relaying position, communicationposition, communication between “Son” between and “Son” GCS and was GCS re-established was re-established automatically automatically and wasand notwas lostnot lost again beforeagain the before end the of the end mission. of the mission.

(a) (b)

(e) (f)

Figure 22. A rundown of flight test (a) Pre-takeoff setup of flight test; (b) UAV “Son” starts the Figure 22. A rundown of ﬂight test (a) Pre-takeoff setup of ﬂight test; (b) UAV “Son” starts the mission; mission; (c) “Son” is out of radio range and communication with it is lost; (d) UAV “Mom” is deployed (c) “Son” is out of radio range and communication with it is lost; (d) UAV “Mom” is deployed to the to the relay position; (e) Communication with “Son” is re-established after being relayed through relay position; (e) Communication with “Son” is re-established after being relayed through “Mom”; “Mom”; (f) “Mom” and “Son” return to launch position; the mission is completed. (f) “Mom” and “Son” return to launch position; the mission is completed. Eight flight tests were conducted. In all of the tests, the system was able to re-establish communication between “Son” and “Mini GCS” through the relay of “Mom”. When “Mom” reached the designated relaying position, the communication connection between “Son” and “Mini GCS” was

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SensorsEight 2016, 16 ﬂight, 1696 tests were conducted. In all of the tests, the system was able to re-establish18 of 20 communication between “Son” and “Mini GCS” through the relay of “Mom”. When “Mom” reached adequatelythe designated stable relaying for mission position, operations. the communication The successful connection rate of the between communication “Son” and re-establishment “Mini GCS” was isadequately 100%. The stable tests for demonstrated mission operations. that: (1) The the successful system rateis capable of the communication of maintaining re-establishment uninterrupted communicationis 100%. The tests between demonstrated GCS and an that: end node (1) the once system the relaying is capable node of is maintaining properly deployed; uninterrupted and (2) connectivitycommunication between between GCS GCS and andthe end an end node node can oncebe automatically the relaying re-established node is properly when deployed; the relaying and (2)node connectivity reaches the between proper GCSrelaying and theposition, end node even can if it be has automatically been previously re-established lost due whento an theinadequate relaying relay.node reaches the proper relaying position, even if it has been previously lost due to an inadequate relay.

6. Conclusions Conclusions and and Future Future Development Development In thisthis study,study, major major attention attention was was given given to implementing to implementing a functional a functional and reliable and reliable proof-of-concept proof-of- conceptUAV communication UAV communication relay system relay that system uses that commercially uses commercially available hardwareavailable hardware components components combined withcombined customized with customized software. Insoftware. the prototype In the protot system,ype two system, quadrotors, two quadrotors, each mounted each mounted with a Pixhawk with a Pixhawkflight controller flight controller and a Raspberry and a Raspberry Pi 2 Model Pi B2 microcomputerModel B microcomputer were used were as the used aerial as platforms.the aerial platforms.The onboard The relay onboard control relay protocol control and protocol program and were program developed were to realizedeveloped UAV to communication realize UAV communicationextension and cross-obstacle extension and communication.cross-obstacle commu The GSCnication. software The GSC was software also developed was also to developed facilitate tothe facilitate two-UAV the relay two-UAV operations. relay operations. A series of A indoor series of and indoor outdoor and testsoutdoor were tests carried were out.carried The out. indoor The indoortests aimed tests to aimed determine to determine the suitable the data suitable transmission data transmission rate, while therate, outdoor while teststhe demonstratedoutdoor tests demonstratedreliable communication reliable communication performance performance in two proposed in two application proposed scenariosapplication in scenarios Hong Kong in Hong and Kongone BLOS and missionone BLOS simulation mission insimulation Taiwan. Allin Taiwan the outdoor. All the tests outdoor showed tests the system’sshowed the capability system’s of capabilitymaintaining of goodmaintaining quality real-timegood quality communication real-time communication through relays, through and automatically relays, and recovering automatically from recoveringcommunication from losses.communication losses. The current work focused only on communication relay relay through through a a single single relaying relaying node node (UAV (UAV “Mom”).“Mom”). Relays through multiple relaying nodes, although feasible, will be conducted in the near future. The The current current configuration configuration of of the the system system allows allows the the system system to to be extended by adding more relaying nodes. A possible configuration configuration is illustra illustratedted in Figure 23,23, in which multiple relaying nodes (“Mom 1” to “Mom N”) are connectedconnected inin series,series, formingforming aa bidirectionalbidirectional relayrelay system.system. The current design of the data frame permits, at most, 13 relaying nodes between “Son” and the GCS. When the format of the data frames was designed, four binary bits were assigned to represent the “ID” of the UAV. WithWith fourfour bitsbits toto use,use, atat mostmost 16 unique IDs could be represented, among which which UAV UAV “Son”, “Son”, GCS, and a special broadcasting ID occupied three places. Therefore, Therefore, there there were were 13 13 unique unique IDs IDs left, left, which could then be taken up by 13 multiple UAV “Moms”, theoretically. However, the maximum number of relaying nodes in practice may also be subjected to the constraint of the data rate. With more UAVs in the system, more information will be exchanged. If If the data rate is kept unchanged, communication between UAVs andand GCSGCS mightmight experienceexperience longerlonger delays.delays.

Figure 23. ExtendingExtending the relay system by adding relaying nodes.

Acknowledgments: This work is sponsored by Innovation and Technology Commission, Hong Kong under Contract No. ITS/334/15FP. We also appreciate Hiu Fung Wan for his help in mechanical parts of this work.

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Supplementary Materials: The following is available online at https://www.youtube.com/watch?v=HxN0oafNmzw, Video S1: Multiple UAVs Telecommunication Demonstration. https://github.com/YifanJiangPolyU/UAV- communication-relay, Source code 1: Mission control software source code. https://github.com/HKPolyU-UAV/ Mini_GCS, Source code 2: Mini-GSC source code. Acknowledgments: This work is sponsored by Innovation and Technology Commission, Hong Kong under Contract No. ITS/334/15FP. We also appreciate Hiu Fung Wan for his help in mechanical parts of this work. Author Contributions: Y.J. and J.S. conceived the relay system. B.L. built up the hardware platform. Y.J. developed the protocol and onboard software. L.C. developed the G.C.S. software. B.L. and Y.J. performed the tests and wrote the paper. C.W. was in charge of the whole project management. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Abbreviations The following abbreviations are used in this manuscript: BLOS Beyond-Line-of-Sight UAV Unmanned Aerial Vehicle GCS Ground Control Station RSSI Received Signal Strength Indication FHSS Frequency-Hopping Spread Spectrum TDM Time Division Multiplex UART Universal Asynchronous Receiver and Transmitter CRC Cyclic Redundancy Checking BER Bit Error Rate GUI Graphic User Interface

References

1. Nonami, K. Prospect and recent research & development for civil use autonomous unmanned aircraft as UAV and MAV. J. Syst. Des. Dyn. 2007, 1, 120–128. 2. Francis, M.S. Unmanned air systems: Challenge and opportunity. J. Aircr. 2012, 49, 1652–1665. [CrossRef] 3. Colomina, I.; Molina, P. Unmanned aerial systems for photogrammetry and remote sensing: A review. ISPRS J. Photogramm. Remote Sens. 2014, 92, 79–97. [CrossRef] 4. Bekmezci, I.;˙ Sahingoz, O.K.; Temel, ¸S.Flying ad hoc networks (fanets): A survey. Ad Hoc Netw. 2013, 11, 1254–1270. [CrossRef] 5. Arena, A.; Chowdhary, G.; Conner, J.; Gaeta, R.; Jacob, J.; Kidd, J. Development of an Unmanned Aircraft Systems (UAS) Option at the Graduate Level. In Proceedings of the 51st AIAA Aerospace Sciences Meeting Including the New Horizons Forum and Aerospace Exposition, Grapevine, TX, USA, 7–10 January 2013. 6. Gupta, L.; Jain, R.; Vaszkun, G. Survey of important issues in UAV communication networks. IEEE Commun. Surv. Tutor. 2016, 18, 1123–1152. [CrossRef] 7. Andre, T.; Hummel, K.A.; Schoellig, A.P.; Yanmaz, E.; Asadpour, M.; Bettstetter, C.; Grippa, P.; Hellwagner, H.; Sand, S.; Zhang, S.W. Application-driven design of aerial communication networks. IEEE Commun. Mag. 2014, 52, 128–136. [CrossRef] 8. Pinkney, F.J.; Hampel, D.; DiPierro, S. Unmanned Aerial Vehicle (UAV) Communications Relay. In Proceedings of the Milcom 96, McLean, VA, USA, 21–24 October 1996; Volumes 1–3, pp. 47–51. 9. Rangel, R.K.; Kienitz, K.H.; de Oliveira, C.A.; Brandao, M.P. Development of a Multipurpose Tactical Surveillance System Using UAV’S. In Proceedings of the 2014 IEEE Aerospace Conference, Big Sky, MT, USA, 1–8 March 2014; pp. 1–9. 10. Orfanus, D.; de Freitas, E.P.; Eliassen, F. Self-organization as a supporting paradigm for military UAV relay networks. IEEE Commun. Lett. 2016, 20, 804–807. [CrossRef] 11. Choi, Y.; Pachter, M.; Jacques, D. Optimal Relay UAV Guidance—A New Differential Game. In Proceedings of the 50th IEEE Conference on Decision and Control and European Control Conference (CDC-ECC), Orlando, FL, USA, 12–15 December 2011; pp. 1024–1029. 12. Zhan, P.C.; Yu, K.; Swindlehurst, A.L. Wireless relay communications with unmanned aerial vehicles: Performance and optimization. IEEE Trans. Aerosp. Electron. Syst. 2011, 47, 2068–2085. [CrossRef] 13. Cetin, O.; Zagli, I. Continuous airborne communication relay approach using unmanned aerial vehicles. J. Intell. Robot. Syst. 2012, 65, 549–562. [CrossRef] Sensors 2016, 16, 1696 20 of 21

14. Cetin, O.; Zagli, I.; Yilmaz, G. Establishing obstacle and collision free communication relay for UAVs with artiﬁcial potential ﬁelds. J. Intell. Robot. Syst. 2013, 69, 361–372. [CrossRef] 15. Hauert, S.; Zufferey, J.-C.; Floreano, D. Evolved swarming without positioning information: An application in aerial communication relay. Auton. Robot. 2009, 26, 21–32. [CrossRef] 16. Jiang, F.; Swindlehurst, A.L. Dynamic UAV Relay Positioning for the Ground-to-Air Uplink. In Proceedings of the 2010 IEEE Globecom Workshops, Miami, FL, USA, 5–10 December 2010; pp. 1766–1770. 17. Marinho, M.A.M.; de Freitas, E.P.; da Costa, J.P.C.L.; de Almeida, A.L.F.; de Sousa, R.T. Using Cooperative Mimo Techniques and UAV Relay Networks to Support Connectivity in Sparse Wireless Sensor Networks. In Proceedings of the 2013 International Conference on Computing, Management and Telecommunications (Commantel), Ho Chi Minh City, Vietnam, 21–24 January 2013; pp. 49–54. 18. Ben Moussa, C.; Gagne, S.; Akhrif, O.; Gagnon, F. Aerial Mast vs. Aerial Bridge Autonomous UAV Relay: A Simulation-Based Comparison. In Proceedings of the International Conference on New Technologies, Dubai, United Arab Emirates, 30 March–2 April 2014; pp. 3118–3123. 19. Lee, K.; Noh, H.; Lim, J. Airborne relay-based regional positioning system. Sensors 2015, 15, 12682–12699. [CrossRef][PubMed] 20. Brown, T.X.; Argrow, B.; Dixon, C.; Doshi, S.; Thekkekunnel, R.-G.; Henkel, D. Ad Hoc UAV Ground Network (Augnet). In Proceedings of the AIAA 3rd Unmanned Unlimited Technical Conference, Chicago, IL, USA, 20–23 September 2004; pp. 1–11. 21. Brown, T.X.; Doshi, S.; Jadhav, S.; Himmelstein, J. Test Bed for a Wireless Network on Small UAVs. In Proceedings of the AIAA 3rd Unmanned Unlimited Technical Conference, Chicago, IL, USA, 20–23 September 2004; pp. 20–23. 22. Jimenez-Pacheco, A.; Bouhired, D.; Gasser, Y.; Zufferey, J.C.; Floreano, D.; Rimoldi, B. Implementation of a Wireless Mesh Network of Ultra Light MAVs with Dynamic Routing. In Proceedings of the IEEE Globe Work, Anaheim, CA, USA, 3–7 December 2012; pp. 1591–1596. 23. Morgenthaler, S.; Braun, T.; Zhao, Z.L.; Staub, T.; Anwander, M. Uavnet: A Mobile Wireless Mesh Network Using Unmanned Aerial Vehicles. In Proceedings of the IEEE Globe Work, Anaheim, CA, USA, 3–7 December 2012; pp. 1603–1608. 24. Asadpour, M.; Giustiniano, D.; Hummel, K.A. From Ground to Aerial Communication: Dissecting WLAN 802.11N for the Drones. In Proceedings of the WiNTECH 013 8th ACM International Workshop on Wireless Network Testbeds, Experimental Evaluation & Characterization, Miami, FL, USA, 30 September–4 October 2013; pp. 25–32. 25. Asadpour, M.; Giustiniano, D.; Hummel, K.A.; Heimlicher, S.; Egli, S. Now or Later?—Delaying Data Transfer in Time-Critical Aerial Communication. In Proceedings of the 2013 ACM International Conference on Emerging Networking Experiments and Technologies (Conext ‘13), New York, NY, USA, 9–12 December 2013; pp. 127–132. 26. Yanmaz, E.; Kuschnig, R.; Bettstetter, C. Achieving Air-Ground Communications in 802.11 Networks with Three-Dimensional Aerial Mobility. In Proceedings of the IEEE Infocom Ser, Turin, Italy, 14–19 April 2013; pp. 120–124. 27. Yanmaz, E.; Hayat, S.; Scherer, J.; Bettstetter, C. Experimental Performance Analysis of Two-Hop Aerial 802.11 Networks. In Proceedings of the 2014 IEEE Wireless Communications and Networking Conference (WCNC), Istanbul, Turkey, 6–9 April 2014; pp. 3118–3123. 28. Johansen, T.A.; Zolich, A.; Hansen, T.; Sørense, A.J. Unmanned Aerial Vehicle as Communication Relay for Autonomous Underwater Vehicle—Field Tests. In Proceedings of the Globecom 2014 Workshop—Wireless Networking and Control for Unmanned Autonomous Vehicles, Austin, TX, USA, 8–12 December 2014. 29. Guo, W.; Devine, C.; Wang, S. Performance Analysis of Micro Unmanned Airborne Communication Relays for Cellular Networks. In Proceedings of the 9th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP), Manchester, UK, 23–25 July 2014; pp. 658–663. 30. Deng, C.; Wang, S.; Huang, Z.; Tan, Z.; Liu, J. Unmanned aerial vehicles for power line inspection: A cooperative way in platforms and communications. J. Commun. 2014, 9, 687–692. [CrossRef] 31. Willink, T.J.; Squires, C.C.; Colman, G.W.K.; Muccio, M.T. Measurement and characterization of low-altitude air-to-ground mimo channels. IEEE Trans. Veh. Technol. 2016, 65, 2637–2648. [CrossRef] 32. 3DR Pixhawk. Available online: https://store.3dr.com/products/3dr-pixhawk (accessed on 30 June 2016). Sensors 2016, 16, 1696 21 of 21

33. Sik Telemetry Radio. Available online: http://ardupilot.org/copter/docs/common-sik-telemetry-radio. html (accessed on 30 June 3016). 34. Dronecode/Sik. Available online: https://github.com/Dronecode/SiK (accessed on 30 June 2016). 35. Sik Radio—Advanced Configuration. Available online: http://ardupilot.org/copter/docs/common- 3dr-radio-advanced-configuration-and-technical-information.html#common-3dr-radio-advanced- configuration-and-technical-information (accessed on 15 August 2016).

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