# Design and Implementation of a Tether-Powered Hexacopter for Long Endurance Missions
## 1 Introduction  
Endurance is a crucial performance index of electrical rotary-wing unmanned aerial vehicles (UAV). Intuitively, bigger batteries lead to longer flight time; however, lifting heavy batteries requires more electricity and thus decreases the endurance. Therefore, an optimal battery capacity exists to achieve the maximum endurance. The endurance of battery-powered UAVs has been investigated thoroughly [1; 2; 3; 4; 5; 6]. Methods of increasing maximum endurance can also be found in [7; 8; 9; 10; 11; 12; 13; 14; 15]. Methods of increasing maximum endurance include optimizing the design of UAVs according to their applications [8; 9; 10; 11], deposition of empty batteries [6; 7], reducing energy loss and cost during operation [12; 13], and combining with other energy sources [14; 15]. The performance of typical battery-powered UAVs is summarized in Table 1.  
For several applications, such as precision smart agriculture, intelligent surveillance, and temporary telecom hotspots, for emergencies which require UAV working within a specific area for a long time, tether-powered UAVs have a great potential to demonstrate much longer endurance than battery-powered UAVs.  
A tether-powered UAV usually contains a multicopter in the air and a ground station, with a conductive tether connecting both of them [16; 17]. The electrical power is transmitted form the ground station to the multicopter. Tether-powered multicopters are able to extend their endurance to \"near infinite\" since the ground station is powered by the grid, a fuel generator, or a huge battery that contains much more energy than can be lifted in the air. In previous researches, a tether-powered multicopter can either be directly powered by a tether [16; 18; 19] or through an additional electricity converter installed onboard [17; 20; 21]. Usually, one with an electrical converter is able to achieve longer tether length than those being directly powered by a tether. Tether-powered multicopters can operate with only small batteries onboard or no battery at all. Onboard batteries only serve as a backup power to prevent a possible multicopter crash in case of power failure [22; 23]. Several researches on the power tether can be found in [18; 19; 21; 24]. Our previous work provides a math model to optimize tether parameters for different missions [18]. A power-over-tether system is presented to control the tether tension automatically [19]. A tether structure thatcan transmit both power and data is invented in [21]. An optimum design of multi-core tether to transmit AC power while considering the skin effect is introduced in [24], but skin effect is not an issue for our DC-powered UAV.  
This research work focuses on the system design, tether selection, and outdoor implementation. A 4-h mission with the operation altitude up to 50 m will be executed to demonstrate the longest endurance and the highest altitude among surveyed experiments about tether-powered UAVs. The structure of this paper is organized as follows. Section 2 introduces the composition of our tether-powered hexacopter, detailed design parameters, power consumption model, tether selection, and design considerations. Section 3 describes experimental results. Section 4 describes the analysis of test results. Section 5 presents the conclusion of this article and improvements that can be done in the future.