## 1 Introduction  
Unmanned aerial vehicles (UAV), perhaps most notably represented by multicopters, have recently been used to provide diverse services, including the monitoring of crops and forests, measuring area, transporting commercial goods, relaying video and data, and inspecting tunnels, bridges, and buildings. Among the several uses of UAVs, wireless relay systems for high altitudes using fixed-wing UAVs [1; 2], or high-altitude platform stations (HAPS) [3; 4; 5; 6; 7], are some of the most promising applications.  
The concept of HAPS was formulated at the World Radiocommunication Conference in 1997, organized by the International Telecommunication Union (ITU) [3; 7]. Since the 1990s, HAPS have been actively studied as radio relay stations using airships and airplanes that fly in the stratosphere where the airflow is relatively stable. Now that recent improvements in aircraft performance have reinvigorated the study of the technology, HAPS are drawing attention from telecommunication industries again and anticipated to support disaster-resistant and economical networks by performing wireless relay over a wide range, as shown in Figure 1. The fixed UAV flies and circles over the designated area in the stratosphere, with which communication equipment is equipped and the wireless communication link between mobile station and fixed station is established.  
Generally speaking, fixed-wing UAVs have an advantage over multicopters in terms of long flight duration and endurance that makes them suitable for wireless relay that re-quires long-term flight. However, because fixed-wing UAVs cannot hover in the same po-sition, but have to constantly turn to stay midair, the antennas onboard UAVs have to be directed toward the designated ground station regardless of the turning motion of the UAVs.  
There are many papers concerning payload pointing system [8; 9; 10; 11]. Most of the systems are mounted on spacecraft, such as observation satellites [8; 9; 10] and space telescopes [11].  
Therefore, the attitude motion of the spacecraft is very low, and its angular velocity is near zero. Besides, commands into the control system are almost constant, and all sensors onboard the spacecraft have a few noises compared to those of the UAV.  
In contrast, in case of the fixed wing UAVs, the UAVs are subject to a large disturbance represented by winds, and the disturbance causes large deviations of attitude angles and large attitude angular velocities of the UAV, and the body of the UAV may block the antenna direction. In addition, as the positions and attitudes of the UAV greatly change during its flight, the commands into the antenna pointing control system also greatly change every moment and should be generated by using measured attitude angles with relatively large noises.  
In this situation, it is very important to clarify conditions both to avoid blocking the antenna direction by aircraft bodies and to realize highly accurate antenna pointing control systems for the designated ground station.  
This paper is configured as follows. Section 2 describes the antenna pointing control system using mechanical gimbals. Next, Section 3 describes the blocking analysis of the antenna's pointing direction by a UAV during its turns in flight. Section 4 then explains how to determine an antenna's size and pointing accuracy, after which Section 5 de-scribes the configuration of the antenna pointing control system and its design method. Section 6 describes the results of simulations, and Section 7 concludes the paper and de-scribes future outlook on study.