###### Abstract  
Among the several usages of unmanned aerial vehicles (UAVs), a wireless relay system is one of the most promising applications. Specifically, a small fixed-wing UAV is suitable to establish the system promptly. In the system, an antenna pointing control system directs an onboard antenna to a ground station in order to form and maintain a communication link between the UAV and the ground station. In this paper, we propose a sensor system to detect the direction of the ground station from the UAV by using color marker patterns for the antenna pointing control system. The sensor detects the difference between the antenna pointing direction and the ground station direction. The sensor is characterized by the usage of both the color information of multiple color markers and color marker pattern matching. These enable the detection of distant, low-resolution markers, a high accuracy of marker detection, and robust marker detection against motion blur. In this paper, we describe the detailed algorithm of the sensor, and its performance is evaluated by using the prototype sensor system. Experimental performance evaluation results showed that the proposed method had a minimum detectable drawing size of 10.2 pixels, a motion blur tolerance of 0.0175, and a detection accuracy error of less than 0.12 deg. This performance indicates that the method has a minimum detectable draw size that is half that of the ArUco marker (a common AR marker), is 15.9 times more tolerant of motion blur than the ArUco marker, and has a detection accuracy error twice that of the ArUco marker. The color markers in the proposed method can be placed farther away or be smaller in size than ArUco markers, and they can be detected by the onboard camera even if the aircraft's attitude changes significantly. The proposed method using color marker patterns has the potential to improve the operational flexibility of radio relay systems utilizing UAVs and is expected to be further developed in the future. f 2025 12 disasters. Specifically, we envision using a small fixed-wing UAV that remains airborne while circling, as shown in Figure 1, as a wireless relay system for video transmission. The purpose of this system is to be deployed in disaster areas where network infrastructure has been disrupted in order to establish communication lines. To link an antenna onboard the UAV with a ground station, it is imperative that the onboard antenna is accurately controlled to point toward the direction of the ground station. To achieve this, a highly accurate control system must be configured. Accordingly, the control system requires a direction detection system for the ground station, which is necessary to measure the deviation in the antenna angles from the ground station direction and provide feedback to the control system. For a ground station direction detection system, the authors consider an approach to detect the direction of ground stations using image data obtained from cameras mounted on small fixed-wing UAVs. The most reliable method for detecting ground stations from image data is to place a distinctive marker on the ground station. Through the accurate recognition of the marker and detection of its position, the direction of the ground station can be determined with high precision. Furthermore, when operating a wireless relay system, the marker needs to be placed only on the ground station, which offers the advantage of not impairing the flexibility of the UAV. On the other hand, the detection performance of markers is highly dependent on the operating environment. Two factors can affect marker detection performance: first, the resolution of the marker, which can hinder detection due to the smaller size of the marker and the reduced number of pixels representing it at a distance; and second, motion blur, which occurs when a UAV is in flight and its posture is constantly changing. This motion blur distorts the markers, which can interfere with marker detection.