###### Abstract
There are also methods for detecting the direction of ground stations without using markers or cameras. One example is a radio frequency (RF) sensor that directs the antenna of a geostationary communications satellite toward the ground station [6]. Since RF sensors only need to detect beacon signals, they can detect ground stations relatively easily, even from flying UAVs. However, because RF sensors are composed of multiple power feed systems, high-precision RF sensors are heavy and consume significant amounts of power. Furthermore, a beacon transmitter must be installed at the ground station for the RF sensor to detect its direction, which reduces the flexibility of UAV operations. Considering the trade-offs of each approach, the increase in weight and power consumption associated with the higher accuracy of RF sensors presents a major disadvantage for small flight systems. Therefore, as part of a wireless relay system for small UAVs, it is essential to develop a sensor system that detects the direction of the ground station using a camera and a marker, while being resilient to the effects of the operating environment. A key factor is the marker detection method's ability to tolerate the influences of the operating environment. The sensor must meet the following three performance requirements:  
* Detection of low-resolution, long-range markers;  
Figure 1: Wireless relay system using a small fixed-wing UAV. * Accuracy of marker detection;
* Robust marker detection against motion blur. Among these, the accuracy of detecting the direction angle toward the ground station is the most important for a highly accurate antenna control system. Moreover, motion blur, which is proportional to the angular velocity generated by the UAV's motion, occurs in the acquired image. This motion blur can interfere with marker detection. Conventionally, an Augmented reality (AR) marker is used in position detection. The detection of AR markers is carried out by using the contour information inherent to the shape of the marker. However, for a wireless relay system in a UAV operating environment, contour information may degrade due to low resolution and motion blur, leading to poor detection performance. Therefore, a method for marker detection that does not rely on contour shape is necessary. We have adopted color markers that utilize color information as a marker and avoid the use of contour information. The rationale behind this approach is that the degradation of color information is minimal in motion blur and low-resolution markers. However, color information alone is not enough to distinguish markers from objects or noise that has similar colors to those of the markers. For example, if a red color marker is used, color information alone would not be sufficient to distinguish the marker from a leaf of the same red color. To solve this issue, we propose a new method to detect only multiple color markers using two types of information: the predicted multiple color marker placement information and the color information. Based on the methods presented above, this study presents an image processing-based direction detection sensor that can be applied in a small fixed-wing UAV. This paper is configured as follows: Section 2 gives a brief overview of conventional direction detection technologies, which have been or can be applied to ground station direction detection. Section 3 describes the proposed method in detail, and Section 4 presents the performance requirements of the proposed method. Section 5 presents the experimental results and analysis, and finally, Section 6 concludes with discussing the validity of the proposed method.