# A Study on a Radio Source Location Estimation System Using High Altitude Platform Stations (HAPS)
## 1 Introduction  
In recent years, mobile communication technology has made great progress, and is used not only in public places but also in our everyday life. For example, mobile communication technology is indispensable in the field of IoT, which is used in smart phones and electrical appliances that everyone has today, and in the field of automatic driving. In fact, the penetration rate of mobile devices is increasing every year [1]. The number of wireless stations has been increasing along with this growth, and as of the end of September 2022, a total of nearly 3 million stations were in operation nationwide in Japan, as shown in Figure 1[2]. Figure 1: Number of wireless stations in Japan. As time has progressed, technologies such as cell phones and IoT have been developed, and mobile communication devices have become more familiar to us, the general public. However, perhaps due to these developments, the number of illegal radio stations (which use power in excess of the limit or illegal frequencies without a license) has been increasing in recent years, as shown in Figure 2[3]. Today, these illegal radio stations are searched in various ways, and one of them is called a radio wave monitoring system. This system monitors radio waves received at sensor stations and estimates the direction of the radio wave source. Based on this, the location of illegal radio stations is identified by remotely operating sensor stations installed at various locations and sensor stations mounted on vehicles, from a center station installed at each general communications station [3]. However, in urban areas where many buildings are densely built or where there is a lot of vegetation, these systems may not work well because there may be no line of sight from the sensor station to the illegal radio station, or many reflected waves may be generated. In recent years, research has been conducted using unmanned aerial vehicles (UAVs) such as drones to gain some altitude in order to improve the location estimation accuracy of the source of radio waves by making it easier to see the source of radio transmission [4]. However, the service area of this system is limited by the height of the UAVs and actual implementation has not yet been achieved. As for radio monitoring technology, advanced spectrum sensing techniques are still being developed to enhance the detection of illegal radio transmissions in harsh environments [5]. Many other studies on radio monitoring technology [6; 7] still exist today. To address this issue, this paper focuses on the use of the High Altitude Platform System (HAPS) which is a generic term for a system that provides communication services over a wide area by operating an unmanned aircraft, such as an aircraft flow in the stratosphere, like a communication base station [8; 9]. As explained here, the HAPS mainly operates unmanned aircraft as base stations, and papers such as \"Research and Development of HAPS Mobile Communication System for Rapid Disaster Recovery\" [10] have been published. The HAPS has the advantage of being able to deploy unmanned base stations regardless of ground traffic conditions or environments such as the sea. So far, research has been conducted with the idea of operating unmanned aircraft as the main base station. However, if the HAPS is operated as an illegal radio surveillance system, it will be possible to monitor a wider area with less influence from reflected waves and at a lower cost than conventional surveillance systems. If the HAPS can also be operated as a base station in times of disaster, it will be effective in estimating the location of mobile terminals in times of disaster. Figure 2: Statistics of illegal radio stations in Japan. This paper examines the operation of the HAPS as an illegal radio monitoring system and a location estimation system for mobile terminals during disasters, and demonstrates its effectiveness mainly through numerical simulations using MATLAB [11].