# Magnetic Field-Based Vehicle Positioning System in Long Tunnel Environment
## 1 Introduction  
Vehicle navigation services are essential for drivers and can be used with smartphones, regardless of time, weather, or location (under coverage). In general, most of these services are available for free and operate based on a global navigation satellite system (GNSS) [1], through which users can accurately determine their current location in real time. GNSS can accurately estimate the user position for a clear line of sight between the user and satellite. For instance, numerous multipaths occur in an urban area with several tall buildings, and the GNSS performance is degraded. Similarly, accurate positioning cannot be determined in places where GNSS signals are not received such as indoors or tunnels [2]. As the current navigation system relies on only the GNSS, the navigation service is restricted in places with inferior GNSS signal quality. Seamless navigation is an aspect required in future navigation systems. Currently, navigation applications in smartphones do not provide seamless navigation as only the GNSS is utilized. For instance, navigation ceases when a vehicle enters an underground parking lot from outdoors [3], especially in airports, large shopping malls, and hospitals, which usually have large underground parking lots, and first-time visitors face difficulties in locating their position. If the vehicle position in the underground parking lot can be estimated using other infrastructure, users can experience a new navigation service. Moreover, the current navigation system needs to be supplemented in the tunnel environment, as it provides position information inside the tunnel by simply calculating the position using the initial entry velocity of the vehicle. It is thought that this situation may not cause much inconvenience to the driver, but in fact, it is not. The positioning erroraccumulates as the vehicle velocity alters inside the tunnel. Even if the vehicle stops due to traffic jams or accidents inside the tunnel, the navigation screen simply indicates that the vehicle continues to move. In addition, navigation should provide accurate guidance when the vehicle approaches and passes through an exit located at the middle of the tunnel. However, accurate guidance is not possible, because the current navigation system cannot estimate the vehicle position in real time inside the tunnel. It is difficult to apply the existing infrastructure-based indoor positioning system [4; 5; 6; 7; 8; 9] to the vehicle in tunnel environment because it is not easy to install and manage the infrastructure in the tunnel of several kilometers. In several prior studies, position estimation in tunnel environments has been conducted using Wi-Fi [11], wireless sensor networks (WSN) [12], radio frequency identification (RFID) [13] and visible light communication (VLC) [14; 15]. In [16], an angle offset-assisted positioning method was proposed for railway vehicle positioning in tunnel environments. These technologies require a very dense infrastructure for estimating a position, in addition to the expensive infrastructure maintenance required for ensuring consistent functioning. Such approaches are unsuitable for vehicle navigation as a separate receiver is required for position estimation. In general, the installation of intensive infrastructure for positioning is challenging for tunnels that are several kilometers long. In this paper, we present the positioning system using a magnetic field for vehicles passing through a long tunnel. The advantage of using the magnetic field is that there is no need to install the infrastructure. The magnetic field is suitable for tunnel environments as its strength can be measured anywhere with a magnetic sensor in the smartphone. Various localization experiments have already been carried out in the indoor environments through the magnetic field [17; 18; 19; 20], and will be applicable to vehicles in the tunnel environment.