## 1 Introduction  
Along with the recent widespread expansion of aerospace applications, the demand for a variety of wireless communication technologies is gradually increasing in rocket launch events [1; 2; 3]. The wireless communication systems are generally used for collecting the information about the rocket status [4; 5; 6]. If the communication link between the rocket and the main station is disconnected, the rocket's position can be missed. Then, the rocket should self-destruct for safety issue with a termination command. Thus, maintaining a stable communication link is critical for successful rocket launch events. However, due to the reflection and diffraction from complex terrain and buildings, the radio propagation characteristics may be significantly degraded in outdoor environments [7; 8; 9; 10; 11]. Moreover, the environment where the communication is taking place changes rapidly depending on the altitude of the launched rocket. Previously, electric field distributions by wireless communication systems were analyzed in outdoor wave propagation studies [12; 13], and a path loss model for the urban area was proposed [14; 15; 16]. Channel characteristics analysis for moving targets including line of sight (LoS) and non-line of sight (NLoS) situations was also executed [17; 18; 19]. However, research on the propagation characteristics from the launched rocket to the base station or the relay station according to the altitude of the rocket has not been fully carried out yet. In this paper, we propose a novel estimation method to optimize a relay station position of the S-band telemetry system in outdoor environments. The estimation method using the EM simulation needs four steps: first, the model for the nearby environment such as the terrain and building is included in the EM simulation. Second, the path loss between the rocket and the receiver according to rocket's altitude is calculated using the EM simulation. Third, the optimal position of the relay station is determined by examining the path loss from the rocket to the relay station and the main station. Finally, the optimum position of the relay station is validated by observing the effect of the radiation pattern and phase variation. To determine an optimal relay station position, a ray tracing-based EM simulation software (Wireless InSite) is utilized [20], complex terrain surfaces and buildings near the Naro Space Center are included in the simulation. The transmitting and receiving antenna patterns are also considered in the simulation. Their radiation patterns are assumed to be a dipole pattern and an ideal isotropic pattern, respectively. The optimal relay station position is then determined using the estimation method by comparing the path losses from the rocket to the main station and some candidate positions of the relay station. The relay station in this work is not portable and should be installed at a fixed position on a building. The rocket's communication link is significantly influenced by the surrounding terrain and buildings, especially when the rocket is at low altitudes. Since the shorter distance between the transmitting and receiving antennas has a lower path loss, the relay station near the rocket is more advantageous for maintaining a better communication link. However, when the rocket is at high altitude, the gain in the LoS direction decreases due to the weak pattern of the antenna in the rocket's tail direction. Thus, it is very important to select the advantageous position of the relay station for a better overall communication link. The results demonstrate that the relay stations near the rocket are more advantageous when the rocket's altitude is low due to less path loss. On the other hand, due to the shaded region by the radiation pattern of the transmitting antenna, the relay station farther away would be better for the rocket at a higher altitude. It means that once the rocket altitude to be covered by the relay station is determined, it is possible to find the optimum position of the relay station.