its location, a StarGazer sensor system is used. A landmark is printed with a pattern reflecting an infra-red light and is attached to the ceiling. The landmark plays the role of an independent coordinate system with an identity number. The StarGazer consists of an infra-red beam emitter and an infra-red image camera. Generally, the sensor system is equipped on the head of the robot. It analyzes the infra-red image of the pattern on the ceiling and calculates its position and the heading angle of the robot.