The technology for aerial robots and mobile robots is developing very rapidly. This technology can be used in many applications: for example, security robots, tour guided robots, home care robots, and restaurant service robots. Positioning systems are necessary for these applications. The global positioning system (GPS) is widely used in many applications. It gives users information on current location immediately in an outdoor environment. However, GPS cannot be used indoors because of the shadowing effect. Consequently, many researchers are studying indoor positioning systems. In this research, we developed an indoor positioning system based on several ultrasonic sensors and the ZigBee network. Visual Basic software was used to design a user interface (UI) which can show the current position of an aerial robot or a mobile robot. Ultimately, an ultrasonic based positioning system was used to guide an omni-directional mobile robot. Several paths, for example squares and circles, were used to measure the positioning error of a mobile robot with the proposed indoor positioning system.