This paper presents an ultrasonic based indoor positioning (ICKON) system for indoor environments. The ICKON uses only ultrasonic signals and calculates the position of the mobile platform at centimeter-level accuracy. In this system, ultrasonic transmitters are located at known fixed positions and periodically send signals. The receiver is on the mobile unit and uses the time difference of arrival (TDOA) to calculate the mobile unit's position. There is no synchronization signal between the transmitter and the mobile unit. The number of the mobile robots that can be used is restricted due to the physical limitations of the environment. Large indoor areas are covered by integrating an ultrasonic ID emission feature to the transmitters. The system uses the existing Local Area Network of the building, therefore it is cost effective. To demonstrate the effectiveness of this positioning system, a prototype is implemented and experiments are conducted using P3-DX mobile robots in various test environments.