This paper describes a TDOA-based positioning system by means of ultra-wideband impulse radio (UWB-IR). The system consists of three reference nodes and a location server. One of the reference nodes transmits an impulse signal to other reference nodes for synchronizing them immediately after receiving an impulse signal sent from a target node. Thus, no additional component for the synchronization is needed in our system. The positioning accuracy was investigated by a simulation and an experiment. The experiment was performed with our UWB-IR transceivers and the results demonstrated that 67% of the position estimates correctly predicted the true positions within 0.36 m.Index Terms -Indoor radio communication, microwave communication, personal area networks, ultra wideband communication, wireless sensor networks.