The wireless sensor network based localization system--Cicada is designed to support the location-awareness in indoor environment. The system is based on TDOA (time difference of arrival) between radio frequency and ultrasonic to measure the distances, uses the SWF(slide window filter) and least square fitting for the rough distance correction, adopts the UKF (unscented kalman filter) to ultimately estimate the coordinates, and then sends the coordinate values to the network to provide location services. The system has several notable merits: it not only can provide the coordinates within 5cm average deviation either for static object or for mobile object, but also can support multiple objects at the same time; it not only has a nearly omni-directional working area, but also be convenient for deployment and portability. Compared with other indoor localization system, the Cicada system adopts a more accurate algorithm and the whole system is more portable and convenient to deploy. Experiment results show the system can well satisfy the requirements in smart space.