Location-awareness is crucial and becoming increasingly important to many applications in wireless sensor networks. This paper presents a network-based positioning system and outlines recent work in which we have developed an efficient principled approach to localize a mobile sensor using time of arrival (TOA) information employing multiple beacon nodes in the line-of-sight scenario. With two-way message exchanges between the beacon nodes and the mobile sensor node, the mobile target sensor node can obtain adequate observations and localize itself automatically. The proposed positioning scheme performs location estimation in three phases: (I) Ultrasonic TOA ranging, (II) Geometrical positioning with kernel-based particle filtering, and (III) Data processing. The experimental results show that the proposed localization system provides a feasible and robust platform for indoor position estimation.