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Rapid growth in the number of wireless enabled devices has led to an increased interest in location-aware services. The core support for such services is provided by a location system. In this paper, we present a time and power based positioning scheme (TPPS) for indoor wireless local area network (WLAN) using an overlay sensor network. TPPS is based on time difference of arrival (TDoA) and received signal strength (RSS) using radio frequency and ultrasonic signals to measure and detect the range differences from the wireless mobile terminal to three sensor nodes. Mobile terminals can use TPPS to effectively estimate their locations based on the range differences. These range differences are averaged over multiple beacon intervals to counter interference and fading before they are combined to estimate the location of a mobile terminal through trilateration. TPPS requires low computation overhead and no time synchronization, as the location estimation algorithm involves only a simple algebraic operation.