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Heterogeneity is the key distinguishing characteristic of wireless sensor networks (WSN) as compared to wireless cellular networks (WCN). It may appear in the form of communication ranges of sensor devices, synchronization, and signal processing capabilities. In this article, we develop a hybrid location estimation scheme for heterogeneous WSN with unsynchronized short-range simple relays and mobile sensors nodes, and synchronized stations. Our scheme employs combination of time difference of arrival (TDOA) and received signal strength (RSS) measurements, whereby the TDOA is used to filter out the clock offset that appears due to the lack of synchronization. We quantify the estimation accuracy of the scheme by deriving the Cramer-Rao bound (CRB), and discuss the performance trade-off between the number of synchronized and non-synchronized devices involved.