I. Introduction
Wireless sensor networks (WSNs) [1] have been widely deployed to acquire some physical data, such as temperature, humidity and acceleration, etc. Localization is an indispensable and fundamental component for WSNs since the data sensed by sensors will be meaningless without location information, attracting increasingly considerable attention in the past few years. As a popular system, Global Positioning System (GPS) [2] has been broadly used in daily life. However, it may be infeasible in WSNs as they are usually deployed in complicated environments (e.g., indoor or underground). What’s more, it is extremely expensive to equip each target a GPS receiver in WSNs.