## 2 Related Work  
Localization in wireless sensor networks is different from the traditional wireless communication technology. It is an important aspect in WSNs as the events detected by sensors usually should contain location of those nodes that detect a target. For example, location of a military tank should be informed to the sink if it is detected by the sensors, which can be achieved through location information of the sensors. Besides, many network operations also depend on the locations of sensors, such as geographic routing, key distribution protocols, and location-based authentication. Incorrect locations may lead to severe consequences. For example, lack of location information of sensors may lead to wrong military decisions on the battlefield and falsely granting access rights to people. Thus it is important and essential to ensure the correctness of sensors' locations. There has been an increasing interest in the localization techniques for WSNs in recent years and many localization algorithms [3; 4; 5] have been proposed. Constraint on limited hardware supports and power supply, sensor nodes can only find its approximate location information. In order to find node's location effectively, various localization algorithms are proposed, which can be further classified into _range-based_ and _range-free_ localization schemes. The range-based localization scheme [6] uses measurements of distance or angle to estimate the node's location. According to signal propagation and receive time, two kinds of technology are mentioned to obtain the distance. They are: time of arrival (TOA) [7], time of difference of arrival (TODA) [8]. TOA method is used to obtain the range between the sender and receiver nodes by signal arrival time. TODA technique is based on the difference in time between two different signals arrival time and is widely proposed as a necessary measurement method in localization solution for WSNs. The algorithms proposed in [9] and [10] are self-organized methods to establish the relative coordinate system on every known nodes through the TODA. Angle of arrival (AOA) technique [11] is another ranged-based localization algorithm. In this algorithm, normal nodes have ability to detect the angle to neighbor nodes by directional antenna or smart antenna. Using the angle information, location of the nodes can also be calculated. However, these three methods require additional equipments and hardware supports, which may incur additional cost and energy consumption. Hence, these protocols seem less suitable for the low-power WSNs. With help of global position system (GPS), few beacon nodes can obtain their absolute location information and other unknown nodes estimate their location information by receiving the beacon packets from the beacon nodes. In [12], authors propose a localization scheme called approximate point in triangular test (APIT) algorithm. In APIT, each beacon node first broadcasts the beacon packet to the neighbor nodes, which is later flooded into the whole network. Then each unknown node determines if it is within a particular triangle formed by a set of beacon nodes. Finally, an unknown node estimates its location by the center of gravity of the overlapped area. Although location information of the unknown nodes can be obtained by this algorithm, still some problems exist in it. First, the accuracy relies heavily on the percentage of beacon nodes, and communication cost is high as each node needs to listen many times to different beacon packets. Besides, the complexity of computations is high when the unknown node estimates the overlapped area. A range-free localization scheme called distance vector hop (DV hop) is proposed in [13, 14]. It uses topological information and number of hops alternative to the real distance. In the beginning, the beacon node floods the packet with hop count and node ID to the rest of the network. Unknown nodes compute the average hop size of their nearest beacon nodes, translate the number of hops into real distance and estimate their position.