Wireless sensor networks (WSNs) have many applications for habitat monitoring, search and rescue, forest-fire monitoring, battle-field surveillance and applications of aerospace.These sensor nodes in WSNs are randomly placed and their exact locations may not be predetermined.Localization is a critical issue in almost all WSNs.For rapid deployment in some complicated environments and composed of a large number of low-cost sensor nodes, the use of GPS[1] or relatively expensive technologies[2-7] is not practical.At present, the algorithms of localization can be divided into two classes: range-based and range-free.