        ! 60               Vibha Yadav, Manas Kumar Mishra, A.K. Sngh and M. M. Gore Department of Computer Science & Engineering, Motilal Nehru National Institute of Technology, Allahabad, INDIA (cs0706, rcs0701, singhak, gore)@mnnit.ac.in ABSTRACT Location awareness among the participating nodes is one of the crucial requirements in designing of solutions for various issues related to Wireless Sensor Networks (WSNs). This paper discusses about a range free localization mechanism for WSN that operate in a three dimensional space. in this scheme, the sensor network is supposed to be comprised of mobile and static sensor nodes. Mobile sensor nodes are assumed to be equipped with GPS enabled devices and are expected to be aware of their position at any instance. These mobile nodes move in the network space and periodically broadcast beacon messages about their location. Static sensor nodes receive these messages as soon as they enter the communication range of any mobile node. On receiving such messages the static nodes calculate their individual position based on the equation of sphere. The proposed scheme gains in terms of computational and memory overhead as compared to existing approaches. The proposed scheme is simulated using Sinalgo, and the performance of this is compared with the chord selection approach. The simulation results validate the gain in localization time, its accuracy, and the resulted overhead. KEYWORDS Localization, Mobile Sensor Nodes, GPS, Range Free, Connectivity Range 1. INTRODUCTION A wireless sensor network consists of a large set of inexpensive sensor nodes with wireless communication interface. These sensor nodes have limited processing and computing resources. Thus, algorithms designed for wireless sensor networks need to be both memory and energy efficient. In most of the algorithms for wireless sensor network, it is assumed that the sensor nodes are aware of their locations and also about the locations of their nearby neighbors. Hence, localization is a major research area in wireless sensor networks. But, this problem has not been studied extensively in three dimensional WSNs because of its complexity reasons. However, in some real world application scenario the deployed sensor network operates over a three dimensional volume rather than in a two dimensional area. Deployment of WSNs for surveillance of terrains, study of underwater ecosystem, space monitoring & exploration, etc; are examples of such applications. But, so far only a few researchers have addressed the problem of localization for these 3D scenarios. Localization in sensor networks can be defined as ``identification of sensor node's position''. For any wireless sensor network, the accuracy of its localization technique is highly desired. The existing algorithm for localization can be broadly classified into two basic categories: