  Localization is widely used in Wireless Sensor Networks (WSNs) to identify
the current location of the sensor odes. A WSN consist of thousands of nodes
that make the installation of GPS on each sensor node expensive and moreover
GPS may not provide exact localization results in an indoor environment.
Manually configuring location reference on each sensor node is also not
possible for dense network. This gives rise to a problem where the sensor nodes
must identify its current location without using any special hardware like GPS
and without the help of manual configuration. In this paper we review the
localization techniques used by wireless sensor nodes to identify their current
location.
