  Wireless sensor networks (WSNs) have recently gained a lot of attention by
scientific community. Small and inexpensive devices with low energy consumption
and limited computing resources are increasingly being adopted in different
application scenarios including environmental monitoring, target tracking and
biomedical health monitoring. In many such applications, node localization is
inherently one of the system parameters. Localization process is necessary to
report the origin of events, routing and to answer questions on the network
coverage,assist group querying of sensors. In general, localization schemes are
classified into two broad categories: range-based and range-free. However, it
is difficult to classify hybrid solutions as range-based or range-free. In this
paper we make this classification easy, where range-based schemes and
range-free schemes are divided into two types: fully schemes and hybrid
schemes. Moreover, we compare the most relevant localization algorithms and
discuss the future research directions for wireless sensor networks
localization schemes.
