## 4 Underwater Sensor Networks  
In [18], [PERSON] [PERSON] at al. present a simple and energy-efficient localization strategy for near surface buoy-based sensors that let them self-calibrate in terms of geographic location without the need for costly and energy consuming GPS-hardware. The approach does not rely on stationary or mobile reference points. It uses the sun as a reference point. Nodes use an accurate clock and light sensors that can regularly sample the cyclic variations of light intensity resulting from the movement of the celestial bodies throughout a day. The light intensity measurements are fitted into a celestial model of the earth motion around the sun. By identifying the trajectory of the sun across the skies, they can accurately determine sunrise and sunset times, and thus extract the longitude and latitude of the sensor. The total accuracy mainly depends on the precision when identifying sunrises and sunsets. Compared to current global localization methods, the proposed system cannot be applied to tasks requiring a high accuracy, such as search and rescue, navigation, _etc._ but it can be sufficient for certain data gathering applications such as water monitoring and climate change monitoring. In recent years, the localization problem in Underwater Wireless Sensor Networks has resulted in many innovative solutions and ideas. [PERSON] _et al._ present in [19] a comprehensive survey of localization algorithms for Underwater Wireless Sensor Networks. The classified localization algorithms into three categories based on sensor nodes' mobility: stationary localization algorithms, mobile localization algorithms and hybrid localization algorithms. Then, they compare the localization algorithms in detail and analyze future research directions of localization algorithms in Underwater Wireless Sensor Networks. They believe that the possible hot research topics are localization algorithms for large scale Underwater Wireless Sensor Networks and realistic models of sensor node's mobility according to the corresponding underwater information (e.g., depth, current velocity, water salinity, _etc._). [PERSON] _et al._ propose a novel 3D centralized localization scheme for mobile underwater wireless sensor network, named Reverse Localization Scheme (RLS) in [20]. RLS is an event-driven localization method triggered by detector sensors for launching localization process that can achieve high average response time in underwater sensor networks. It is a fast and energy efficient localization method to transfer the location estimation from the sensors to the base station. Mobile sensor nodes report the event toward the surface anchors as soon as they detect it. They do not wait to receive location information from anchors. The algorithm requires very low computational complexity and energy consumption. The authors present the design and the development of the message exchange mechanism. The proposed scheme minimizes the number of localization messages while decreases the average response time and keeps the accuracy considering to the mobility feature of water currents. It is suitable for surveillance applications that require very fast reactions to events and could report the location of the occurrence. Simulation results shows that the proposed localization algorithm improves the energy efficiency, involves low message exchange and reduces significantly localization response time with a proper level of accuracy in terms of mobility model of water currents, which can be a vital component in some applications of underwater networks like disaster prevention. In [21], [PERSON] and [PERSON] describe a new range-free Localization with a Mobile Beacon for Underwater Acoustic Sensor Networks called LoMoB. In their proposal, the mobile beacon periodically broadcasts a beacon message in the underwater environment containing its location. Sensor nodes are individually localized by passively receiving the beacon messages without inter-node communications.