# Research on Localization Algorithms Based on Acoustic Communication for Underwater Sensor Networks
## 5 General Challenges and Open Issues  
The localization in UWSNs suffers more challenges than the terrestrial environment. The mobile current, the effect of transportation and other factors lead to the localization problems in the positioning algorithms. Here, we address some main challenges and open issues. **Sound speed variation**: In many localization algorithms, the authors made an assumption that the sound speed is a constant. Not only the pressure and temperature but also the salinity of the sea water may affect the sound speed. In the multilateration algorithm, because of the assumption of the sound speed, the estimation of the coordinates of the sensor node may incorporate an error. Thus, how to construct a common and accurate sound speed model is a question. **Unpredictable underwater environment and intricate network design and deployment**: Many aspects will affect UWSNs. In addition, most of these aspects are unpredictable such as the water pressure, the mobility of the animal, and the activities of the underwater, and uneven depths of the underwater surface. Thus, network design and deployment are intricate. In the constrained communication, the currently tethered technology can be used but with additional cost [2]. **The impact of channel structure**: The underwater acoustic channel is looked at as a frequency selective time-varying channel [38]. If the positioning algorithm requires the distance measures, the structure of the channel will decrease the accuracy of the positioning. In [38], the authors show that the range measurement is based on direct LOS signals. However, the direct signal may be lost because of the presence of structures and obstacles in the underwater channel. Thus, on the side of the receiver, the case that only NLOS signals are detected may happen. This means that the NLOS signals may be processed as LOS signals, which lead to the lower accuracy. **Novel protocols for UWSNs**: In UWSNs, the medium of communication is water but not the air, which is the medium of communication in the terrestrial sensor networks. Thus, the communication protocols of the terrestrial sensor networks do not work efficiently in UWSNs. In addition, the RF signals are mainly considered the short-distance, while the acoustic signals in UWSNs can be used for long-distance communication in water. Thus, a new protocol is needed for UWSNs. Of course, numerous protocols have been put out and we have presented some survey papers of the protocols in UWSNs. However, some improvement and new protocols still need more attention. **Time synchronization**: As previously mentioned, some schemes do not require the time synchronization, but the other designs need it. In the time synchronization scheme, the time synchronization is an important part to be concerned. The surface nodes are time-synchronized via GPS or DNR, while the underwater nodes cannot be time-synchronized, and their clocks are subject to skew as well as offsets [12]. Some protocols of time synchronization have been proposed in [71; 72; 73]. **Physical damage to equipment and unreliable information**: In UWSNs, some sensor nodes are located in the water, which may suffer from physical damage. In [2], the authors have pointed out that the algae collection on the camera lens and salt accumulation may lead to the lower effectiveness of sensors. In addition, in the underwater environment, the mobile current, other animals, and many unpredictable factors may lead to dismantling the network topology and node locations. Thus, the information may be unreliable.