In underwater sensor networks (UWSNs), it is important to determine the location of all the sensors. Localization is the process of estimating the location of each node in a sensor network. There are only a few localization schemes for UWSNs, even though many localization algorithms have been proposed for terrestrial sensor networks. The characteristics of underwater sensor networks are fundamentally different from that of terrestrial networks. The physical layer environments with rigorous bandwidth limitations are the main characteristic of underwater acoustic channels. A unique set of challenges are posed by the long propagation delays and the variable speed of sound under water for localization in UWSNs. This chapter explores the different localization algorithms relevant to underwater sensor networks, and the challenges in meeting the requirements posed by emerging applications for such networks, likes offshore engineering.
TopIntroduction
In recent trends, the deployment of low-cost wireless sensors is proven to be a promising technique for various applications. Underwater applications look into the early warning systems for natural disasters, ecosystem monitoring, oil drilling, and military surveillance. The limited power constraints and processing capability on the sensors makes the management and deployment of wireless sensor networks a greater challenge. The researches related to underwater sensor networks are being done in the recent days. The sensor networks from the physical layer to the application layer have been discussed. Oil drilling generally uses huge exploration vessels that are anchored to the seabed with multiple anchors. These smart sensors can be deployed on the seabed, and it can monitor the system parameters and the environmental parameters. These work together with these Remotely Operated Vehicles (ROV), that are controlled from the ship, or Autonomous Underwater Vehicles (AUV), that can autonomously navigate the deep waters are based on some set of instructions. The ROVs/AUVs, sensors and anchors collect and feed information from the seabed. These can measure parameters such as foundation strength and mooring tensions. They also provide accurate position references to the AUVs. The location of the sensors, anchors and the AUVs are needed to be determined in order to deliver data from the seabed to the ship. This problem is especially challenging for deep water applications. Another challenge in underwater acoustic communication is the limited bandwidth long propagation delay, signal failure issue and sensor node failure due to environmental conditions (Chandrasekhar & Seah, 2006). These nodes must self-configure and it selves to the changes in the ocean environment. Developing efficient underwater communication and networking techniques are involved in the energy conservation of UWSNs.
Figure 1.
2D architecture of underwater sensor network
Figure 2.
An example application scenario for underwater sensor networks
TopLocalization Schemes Survey
A sensor’s location helps in typically interpreting a sensor network data. For e.g. tracking a moving object, monitoring the physical conditions of some region or reporting the occurrence of certain events. The localization underwater is as challenging as Radio Frequency (RF) waves that are heavily attenuated under water. A number of localization schemes have been proposed that takes into account many factors such as device capability, signal propagation model and energy requirements. The underwater localization involves many challenges like node development, node mobility, change in signal strength, time synchronization, variation in sound speed and acoustic channel characteristics (Hahn & Rice, 2005).
Range or bearing information like Time Of Arrival (TOA) and Time Difference Of Arrival (TDOA) is used by the range-based localization schemes for position estimation. Locating some nodes in network is known as anchor node or reference node which is used to localize the unlocalized nodes in the networks.