# Underwater Electromagnetic Sensor Networks, Part II: Localization and Network Simulations
## 1 Introduction  
Underwater wireless sensor networks (U-WSN) are a promising technology to deal with applications that require offshore monitoring, e.g., environmental quality control, seismic analysis, oilfield monitoring and sea robotics [1]. The wireless underwater communication presents, however, a challenging task that needs to be approached depending on the application under consideration. In these two-part papers, we focus on the use of U-WSN in shallow waters. There are three main technologies that can be considered for underwater wireless communications, namely acoustic, optical and radio. Each of them presents its own benefits and drawbacks and can be used in different scenarios [2, 3]. For instance, acoustic communications are valid for medium range distances (several km [4]), but have limited bandwidth and provide poor performance in shallow waters. Acoustic systems near ports or the coastline are affected by multiple sources of noise, such as industries or nearby boats. Furthermore, multi-path associated with different transmission speeds, reverberation and ambient noise also degrade the communication performance. Optical communications present ultra-high bandwidth over short distances, but are susceptible to turbidity, particles or marine fouling. Ambient light is another adverse effect that can degrade the performance of this technology in shallow waters [5]. Furthermore, this technology currently requires directional communication, as omni-directional wireless transmissions are under research [6]. Electromagnetic (EM) communications in the low frequency range (i.e., kHz) are not affected by ambient noise nor light and can be used in omni-directional or directional deployments. Thus, it becomes a worth-studying option to establish communication between nodes in shallow waters, such as in lakes, bays, harbors and areas close to the sea shores. In shallow waters, the medium presents two alternative propagation mechanisms for EM waves: the water-to-air and water-to-seafloor interfaces, e.g., [7; 8]. Since these interfaces present less conductivity than seawater, depending on the relative position of the transceivers and receivers with regards to the surface and seafloor, the communication range of the EM communication in shallow water is usually larger than over a homogeneous medium. In the first part of this paper [9], we performed measurements and characterized the underwater channel for shallow waters in a real setup, described the measurement testbed, provided channel measurements at different frequencies and distances and verified the validity of the results using an EM model of the channel and simulations. In this second part, we focus on analyzing the viability of U-WSN using the mentioned EM propagation mechanisms for localization applications. Although the water medium still has a strong attenuation in the low frequency range, we show that reliable communications are possible in the range of tens of meters, making U-WSN viable for deployment. The research described in this paper is part of a national research project named UnderWorld [10], in which all authors are involved. Our main contribution in this publication is to use the channel characterization from [9; 11; 12] to show that it is possible to use EM-based U-WSN for localization and monitoring applications. First, we introduce some application requirements for the deployment of a U-WSN, which include low connectivity, low data rate and robustness against packet loss; Then, we consider two localization problems: self-localization for the nodes that have been randomly located during the network deployment and localization of an underwater unmanned vehicle (UUV) for aiding navigation. We propose new and existing localization algorithms that solve these problems and study them under the specific restrictions of underwater radio networks, which include low connectivity, a low data rate and considerable packet loss.