# A Mobility-Assisted Localization Algorithm for Three-Dimensional Large-Scale UWSNs
## 1 Introduction  
As an indispensable component of the earth, water covers over 70% of the planet's surface [1]. Due to the development technology of terrestrial resources being relatively mature, in recent years, humans have gradually turned to more abundant marine resources. Underwater wireless sensor networks (UWSNs) have attracted extensive attention in related fields because of their good monitoring effect. Moreover, due to their high military and commercial value, they are widely used in warning systems for natural disasters, oceanographic data collection, underwater environment surveillance, and other underwater-related fields [2; 3; 4]. Underwater sensor node location is the foundation of UWSNs' application in various related fields. Therefore, the innovation and optimization of sensor node localization schemes based on UWSNs promote the development and progress of related fields. UWSNs consist of numerous autonomous and individual sensor nodes, which are spatially distributed in the monitoring waters, to capture and transmit relevant data information [1]. Then, the captured monitoring information is processed by the appropriate positioning or tracking algorithm, to obtain the coordinates or trajectory of the underwater target or sensor node. Unlike terrestrialwireless sensor networks (WSNs), due to the huge attenuation and scattering of the actual underwater environment, radio waves and optical waves cannot transmit signals over long distances underwater [5]. Moreover, the Global Positioning System (GPS) signals do not propagate well in water [6]. Therefore, many terrestrial monitoring and positioning technologies cannot be directly applied to the underwater environment, because acoustic signals provide appropriate conditions for long-distance underwater propagation. In general, therefore, UWSNs use acoustic signals for communication. However, underwater acoustic communication has the disadvantages of low data transfer, low communication bandwidth, and long transmission delay. Furthermore, due to the complexity and particularity of the actual underwater environment, the localization of the sensor node still has many challenges, such as localization accuracy, energy restriction, the mobility of sensor node, time synchronization, and so on. Therefore, it is necessary to propose innovative or optimized localization algorithms for UWSNs. Underwater localization algorithms based on UWSNs can be divided into two categories, namely, range-based and range-free [7]. The range-free localization scheme does not need the range measurement, and it employs the local topology and locations of the surrounding anchor nodes to estimate the location of the sensor node. This type of underwater localization scheme has the advantages of low energy consumption, low communication overhead, and low computational complexity. However, the localization accuracy of the sensor node is relatively low. Therefore, this kind of localization algorithm is suitable for the scene where the accuracy of node location is not high. To achieve high sensor-node-localization accuracy, in addition, most related researchers usually choose range-based underwater localization schemes. In this paper, therefore, we concentrate on range-based algorithms. Generally, as shown in Figure 1, range-based localization algorithms are based on time of arrival (TOA), time difference of arrival (TDOA), angle of arrival (AOA)/direction of arrival (DOA), and received signal strength indicator (RSSI) [8]. Due to the absorbability of the underwater environment to acoustic signals and other factors, the location error of the RSSI-based localization algorithm is relatively large. Furthermore, the computational complexity and cost based on the AOA localization algorithms are relatively high. In addition, time synchronization plays an important role in some localization schemes [9; 10; 11]. Underwater localization algorithms based on one-way TOA usually assume time synchronization.