# A Mobility-Assisted Localization Algorithm for Three-Dimensional Large-Scale UWSNs
## 3 System Model
### Network Structure  
The vertical section of the 3D UWSNs monitoring area is shown in Figure 2. In this paper, we consider that the underwater sensor network model consists of two types of nodes, namely, reference nodes (mobile anchor nodes and sensor nodes that have been located) and ordinary sensor nodes.  
Reference nodesThe reference node can propagate its position message. In this paper, there are two types of reference nodes, respectively, mobile anchor nodes and sensor nodes that have been located. Mobile anchor nodes dive by attaching additional weights. Then, at a certain depth, the weight is released, and the anchor nodes rise. The mobile anchor node is equipped with GPS, and it can receive coordinate information when it comes to the surface. Moreover, it can obtain depth information in real-time by utilizing the configured underwater pressure sensor. During the underwater dive and rise, the anchor node periodically propagates the current coordinate information. Moreover, if the ordinary sensor node that has been located has a high confidence value in the first phase, it can become a new reference node in the second phase.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Ref.** & **NS** & **CA** & **RM** & \begin{tabular}{c} **Auxiliary** \\ **Equipment** \\ \end{tabular} & **AR** & \
\begin{tabular}{c} **Anchor** \\ **State** \\ \end{tabular} &
\begin{tabular}{c} **Ordinary** \\ **Node State** \\ \end{tabular} & **SR** & **CBNs** & **ME** \\ \hline
[19] & L-SN & D & Rb & Mobile anchor & Ab & Mobile & Static; free & Sy & Ss & S \\
[20] & S-SN & D & Rb & Mobile anchor & Ab & Mobile & Static & As & Ss & S \\
[21] & L-SN & D & Rb & Mobile anchor & Ab & Mobile & Static & As & Ms & A \\
[22] & L-SN & D & Rb & Mobile DET & Ab & Static & Static & Sy & Ms & S \\
[12] & L-SN & C & Rf & Mobile DET & Af & — & Static & As & Ss & A \\
[23] & L-SN & C & Rf & Mobile DET & Af & — & Static & As & Ss & A \\
[24] & L-SN & C & Rb & Static anchor & Ab & Static & Static & As & Ss & A \\
[25] & L-SN & D & Rb & AUV & Af & — & Static & Sy & Ms & S \\
[26] & S-SN & D & Rf & AUV & Af & — & Static & As & Ss & S \\
[27] & L-SN & D & Rb & Static anchor & Ab & — & Static & As & Ms & A \\ Our & L-SN & D & Rb & Mobile anchor & Ab & Mobile & Static; free & As & Ms & A \\ \hline \hline \end{tabular} Notes: Reference (Ref.), network scale (NS), lager-scare network (L-SN) or small-scale network (S-SN), computation algorithm (CA), centralized or distributed (C or D), range measurement (RM), range-free or range-based (Rf or Rb), anchor requirement (AR), anchor-free or anchor-based (Af or Ab), synchronization requirement (SR), synchronization or asynchronization (Sy or As), communication between nodes (CBNs), single-stage or multistage (Ss and Ms), message exchange (ME), and active or silent (A or Si).  
\end{table}
Table 2: Comparison of related underwater Localization algorithms.  
Ordinary sensor nodes.Ordinary sensor nodes account for the largest proportion in UWSNs, and their coordinate positions are unknown. In this paper, ordinary sensor nodes have autonomous computing capability, and the network is a distributed UWSN. It can locate itself by receiving information from the anchor node. Moreover, since all sensor nodes are assumed to be equipped with UPS, only three non-collinear anchor node position messages need to be received to estimate their position.