# Research on Localization Algorithms Based on Acoustic Communication for Underwater Sensor Networks
## 3 Localization Algorithm
### The State of the Nodes
#### 3.4.2 Mobile  
We still first consider the active messaging properties based on the stationary state and collect several papers to describe it. In [55], the time synchronization and localization based on the semi-periodic property of seawater movement scheme is called SLSMP. The deployment is described as follows: the reference nodes attached to the buoys and the sensor nodes equipped with tiny gyro and acceleration sensors are used. The reference nodes can obtain the global time and real-time position on the GPS. The SLSMP protocol contains three phases, namely, Sending Point (SP) selection based on trajectory tracking, message exchange, and time synchronization/localization. In addition, the knowledge about sensor deployment, inertial navigation system (INS) and filter technique is employed to ameliorate the localization accuracy.  
A Joint Solution for Localization (JSL) is shown in [56]. This scheme consists of four steps, namely, data collection, synchronization, localization, and iteration. In the first step, data collection means that an ordinary node can obtain the reference time and positioning information. In the second period, based on the information obtained in the first phase, the ordinary node performs the synchronization process. In the third step, based on the estimated propagation delays, the localization process is carried out. In the fourth phase, the iteration process acts as if the estimated position in the third stage is regarded as the input to the second phase to update the rough position in the next round of message exchange.  
In this part, we discuss the silent messaging properties based on the mobile state. A multi-stage localization scheme that employs mobile beacons into UWSNs is presented in [57]. Mobile beacons periodically rise and dive in the underwater. GPS coordinates will be received when mobile beacons resurface. Then, these mobile beacons fall underwater to broadcast these coordinates. If the sensors are localized, the sensors are looked at as the proxy beacons and broadcast their positions. In this method, the authors use proxies instead of the beacons to minimize the number of the beacons.  
In [58], the authors employ an AUV aiding in the localizing process and the priority infrastructure or synchronization between nodes are not required. In this scheme, three messages, namely, wake-up, request and response messages are contained. Because of asynchronization between the nodes, the authors use the request/response message pair to measure the Round Trip Propagation Delay (RTPD). First, the AUV sends a wake-up beacon to the sensor node and the sensor node that is in the communication range receives the wake-up message and then starts to measure range by sending a location request packet. When the AUV received the request, it replies a response packet with its coordinates. The authors consider two methods, namely, triangulation and bounding box. In the triangulation method, obtaining n-dimension coordinate needs n+1 equations and then solving the linear independent equations can get the estimation of the sensor location. Because the z-coordinate of the sensor can be obtained from the pressure sensor, we need three messages to estimate the 2D positions of the nodes and can be described as  
\[(x-x_{i})^{2}+(y-y_{i})^{2}+(z+z_{i})^{2}=d_{i}^{2}, \tag{4}\]  
where \(x,y,z\) are the coordinates of the unknown sensor node. \(x_{i},y_{i},z_{i}\,(i=1,2,3)\) are the AUV coordinates. \(d_{i}^{2}\) is the square of the measured range. The boundary method draws a rectangular area with the intersection of the estimated distances. The intersection of the diagonal lines indicates the position of the sensor node.