## 2 Navigation and Localization
### Acoustic Navigation
#### Acoustic Ranging
This filter considers an underwater vehicle moving in a scenario where there is a set of fixed landmarks installed in an LBL configuration and the vehicle is equipped with a USBL acoustic positioning system. The filter achieves good performance even in the presence of sensor noise under a simulated environment. The resulting solution ensures a quick convergence of the estimation error to zero for all initial conditions. However, it could not be a practical solution for some cases since it requires a complex infrastructure. A coordinated navigation of surface and underwater vehicles is proposed by [PERSON] et al. [48]. The proposed scheme has the purpose to serve as a first-responder monitoring team on environmental disasters at oceans. The USV is connected to a ground station via Wi-Fi for control and monitoring; meanwhile, acoustic communication is used to send instructions to the AUV and to retrieve information from it. To locate the vehicles, a Global Positioning System (GPS) is mounted on the USV so it can get a position on geographic coordinates. Once the USV gets a location, a USBL system is used to get a relative location of the AUV regarding the surface vehicle. An algorithm is run to convert them to a global position so the control station can know where both vehicles are. This allows the precise localization of pollution or any other problems found by the vehicles and is intended to help to plan a rapid response. As long as the USV and AUV remain on a close-range for communication, limitations on the USBL system are not a problem in this scenario. [PERSON] et al. [49] used a digital USBL system for AUV recovery. The AUV was equipped with a receiver array of four transducers and a transponder array was mounted on an USV which served as recovery station. The system proposed is not only capable of estimating the distance between the AUV and the recovery location, but it is also able to measure horizontal and vertical bearings. The system has an update period of 3 s and has an accuracy of less than a meter. Its main limitation is the sensing range, AUV must be 25 m within the target localization, or the system measurements are considered as erroneous. Field experiments showed a success rate of 37.5% at recovering the AUV. Range-only--also known as Single-beacon--localization is another alternative to traditional acoustic localization systems that has gained attention in recent years. The concept of range-only/single-beacon positioning can be divided into two groups depending on the way they are used [50]: (i) as a navigational aid for a moving vehicle, or (ii) localization of a stationary or moving target. All these methods use a set of ranges between a target and different static nodes, known as anchor nodes. Typically, these ranges can be obtained using the time of flight given the speed of sound in water. Then, the unknown underwater target position problem can be solved using trilateration, where in general, three or more points are needed in 2D dimensions and, at least, four points in 3D scenarios. A method for target positioning from a moving vehicle--which periodically measures the range to the underwater target--is represented in Figure 8. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name** & **Type** & **Accuracy Range (m)** & **Operating Depth Range (m)** \\ \hline EvolLogics S2C R LBL [42] & LBL & Up to 0.15 & 200–6000 \\ \hline GeoTag seabed positioning system [43] & LBL & Up to 0.20 & 500 \\ \hline \(\upmu\)PAP acoustic positioning [44] & USBL & Not specified & 4000 \\ \hline SUBSONUS [45] & USBL & 0.1–5 & 1000 \\ \hline \multirow{3}{*}{UNDERWATER GPS [46]} & \multirow{3}{*}{SBL/USBL} & 1\% of distance range & \multirow{3}{*}{100} \\  & & (1 m for a 100 m \\ \cline{1-1}  & & operating range) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Commercial acoustic positioning systems.