# Underwater Acoustic Target Tracking: A Review
## 3 Knowledge of Acoustic Target Tracking Instruments  
To make readers understand the area easily, the basic structure of sensors used for tracking is clarified. Then the most common used acoustic tracking instruments including TASAs, UWSNs, and acoustic imaging sensors are introduced in this section. As is known to humans, acoustic waves are the only medium which can propagate over a long distance in seawater. As a result, people have been using sound waves to navigate, locate, track and recognize fixed or moving underwater targets. Devices with these functions are acoustic sensors. Compared to the tracking based on WSNs, the underwater acoustic tracking algorithms have to consider some special issues such as communication issues, improvement of real-time and tracking mode. However, the issues in tracking can be solved not only by effective algorithms but also by the tracking instruments. Recently, some researchers are focused on designing acoustic modems for sensors for underwater target tracking to solve the issues incurred by the underwater environment. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Survey Content** & **Reference Number** & **Published Year** & **The Related Knowledge** \\ \hline
[11] & The whole process of target tracking & High & 2016 & Detailed (The whole process of tracking, the metrics for analyzing algorithms, the requirement of tracking based on WSNs) \\ \hline
[12] & The management of energy for target tracking & Medium & 2012 & Brief \\ \hline
[13] & The prediction algorithms used in target tracking & Low & 2014 & Brief \\ \hline
[14] & The target recovery techniques in target tracking & Low & 2016 & Brief \\ \hline
[15] & The security of target tracking & Low & 2014 & Brief \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison of survey papers for target tracking based on WSNs. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Reference** & **Taxonomy** & **Reference Number** & **The Newest Reference** & **Standard of Comparison** \\ \hline \multirow{2}{*}{Ours} & Instrument-assisted method & \multirow{2}{*}{High} & \multirow{2}{*}{2017} & Comprehensive (tracking precision, cost, complexity, note, the characteristic of target) \\  & Mode-based method & & & \\ Tracking optimization method & & & & \\ \hline \multirow{3}{*}{[11]} & Network structure & \multirow{3}{*}{High} & \multirow{3}{*}{2015} & Common (prediction, energy management, target recovery) \\  & Problem formulation & & & \\ Number of targets & & & & \\ Type of target & & & & \\ \hline \multirow{3}{*}{[18]} & Network structure & \multirow{3}{*}{Low} & \multirow{3}{*}{2015} & Only summary no comparison \\  & Prediction-based & & & \\ Type of objects & & & & \\ Type of sensors & & & & \\ Type of targets & & & & \\ Recovery & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of survey papers for target tracking. The authors in [19] provide a comprehensive survey of underwater acoustic modems. The sensors are usually equipped with power, processing and communication units. The most common acoustic sensors have two kinds of use. One is to launch sound waves, and the second is to receive sound waves [20]. The acoustic sensor is usually utilized to transmit and receive sound waves at the same time, while the one especially used just for receiving signals is called a hydrophone. There are varieties of acoustic tracking systems which can be summarized as an active system or a passive system according to the working mode of sensors. Acoustic systems composed of active sensors can be further classified into monostatic systems, where the single sensor serves as a transmitter as well as a receiver; multi-static systems, where the sensor only undertakes the task of transmitting or receiving sound waves. The bistatic system is the simplest type of multi-static systems with a single source-receiver pair [21; 22]. Usually, acoustic sources are hull-mounted sensor arrays while the common examples of receivers are towed line arrays as Figure 1 shows [23].