# Underwater Acoustic Target Tracking: A Review
## 6 Discussion, Challenges, Countermeasures, and Lessons Learned
### Discussion  
Underwater target tracking plays a crucial part in modern civil and military systems. Therefore, numerous research results have been reported. Due to the special characteristics of the marine environment, the acoustic wave becomes the only appropriate carrier to realize underwater target tracking. In this paper, we make a comprehensive study of the underwater acoustic target tracking algorithms and propose a new taxonomy based on the methods used, namely, instrument-assisted method, mode-based method, and tracking optimization method. The varieties of underwater target tracking algorithms introduced in Section 5 are summarized in Tables 6 and 7.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Instrument-Assisted** & **Optimization** & \multicolumn{3}{c}{**Target**} \\ \hline
[37] & UWSNs & EKF & Single & 3-D & No \\
[56] & TASA & EKF & Single & 2-D & Yes \\
[58] & TASA & — & Multiple & 3-D & No \\
[63] & UWSNs & KF & Single & 2-D & No \\
[67] & UWSNs & PF, Quantization & Single & 2-D & No \\
[73] & UWSNs & UKF & Single & 2-D & Yes \\
[74] & TASA & Arithmetic average & Single & 2-D & No \\
[75] & TASA & Sage-Husa model & Single & 2-D & No \\
[77] & TASA & KF & Multiple & 2-D & No \\
[78] & — & KF & Single & 2-D & Yes \\
[80] & — & EKF & Single & 3-D & Yes \\ \hline \hline \end{tabular}
\end{table}
Table 6: Summary and comparison of underwater active acoustic target tracking algorithms.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Reference** & **Instrument-Assisted** & **Optimization** & \multicolumn{3}{c}{**Target**} \\ \hline
[38] & TASA & — & Single & 2-D & Yes \\
[49] & Acoustic imaging sensors & — & Multiple & 3-D & Yes \\
[50] & Acoustic imaging sensors & EKF & Multiple & 3-D & Yes \\
[52] & Acoustic imaging sensors & KF & Single & 3-D & No \\
[53] & Acoustic imaging sensors & — & Multiple & 3-D & No \\
[55] & TASA & PF & Single & 2-D & No \\
[61] & UWSNs & KF & Single & 2-D & No \\
[62] & UWSNs & KF & Single & 2-D & Yes \\
[65] & UWSNs & KF & Single & 2-D & No \\
[66] & UWSNs & EKF & Single & 3-D & Yes \\
[68] & UWSNs & — & Single & 3-D & No \\
[81] & — & KF & Single & 3-D & Yes \\
[82] & UWSNs & — & Single & 3-D & Yes \\
[83] & UWSNs & — & Single & 3-D & Yes \\
[84] & UWSNs & KF & Single & 3-D & Yes \\
[87] & — & EKF & Single & 3-D & No \\
[88] & — & EKF & Single & 2-D & Yes \\
[94] & Acoustic imaging sensor & PF & Single & 3-D & Yes \\  & & PF, Quantization & Single & 3-D & Yes \\ \hline \hline \end{tabular}
\end{table}
Table 7: Summary and comparison of underwater active acoustic target tracking algorithms.  
Different from the tables in Section 5, we summarize and compare all these algorithms based on some other aspect: the number of the target, the dimension of the target state, tracking maneuvering or non-maneuvering target. Due to the large amount of papers, we split the summary table into two, based on the tracking mode. As Tables 6 and 7 show, most tracking algorithms employ active mode, which seems contradictory to what was introduced in Section 2, but actually it is not so, and most researchers prefer using active mode because it is much easier to implement compared that adopting passive mode. However, it is more advocated for researchers to consider the applicability of the algorithm in the actual environment not just for simplicity.