# Underwater Acoustic Target Tracking: A Review
## 7 Conclusions  
With the increasing interest in the exploration of marine resources, underwater target tracking technology has aroused wide attention. Due to the specificity and complexity of the marine environment, acoustic waves have become the most widely used medium for tracking underwater targets. Numerous research papers have reported their tracking results. In this paper, we survey many acoustic tracking algorithms based on a new taxonomy, namely, instrument-assisted methods, mode-based methods, and tracking optimization methods. The contributions of this paper include: (1) It is the first review article about underwater acoustic target tracking; (2) the relevant knowledge is clarified in detail, which makes it easier for the readers to understand the area of underwater target tracking; (3) comprehensive comparisons have been made to make the readers quickly find a suitable method to improve tracking performance; (4) to compensate for the existing shortcomings, review papers are only centered on target tracking based on WSNs.  
The future work in this area can focus on several aspects: (1) _Multiple target tracking_. Few of the algorithms previously mentioned in Tables 6 and 7 can be used to track multiple targets. However, multiple target tracking methods are important in real application. For example, according to the developing trend of modern naval warfare, ships and submarines in the future will be in the form of unit operations. Hence, objects like torpedoes will present in multiple forms. The multiple underwater target tracking technology is the future research area for the scientific and technological personnel. (2) _Enhancing measurement accuracy of passive tracking_. In ASW applications, the target of interest is usually far from the tracker; the tracker can only passively listen to the signal radiated from the target for concealment. Furthermore, the development of silent technologies makes it hard to receive signals from target for passive tracking. Even if the signal is detected successfully, if the passive tracker has a low accuracy when measuring bearing information about the target, the tracking performance will degenerate significantly. (3) _Data fusion_. The tracking system will tend to the multi-state system. Therefore, how to make an effective fusion of homogeneous or heterogeneous measurements information to achieve more robust and accurate tracking will be a research hot issue. (4) _Motion strategies for mobile sensors_. Underwater target tracking is usually realized in large scale areas. Sensors with mobility outperform static sensors. Considering the influence of topology, how to design effective coordinate motion strategies for sensor is a hot issue. On this basis, the adaptive motion strategies based on local information can be developed. (5) _Quantized methods_. As mentioned in Section 5.3, there are few underwater acoustic target tracking algorithms that consider the quantization issue. However, the limited bandwidth in underwater channel makes it impossible to transmit raw measurements.  
This work was supported in part by the National Natural Science Foundation of China and the Civil Aviation Administration of China under Grant U1733110, in part by the Overseas Academic Training Funds, University of Electronic Science and Technology of China under Grant 201506075013, and in part by the Program for Science and Technology Support in Sichuan Province under Grant 2014 GZ0100 and Grant 2016 GZ0088. [PERSON] conceived of and designed the research. [PERSON] performed the experiments and analyzed the result. [PERSON] wrote the paper. [PERSON] and [PERSON] revised this paper. The authors declare no conflict of interest.