# Underwater Acoustic Target Tracking: A Review
## 2 Related Surveys and Contributions  
In this section, related surveys about target tracking are introduced. In view of the fact that most survey papers are restricted to tracking based on WSNs, the difference between underwater acoustic target tracking and target tracking using WSNs is clarified. Moreover, compared to other surveys, the original contributions of this paper are illustrated. To our knowledge, there are only a few review articles about target tracking, and they are all focused on target tracking based on WSNs. In [11], the authors are focused on introducing the related knowledge of target tracking for sensor networks, rather than analyzing and comparing different target tracking algorithms. Some other review articles are only centered on one aspect of the tracking algorithms based on WSNs such as energy-efficiency [12], prediction algorithms [13], target recovery techniques [14], and security [15]. As the extension of traditional target tracking in the application of the underwater environment, underwater acoustic target tracking using UWSNs include all the aspect mentioned before. However, due to the special nature of the underwater environment, there exist numerous differences between underwater acoustic target tracking based on UWSNs and target tracking using WSNs. * _Communication issues_: The underwater channel is a complex channel that is variable over time and space due to the characteristics of the transmission media and the environment [16]. The common communication issues existing in underwater acoustic target tracking are large propagation delay, limited bandwidth and high bit error rate (BER). The speed of acoustic wave in underwater is 1500 m/s, which is five orders of magnitude lower than radio wave transmission. Moreover, the speed is variable with respect to the water temperature, sanity and depth [17]. Therefore, tracking based on distance acquired from time must consider the variation of acoustic speed. The limited bandwidth in underwater acoustic target tracking has motivated researchers to design effective tracking algorithms using quantized measurements. The underwater channel may experience BERs on the order of \(10^{-2}\), resulting in frequent transmission loss. Therefore, robust data fusion techniques are needed in light of the packet loss phenomenon. * _Real-time_: In underwater tracking, the sensors are usually deployed in 3D-space and drift with the current. The tracking is highly dependent on the accuracy of the sensor positions, which poses more stringent real-time requirements for underwater acoustic tracking algorithms compared with those based on WSNs. The limited bandwidth results in low communication rates which also pose a great challenge for real-time underwater acoustic target tracking. * _Tracking mode_: Unlike other modems, acoustic modems consume order of tens of Watts of energy during the transmitting process, and tens of milliwatts during receiving processes. The apparent asymmetry in power consumption makes it preferable for underwater acoustic tracking sensors to work in passive mode for energy-efficiency. Furthermore, the acoustic target tracking not only can be realized by WSNs but also can be dependent on TASA and acoustic imaging sensors which will be introduced in Section 3. However, these surveys about target tracking based on WSNs can provide readers with a comprehensive understanding of the area of target tracking and contrast present tracking methods. Thus, we give a comparison in Table 1. We also compare our papers with other survey papers [11,18] in Table 2, which provides a relatively complete overview of target tracking. With the increased interest in marine resources, the underwater target tracking technology will only get more attention. We believe that a comprehensive survey and detailed introduction of the related knowledge of underwater target tracking algorithms can improve the development of the technology. Our contributions can be summarized as follows:  
* Our paper is the first review article centered on underwater target tracking.