# Underwater Acoustic Target Tracking: A Review
## 1 Introduction  
With the exhaustion of land resources, humans have started shifting their focus to the exploitation and utilization of marine resources for the sake of achieving sustainable development [1]. As a significant component of ocean exploration, underwater target tracking has been a hot area of research in recent years. Tracking is a sophisticated process of estimating the state (i.e., position, velocity, and acceleration) of single or multiple moving targets quickly and as close to the true state as possible by using the available measurements collected from various sensors. It is critical in the war environment for two principal reasons. One is to prevent ourselves from being attacked, and the second is to destroy the enemy. To some extent, the accuracy of acquired information can determine the success or failure of a war. In civil systems, the salvage and rescue of underwater targets and marine life tracking cannot be realized without the techniques of underwater target tracking. Therefore, it is necessary to provide an overview of the state-of-the-art algorithms in underwater target tracking. A considerable amount of literature has investigated well the problem of target tracking in the terrestrial environment. Among them, the systems based on varieties of sensors can be adopted to detect and track the interested target [2]. In [3], the acoustic sensors are utilized to detect and track targets by judging whether the strength of the received acoustic signal exceeds a predefined threshold. Since vibrations can be used to distinguish targets with different speeds and weights, the system making use of the properties of seismic and passive infrared sensors for detection and classification of humans, animals, and vehicles is proposed in [4]. Magnetometers can be used to detect metallic targets and achieve good accuracy [5]. A target tracking system integrating radio-frequency identification(RFID) with wireless sensor networks (WSNs) is proposed in [6]. Each target must carry an active RFID as the identity classification, which limits its applications in most cases of target tracking. Similarly, the authors in [7] propose a person-tracking algorithm based on luminosity sensors requiring the target must be equipped with a light source, which is also impractical. Different from aforementioned sensors, the authors in [8; 9] use sensors providing video images to detect and track targets. Due to the fact they are absorbed and attenuated significantly in the ocean, many detecting media such as electromagnetic waves and laser waves which can be used to implement terrestrial target tracking methods have poor performance in marine environments [10]. As the only carrier that can propagate over a remote distance in the marine environment, acoustic waves have become the most widely used medium in underwater target tracking. Therefore, the acoustic tracking instrument using acoustic waves for underwater target detection and tracking was born. In this paper, we present a classification criterion for underwater acoustic target tracking algorithms based on the methods used, namely, instrument-assisted methods, mode-based methods, and tracking optimization methods. To allow readers to have a good understanding of this topic, we provide an overview of target tracking theory and introduce three frequently-used acoustic tracking instruments in detail. This paper concentrates on a comprehensive survey of underwater target tracking algorithms and is organized as follows: Section 2 presents some related surveys and our contributions. In Section 3, we introduce three acoustic tracking instruments, including traditional acoustic sensor arrays (TASAs), acoustic imaging sensors, and underwater wireless sensor networks (UWSNs). The process of underwater target tracking and these main components are clarified in detail in Section 4. Section 5 presents the classification of underwater acoustic target tracking algorithms. Moreover, the discussion, challenges, countermeasures, and lessons learned are presented in Section 6.