# Underwater Acoustic Target Tracking: A Review
## 6 Discussion, Challenges, Countermeasures, and Lessons Learned
### Countermeasures  
_Improve low target detection probability:_ In most tracking systems, the target is always at a large distance relative to the detecting sensors. With the development of acoustic concealment technology, detecting and tracking targets with low SNR has become an urgent problem. Compared with detect-before-track (DBT), TBD is the feasible technology to solve the problem [96]. Aimed at solving the problem of real-time detecting and tracking multi-target, a non-motorized multi-weak target detection algorithm based on PF-TBD is proposed in [104], which transfers TBD from batch processing to recursive processing. Moreover, the histogram probably multiple hypothesis trackers (H-PMHT) which develops based on traditional TBD has higher real-time [105]. In addition, finite set statistics (FSS) is introduced in [106] to realize number-varying multi-target TBD. _Sound speed model:_ The undefined parameters in water environments such as depth, temperature, and salinity variations have an obvious impact on the sound speed change. The relationship between sound speed variation and these factors is complicated. The authors in [87] adopt one of the relationships described in [107] to construct a sound speed model. The simulation result demonstrates that the tracking accuracy of this algorithm is significantly improved compared with that using constant speed sound. However, this model has limitations of universality. A new calculation of sound speed suitable for all ocean environments is proposed in [108], which is a function of temperature, depth, salinity, and latitude. The authors in [109] have demonstrated the efficiency of the calculation in underwater localization. Hence, the problem of sound speed variation in underwater target tracking can be addressed using the proposed equation in [108]. _Eliminate PS ambiguity_: There are some papers focused on addressing this problem using a multiline array, e.g., twin array and triplet array [110; 111]. Nevertheless, the multiline array requires a larger number of hydrophones, which is not acceptable in ASW applications, especially in the scenario using AUVs. The single linear array is preferred in underwater applications for the simplicity of deployment. Firstly, the ambiguity can be eliminated using a single platform by turning the heading angle. For example, if the angle of the target relative to the array becomes larger when the array turns some degrees to the right, the target can be determined on the port side of the array, and vice versa. This approach has the requirement of the target moving at a low speed and in a long-range relative to the tracking platform. Another approach adopts multiple platforms towing line array to remove the ambiguity using the spatial diversity in obtained data. True contacts always locate around the target, while ghost contacts are at the specific position with respect to the heading of the array. By using multiple arrays, the contacts locating at different positions in the region can be classified as ghost contacts. In addition, the authors in [38] propose an optimal disambiguation way by deriving the full Bayesian posterior PDF of the target without bringing any extra cost. In future, the active sensor array can be used to eliminate PS ambiguity by obtaining range information. _Keep a balance between energy consumption and tracking accuracy_: The authors in [65; 66] consider the energy consumption and tracking accuracy simultaneously. In most of tracking algorithms, the calculated position of the target has an inevitable error. Improving the tracking accuracy is more significant in practical applications and practical needs. However, improving the accuracy means fusing more sensor data, which brings large energy consumption. Therefore, we should construct a weighted equation between the energy consumption and the accuracy when designing the tracking algorithms. Through large simulation experiments, the approximate relationship between both factors can be derived.