# Underwater Acoustic Target Tracking: A Review
## 5 Classification Underwater Acoustic Target Tracking Algorithms
### Classification Based on Instrument-Assisted Method
#### 5.1.2 Taas  
The sound pressure hydrophone itself has no directivity so it is unable to determine the position of the target on its own. TASA usually uses sound pressure hydrophones forming a linear array model to obtain the bearing information of the target through the method of array signal processing. However, due to the cylindrical symmetry, single line array receivers have the problem of port-starboard ambiguity for tracking, which means generating a ghost track of the target symmetrically displaced with respect to the heading of the array as Figure 6 shows. Although most algorithms adopt a heuristic approach to address the aforementioned problem [54], the authors in [55] eliminated the port-starboard ambiguity in an optimal way by introducing the ambiguity into the analytical observation model and then the full Bayesian posterior distribution of the target state is derived. Additionally, another novelty of this paper lies in that large amount of AUVs towing acoustic sensor array constitute a novel UWSNs to estimate the target kinematic state at each time scan. During the data fusion process, different from classic \"track-to-track\" formulation, the fusion and filtering are completed at the same time because the estimated tracks obtained from independent sensors are still dependent due to the common process noise. After obtaining the definite bearing information of the target, the unobservability of the target still exists for the absence of range information especially for the single tracking platform towing or hull mounted the sensor array in passive mode. Some papers focus on designing a proper maneuvering plan for the tracking platform. Nevertheless, in [56], the authors explore the possibilities of completing target tracking using bearings-only measurements without a complex maneuver. In this article, the hull-mounted array (HA) and towed array (TA) are used to obtain bearing measurements from the target as Figure 7 shows. Here \(X_{1}\) and \(Y_{1}\) are known by deploying the towed array at a specific place. The central computer adopts data fusion technologies to process the measurements and then the state and trajectory of the target can be estimated by the filtering algorithm. Moreover, when a sensor array is used to obtain the bearing information, the quality of tracking is highly dependent on the relative position between the interest target and the array. A target at the broadside brings the highest accurate bearing information, while at the end-fire will cause poor bearing resolutions. Therefore, the authors in [38] propose an adaptive control strategy for the vessel towing the array to keep the target at the broadside as much as possible by the acquired measurement information. The matched-field processing (MFP) method is also can be applied to the area of underwater tracking. MFP determines the position of a target based on matching the complex acoustic field observed by a sensor array with the replica of an acoustic field computed numerically according  
Figure 6: Ambiguity of sensor array. Figure 7: HA and TA tracking sketch. to the acoustic propagation model [57]. The most challenge problem in MFP is the environmental mismatch. In [58], a Bayesian approach is developed for tracking multiple acoustic sources using MFP. When the properties of the ocean environment are not well-known, the proposed algorithm shows great performance. Firstly, the problem of environmental mismatch is formulated in terms of PDF over the model parameters. Then, PDF can be marginalized over all parameters to obtain the probability ambiguity surfaces (PAS) which is the probability distributions over source range and depth. Tracking estimates can be extracted from the PAS by the Viterbi algorithm. In this section, different tracking algorithms based on TASA are reviewed. The problem of estimating the state of a target is assumed to be two-dimensional in [55], which is not practical in underwater target tracking.