# Underwater Acoustic Target Tracking: A Review
## 5 Classification Underwater Acoustic Target Tracking Algorithms
### Classification Based on Mode-Based Method
#### 5.2.2 Active Tracking  
The tracking algorithms based on active tracking are aimed at estimating the state of the target according to the bearings and range information provided by the echoes from the target, and the echoes are originated from the system itself. Therefore, systems of active tracking contain transmitters and receivers. However, systems containing active sources have poor governance in anti-submarine warfare (ASW) due to the concealment. The position of the target is also easily obtained because of the available range information provided by active measurements. Therefore, the tracking algorithm based on active measurements is not the attractive research center of the underwater tracking area, especially in their applications in ASW. There are few papers researching the active tracking. In [38], a multi-static system, where a sonar source (transmitter) located on a stationary buoy or ship emits a ping and the echoes reflected from the target are collected by multiple receivers (AUVs towing sensor array), is used to generate bearing-range contacts fed into the tracker to produce tracks. In [81], a bistatic system is used to obtain active measurements of the target. The transmitter sends the signal with the certain strength to the target, then the target reflects the signal to the receiver. The receiver estimates the relative range between it and the target according to the change of the strength. The mathematical model is expressed as follows:  
\[SE=SL-TL_{1}-TL_{2}+TS-L_{e}-Nrd_{n} \tag{12}\]  
In Equation (12), \(SE\) and \(SL\) denote the strength of the signal received by the receiver and that transmitted by the transmitter respectively. \(L_{e}\) represents the level of noise out of beam-former and \(Nrd_{n}\) is related to the ability of receivers to recognize noise. \(TL_{1}\) and \(TL_{2}\) represent the transmission loss from the transmitter to the target and that from the target to the receiver respectively. The range information is implicit in the transmission loss \(TL_{1}\) and \(TL_{2}\):  
\[TL_{1} =20\log(R_{tt}) \tag{13}\] \[TL_{2} =20\log(R_{tr}) \tag{14}\]  
Figure 9: The tracking model of [PERSON]. \(R_{tt}\) in Equation (13) is the relative range between the transmitter and the target, and \(R_{tr}\) in Equation (14) is that between the target and the receiver. The range information can be obtained according to the SE using the figure of merit (FOM). In other words, the point of SE curve crossing with the FOM indicates the estimated range of the target with respect to the receiver. However, the SE is acquired with some degree of noise, which results in the tracking error that cannot be ignored. To this end, the fuzzy c-means method (FCM) and KF are employed to classify the source of noise and compensate the tracking error respectively. In [82], the authors propose a 3-D underwater target tracking (3 DUT) algorithm based on UWSNs. By sending acoustic pulses actively, the distance from the target to the sensor node is estimated and calculated according to the time-delay measurements. Then the location of the target is determined at the sink node using trilateration. To achieve tracking, the process of calculating the location of the target should be performed continuously. In this paper, each sensor node in UWSNs is equipped with the capabilities of actively sending pulses and passively listening to the echoes from the target. Among activated sensors which have the probability of detecting the targets, only one sensor named as a projector node is selected to transmit pulses periodically during the tracking process. Other activated sensors only receiving echoes are called hydrophone nodes. Different from that the active source in [38; 81] is stationary, the projector node in this paper serving as the active source can be changed with the trajectory of the target. Like [42], the authors in [83] also use trilateration to calculate the position of the target.