## 3 Related Work  
Many researchers have made their efforts to address this problem, but all these previously developed techniques have different drawbacks along with their strengths. Here, we are discussing the already known algorithms related to this address problem. Various localization algorithms are discussed for the localization of UNSNs in [9]. In which the authors have divided the localization techniques into three categories such as, stationary localization algorithms, mobile localization algorithms, and hybrid algorithms. Although these algorithms have their individual strength, mostly they are unable to handle the stratification effect, which means that sound waves do not travel in a straight line underwater. Similarly, different time synchronization algorithms are given by [10] which are not so accurate due to the negligence of propagation delay underwater. As considered \"joint time synchronization and localization design for mobile underwater sensor networks\" (JSL) was the first effort which is made by the researcher for the joint procedure of localization and synchronization with the consideration of the stratification effect. This joint technique was given by [11] which only compensated the stratification effect in the underwater medium along with this assumption that sound waves travel in a straight line. This method jointly improves the accuracy by using the advanced tracking algorithm and to increase the accuracy in mobile node localization, interactive multiple models (IMM) are implemented. [PERSON]'s law has been used by the author to calculate the stratification effects, but this method gives low accuracy because of the assumption that sound wave travels in a straight line. JSL localized and synchronizes the nodes with compensation of stratification effect which works in four phases. Phase I is the partitioning of 3D underwater sensor networks in a short latency, phase II performs the localization, phase III gives the time synchronization, and finally, phase IV is the refinement procedure. Another method was proposed by [12] that applies atomic multilateration and iterative multilateration method to localize and synchronize the sensors node. This method initially makes the partitioned of 3D space, but this process gives poor synchronization since it does not consider the clock skew during synchronization and stratification effect. In the network architecture, in this method, there are four or more anchor nodes with a known location on the water surface that sends their time and location information to ordinary sensor nodes to perform the process of localization and time synchronization. After getting messages from five anchor nodes, sensor nodes can estimate their location and time information. Due to the ignorance of mobility and clock skewness, these techniques give low efficiency with the consideration of constant sound speed. A sequential time synchronization and localization (STSL) was given by [13] which showed better results. However, it (STSL) did not consider the stratification effect which gives low accuracy in localization, but it improves synchronization by taking variable sound speed in a dynamic underwater environment. In STSL each node is equipped with directional navigation systems for the accurate estimation of short-term motion which makes this procedure very costly to implement. Although this procedure gives better localization accuracy with the consideration of mobility of the sensor as well as anchor nodes. Another drawback of STSL is that the anchor nodes are not considered as time-synchronized during this process. The main point of this scheme is that it functions with only one anchor node. It also performs a self-evaluation of localization accuracy, by estimating the propagation speed and checking its validity, relying on known model boundaries for it. Although this method is use full for the estimation of nodes that possess short-term motion for the long-term mobile nodes this method is quite expensive.