## 3 Related Work
Another effort was made for a joint procedure of localization and time synchronization based on the least square method that consists of five phases that iteratively compensate for the stratification effect [14]. Phase I is the message exchange procedure between the anchors and sensor node. Phase II estimates the position and clock parameters, phase III estimates the stratification effect compensation, phase IV is the calculation of sound velocity and finally, phase V performs the iteration process. This approach is based on the assumption that rays travel in a straight line which radially decreases its efficiency, but the iterative procedure supports improving the accuracy of localization and time synchronization. In this joint effort, the stratification effect is calculated along with the sound effect with the help of an iterative method, but this paper did not calculatethe continuous deflection of sound wave angle it only measures the initial angle of deflection of sound waves. [PERSON] et al. (2015) has given a quite successful joint technique of localization and synchronization in UWSNs which is considered the stratification effect and to solve this problem maximum likelihood estimator has derived. Like STSL, this procedure also works in two-phases, during the first phase initial clock skew is ignored due to its small value. Then LS estimator is employed to get localized and synchronized by transforming the nonlinear equations into a linear one. With the help of another least square (LS) estimator, the coarse estimation is further refined in the second phase. However, this method is not economical and has poor scalability and like other joint techniques, this method did not consider the stratification effect. This method works on the concept of iteration that gradually improves efficiency. This method can't provide any guarantee for the achievement of accurate results. As defined earlier that the calculation of propagation delay is very essential for time synchronization, this joint effort called \"Energy-Efficient Time Synchronization and Localization Algorithm for Underwater Acoustic Sensor Networks\" TSLA ([PERSON] et al., 2016) ignores both the stratification effect and propagation delay although the provided technique is energy efficient due to the one-way packets exchange and node are in duty cycle which make this technique energy efficient. Low cost distributed networked localization and time synchronization were achieved by ([PERSON] et al., 2017) and are robust to noisy measurement. Implemented in a testbed based on Teledyne Bentnos Telesonar SM-975 underwater modems, these solutions possess no additional overhead. Later, this technique was tested extensively in Lake LaSalle at the University at Buffalo. This method consists of four phases, with localization first, and the output of this process is the input of synchronization for each round of message exchange. This technique fails to give accuracy due to the consideration that sound wave travels in a straight line. Also, this technique only works for the low energy budget. A method called \"Ameliorated Joint Synchronization and Localization for 3D Underwater Sensor Networks\" (AJSL) ([PERSON] et al., 2017) uses iterative weight least square estimator for 3D underwater sensor networks to ameliorated joint localization and synchronization. This method uses the benefit of projection procedure which inherent relationship between localization and synchronization with known depth. As far as the drawback of this technique is concerned this process is less energy efficient which uses an iterative least square method that radially increases the computational complexity. Also, this technique does not compensate for the stratification effect. The work proposed in ([PERSON] et al., 2017) is a unified framework to execute synchronization and localization simultaneously by considering the stratification effect which is the most essential factor to get accuracy in time the process of synchronization and localization.