## 1 Introduction  
During the last decades, there has been a rapidly growing interest in monitoring aqueous environments for scientific exploration, ecosystem monitoring and early warning systems for natural disasters like tsunamis. The ideal means for this type of extensive monitoring is a distributed underwater system with networked wireless sensors, referred to as Underwater Wireless Sensor Network (UWSN) [1]. In some of these applications such as oil drilling, target or animal tracking and disaster prevention the sensed data should be tagged with location information because applications associate the sampled values with when and where they were collected [2]. Localization in terrestrial wireless sensor networks (TWSN) is mature enough, while it is still challenging for UWSN due to some major technical differences. Acoustic communications are the typical physical layer technology in underwater networks. In fact, radio waves propagate at long distances through conductive salty water only at extra-low frequencies (30-300 Hz), which requires large antennae and high transmission power. Optical waves cannot be used because even though they do not suffer from such high attenuation, they are seriously affected by scattering. Furthermore, transmitting optical signals requires high precision in pointing the narrow laser beam. Thus, links in underwater networks are typically based on acoustic wireless communication [3]. Using acoustic communication among UWSN compared to radio links in TWSN presents different challenges and constraints in underwater localization. It is challenging as Radio Frequency (RF) waves are heavily attenuated under water, so employing technology like GPS is not feasible [4]. The acoustic channels are characterized by severely limited bandwidth, high propagation delays and high bit error rates. Hence, it is desired that localization protocols work with minimum possible message exchange. Underwater acoustic communication links can be classified according to their range as _short_, _medium_, _long_ and _very long_. Data rates of acoustic links and typical bandwidths of the underwater acoustic channel for different ranges are given in Table 1[5, 6]. As shown in Table 1, bandwidth and data rates of acoustic underwater communications is very low, but increase for shorter distances. This implies, in a range of applications with using long range acoustic modems, localization protocols with the least possible messages and low data bits should be established. Otherwise, their performance is drastically affected by protocol overhead [2]. This is also constrained by limited power of the underwater sensors and impossibility of changing or recharging the battery [7]. Meanwhile, underwater sensor networks if not anchored, are mobile networks and node locations change continuously. So, the mobility of free-floating nodes brings up another challenge in localization. In such a dynamic aquatic environment, localization process should be run periodically to update the location results, as will dramatically increase the communication overhead and energy consumption [7, 8]. Moreover, in some applications like forewarning disaster and coastline prevention, the localization response time should be fast so that it reports the actual location when data is sensed [9]. In this paper, we present the design and the development of a new message exchange mechanism which can provide an energy efficient localization scheme with the least number of localization applicant messages. The proposed scheme supports the mobility feature of water currents and \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Span** & **Range (Km)** & **Data Rate** & **Bandwidth (KHz)** \\ \hline
**Short range** & \textless{}1 & \textasci{}20 kbps & 20–50 \\
**Medium range** & 1–10 & \textasci{}10 kbps & \textasci{}10 \\
**Long range** & 10–100 & \textasci{}1 kbps & 2–5 \\
**Basin scale** & 3000 & \textasci{}10 bps & \textless{}1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Data rates and typical bandwidth for underwater channel with various ranges.