## 2 Related Work  
In our previous work [3], the current existing underwater localization algorithms are mainly classified into two categories: (1) stationary localization algorithms and (2) mobile localization algorithms. In this section, we first summarize several typical underwater localization algorithms, and then analyze existing research problems of them. In stationary localization algorithms, all the sensor nodes are assumed to be static without influence from dynamic underwater environment. For example, in [6], an efficient Area Localization Scheme (ALS) is proposed for UWSNs, where anchor nodes send localization beacons with different energy levels to unknown nodes. The unknown nodes which receive beacons firstly record ID numbers of anchor nodes and the corresponding energy levels, then send the recorded information to sink node. After receiving the information from unknown nodes, the sink node tries to localize unknown nodes according to the corresponding energy level and the locations of anchor nodes. In [7; 8], an Underwater Positioning Scheme (UPS) is proposed, which localizes unknown nodes by using 4 non-coplanar anchor nodes in three dimensional UWSNs. UPS does not require time synchronization because it uses TDoA (Time Difference of Arrival) technique to measure distances between sensor nodes. However, the localization delay of UPS is quite long, and the localization coverage is limited by the communication range of sensor nodes. In [9; 10], an Underwater Sensor Positioning (USP) scheme is proposed in for sparse three-dimensional UWSNs. In USP, all the sensor nodes are equipped with pressure sensors, thus they can obtain their own depth positions. Then, based on the projection technique, the three dimensional localization problem can be transformed into its two dimensional counterpart. In order to solve the problem of low localization coverage in large-scare UWSNs, the researchers also put forward many solutions, for example, in [11; 12], the strategy of unknown node upgrade is proposed in a Large-Scale Hierarchical Localization (LSHL) approach. That is, the successfully localized unknown nodes can be upgraded as anchor nodes to help neighbor unknown nodes with localization. This strategy can well solve the limited localization coverage problem; however, the main drawback of LSHL is that the location error is accumulated. In real applications, an absolutely stationary network does not exist. The underwater sensor nodes always freely float with ocean current. Therefore, in many research works, ocean current and sensor mobility are taken into account in localization algorithms. For example, in [13], a Collaborative Localization Scheme (CLS) is proposed for mobile UWSNs, where unknown nodes collaborate with each other to determine their positions autonomously without using any anchor nodes. In addition, in mobile UWSNs, mobile anchor node, e.g., Dive and Rise (DNR) anchor nodes [14; 15], Autonomous Underwater Vehicles (AUVs) [16; 17], mobile detachable elevator transceiver (DET) [18], are adopted to improve localization performance. DNR anchor nodes and AUV equipments are equipped with GPS. They can first obtain their positions on the ocean surface, then sink into water and broadcast their positions to localize unknown nodes. In order to avoid position change from ocean flow, the anchor nodes periodically rise to the surface to update their position information. Mobile anchor nodes can move not only in the vertical direction, but also in the horizontal plane. For example, in [19], a Range-free scheme based on Mobile Beacons (RSMB) is proposed for UWSNs, where a mobile anchor node moves on the sea surface at a constant speed following the random way-point (RWP) model and broadcasts localization beacons for unknown nodes. The unknown nodes localize themselves based on the projection technique.