## 2 Research Background
### Underwater Localization Survey  
In this section, we broadly categorize different types of localization issue based on various properties. Figure 1 illustrates the underwater localization survey. * _Underwater Sensor Architecture_: Underwater networks comprise various sensors such as: surface buoys, anchor or beacon nodes, ordinary sensor nodes and Autonomous Underwater Vehicles (AUV). They have different motion capabilities like stationary, mobile or hybrid. In stationary case, nodes have fixed locations while mobile nodes are drifted with water currents or their motion can be controlled by navigation devices. In hybrid architectures, both stationary and mobile nodes are combined [10]. * _Coverage Property_: Based on spatial coverage of underwater sensor networks, localization schemes are classified into two-dimensional or three-dimensional. In the two-dimensional, all of the sensor nodes are assumed to be at the same depth on the ocean surface or the ocean bottom, or they may be floating at a certain depth. In the three-dimensional, each sensor node may be floating at various depths [10]. * _Location Computational Mechanism_: Based on the calculation technique of each sensor node location, two groups are defined: distributed and centralized. Distributed localization techniques allow each sensor to estimate its location individually. The localized sensor nodes do not know  
Figure 1: Localization Survey. their locations, because the location of each sensor node will be calculated in a command center or sink. * _Distance Measurement Method_: The localization schemes based on distance estimation method can be broadly classified into two categories: range-based or range-free. Precise distance or angle measurements made to estimate the location in range-based techniques, while range-free schemes do not use range or bearing information [4]. Instead, connectivity and hop-count metrics are employed to extract the node position. * _Accuracy_: Regarding to the accuracy level of localization method, fine-grained and coarse-grained techniques are available. Fine-grained techniques are referred to that type of algorithms which infer the position of unknown nodes by manipulating ranging measurement techniques like TOA, TDOA, AOA or RSSI. These methods provide highly accurate location estimation. Instead, coarse-grain localization has lower accuracy while they do not employ any ranging metrics. This low accuracy may be sufficient for some applications like animal tracking. * _Message Exchange Property_: In some localization schemes, only anchors are allowed to send beacon messages and the underwater sensor nodes passively listen to these localization signals. They are called silent nodes. Active localization allows underwater nodes to also participate in the localization procedure and send messages for localization. Active methods have higher communication overhead and higher energy consumption. But, silent protocols generally employ more number of anchor nodes with long-range communication capabilities [10]. * _Localized-nodes Cooperation_: based on the cooperation of localized nodes, two groups of schemes are definable: single-stage _vs._ multi-stage. In multi-stage, the localized sensor nodes act as new anchors and they are called reference nodes. Ordinary nodes do not become new reference nodes to help localize other ordinary nodes in single-stage schemes [9]. One of the drawbacks of multi-stage methods is error accumulation. Reference nodes propagate their locations as beacon signals while it may be estimated by errors in distance estimation and so on. * _Anchor-node Deployment_: Localization techniques utilize anchors as location-aware nodes. These nodes and their deployments in the underwater networks have direct impact on localization process. Based on it, two different deployments are available: surface anchors or underwater anchors. On the one hand, to simplify the process of endowing these nodes with their positions, they are placed on the surface as GPS-enable buoys.