## 2 Research Background
### Localization Protocols Proposed for UWSNs
Moreover, the localized nodes provide their estimated locations with its estimation errors. Errors will be accumulated by the nodes which use the localized node's coordination as anchor node's position [10]. In [21], Three Dimensional Multi-power Area Localization Scheme (3D-MALS) is proposed which extends 2-DALS to three dimensional underwater networks. The proposed algorithm combines the idea of variable transmission power levels of ALS [18] and adds the vertical mobility of anchor nodes from DNRL method [19]. The network comprises four types of node, which are surface buoys, Detachable Elevator Transceivers (DETs), anchor nodes and ordinary sensor nodes. In [19] surface buoys are equipped with GPS on the water surface and a DET is attached to a surface buoy [21] and act as a DNR beacon. They broadcast their coordinates at different power levels. Non-localized nodes record mobile anchor location information and their respective lowest power level value and send these to the sink node to estimate the location [10]. It is also a centralized method similar to ALS and also introduces additional overhead by sending the related localization information to the sink where they can already estimate self-location using the periodic anchor messages [10]. A Silent Positioning Scheme is proposed in UPS model by [PERSON] _et al._ in [22]. This method is distributed and works for stationary underwater acoustic sensor networks (UASNs). Silent positioning is a localization scheme for one-hop underwater networks [22]. Four anchors are employed which sequentially send out beacon signals. The underwater nodes do not send any localization messages, hence UPS is silent. The overhead and energy consumption of UPS is low. It is a range-based method which utilizes TDOA range measurement technique, so it does not require synchronization, but UPS is only able to localize the nodes where they reside inside the area enclosed by four anchor nodes [10,22]. Moreover, the anchor locations need to be fixed and their positions require to be known by the sensor nodes which is not possible or hard to gain in practice [10]  
USP is proposed in [15] for underwater localization in sparse 3D acoustic sensor networks. It is a projection-based localization method which is able to map the positions of the anchor nodes to the plane containing a to-be-localized sensor node to transform the problem of 3D underwater localization into a 2D positioning problem. The scheme is composed of two main phases: a predistribution phase and a distributed localization phase. The first phase repeats iteratively. In each iteration, each sensor performs a local broadcast of any new position information that it has. This causes high communication overhead and energy consumption [10,23]. AUV-Aided Localization (AAL) is proposed in [24] for a hybrid, three dimensional UASN. An Autonomous Underwater Vehicle (AUV) is utilized as a mobile beacon. Three different messages are used by the AAL method: wakeup, request and response messages. A wakeup message is broadcast by AUV to declare its presence in the communication range of the sensors. The wakeup message receiver sensor triggers a range measurement and sends a location request message. The response message will be sent by the AUV to answer the request message and it includes the AUV coordinates. The AUV receives GPS signals while floating. The number of necessary response messages differs with the localization algorithm used [24], such as triangulation or bounding-box [25]. This method is silent in terms of messaging properties [10], but it is obvious that they should send a request message to the mobile beacon (AUV), so sending activity is unavoidable in localization, even in silent methods. The accuracy of AAL will be improved by frequently location calibration, however this quickly exhausts the battery of the AUV. Another drawback is high localization delay due to the slow speed of AUVs [10]. Localization with directional beacon [26] is an anchor-free localization method which utilizes an AUV as mobile beacon like the AAL method.