## 2 Background  
### UIUVs and Swarms  
UUVs are decision-making mechatronic systems, which could expand people's underwater activities in the marine engineering construction, underwater search-and-rescue, or ocean ranch operations [5]. They are powerful assets that can operate in underwater missions and operations autonomously and are capable of situation and environment awareness, making their own autonomous decisions, and planning their own trajectories. Moreover, they interoperate with other underwater, surface, and air vehicles or platforms. A UUV can be considered as a knowledge-based autonomous agent [6]. Furthermore, it has the ability of self-management, i.e., self-configuration, self-healing, self-optimization, and self-protection. An interesting, related work reporting on enhancing self-diagnosis and self-adaptation of UUVs, through a \"metacontrol\" framework with the implementation of ontological reasoning, has been recently published [6]. In the work of UX-1 in the UNEXMIN project (a robot developed to survey flooded old mine sites) [7], the authors were motivated by the lack of real-time communication, proposing a framework to enhance its ability for self-diagnosis and self-adaptation. To interconnect these valuable assets and achieve an efficient exchange of information, localization is mandatory. This capability allows a vehicle to determine its position and orientation in the world, both underwater and at the surface.  
Localization ability of UUVs and underwater assets, is more than meaningful in creating vigorous network architectures, due to the facilitation of information sharing. However, this need generates new challenges to overcome. As we have seen in related work [8], there are different localization algorithms that can be applied to a UWSN and are divided in two broad categories, i.e., range-based and range-free schemes. The first category is based on accurate distance and angle measurements, by using different variables such as Time of Arrival (ToA), Angle of Arrival (AoA), etc. While this method is very precise in estimating an asset's position, it relies on strict time synchronization for the exact information transmission and receiving instances. The second one does not use range or bearing information, relying on simplicity, but enhancing the localization error of nodes; this schema is useful for terrestrial sensor networks.  
The technological progress of UUVs is required, towards an efficient operation in the marine environment. They are used to mitigate the risk to human lives and decrease the cost of operations. The need for smart and durable UUVs increases as the maritime operations become more complex and riskier, for instance, supporting the installation of oil and gas facilities or settling underwater cables in depths that were previously unexplored. In addition, in the context of military operations in an IoUT environment, a team of UUVs, each having an individual role and being interconnected, is participating in search-and-rescue operations (SAR) or in mine deactivation operations in Anti-Submarine Warfare [9; 10]. Moreover, the interoperability of the UUVs is an issue of consideration when planning an operation, especially when they operate in a swarm formation. A swarm system of independent assets is a group of self-organizing autonomous agents aiming at the effective accomplishment of various collaborative tasks [11].