# Positioning Systems for Unmanned Underwater Vehicles: A Comprehensive Review
## 1 Introduction
### Remotely Operated Vehicles  
Remotely Operated Underwater Vehicles (ROUVs) are underwater robotic systems controlled by human operators from a surface vessel or platform [11]. These vehicles are typically tethered to the surface via a cable that provides power and enables communication between the ROV and its operator. This connection allows real-time control and monitoring, making ROVs ideal for conducting complex underwater tasks in environments that are too deep or dangerous for human divers [26]. ROVs are equipped with various tools and sensors, including high-definition cameras, sonar systems, and robotic arms, which enable them to perform a wide range of operations. They are commonly used in industries such as oil and gas, marine research, and SAR (Search and Rescue) operations. In these contexts, ROVs are deployed for tasks like underwater inspection, maintenance, exploration, and environmental monitoring. Their ability to deliver live video feeds and data to operators on the surface is critical for real-time decision-making and precise manipulation in underwater environments [26]. One of the key features of ROVs is their versatility [27]. For example, in the offshore oil and gas industry, ROVs are used to inspect and repair pipelines, oil rigs, and other underwater infrastructure, helping to ensure the safety and integrity of critical systems [28; 29]. In scientific research, ROVs allow for the exploration of shipwcks, deep-sea ecosystems, and underwater geological formations. Their robotic arms can collect samples or perform fine manipulations that are impossible for divers to carry out at extreme depths [30; 31]. Unlike Autonomous Underwater Vehicles (AUVs), which operate independently, ROVs require continuous human control, making them particularly useful for missions that demand real-time responsiveness or involve intricate operations. This distinction makes ROVs indispensable for tasks like underwater construction, recovery missions, and even SAR operations, such as diver assistance during rescue operations [19]. The tether that connects ROVs to the surface provides them with a consistent power supply, enabling long operational durations without the need for onboard batteries [32]. This capability allows ROVs to carry out extended missions, often lasting several hours or even days, depending on the specific task [31]. Their ability to perform a wide range of functions in challenging environments has made them a critical tool in various underwater industries and research fields. Remotely Operated Vehicles (ROVs) can be categorized based on their size, capabilities, and the types of missions they are designed to undertake [11]. Observation-class ROVs are the smallest and simplest type, primarily used for visual inspections and light-duty tasks [33]. These ROVs are equipped with cameras and basic sensors to provide real-time video and data from underwater environments. They are ideal for applications that involve monitoring and documenting, such as inspecting underwater structures, observing marine life, or conducting environmental surveys. Their relatively straightforward design makesthem easy to deploy and operate, but they are generally not equipped for heavy-duty tasks or significant manipulations [33; 34]. In contrast, work-class ROVs are larger, more robust vehicles designed to handle more complex and demanding tasks [35]. These ROVs come equipped with advanced robotic arms, specialized tools, and high-resolution sensors, allowing them to perform intricate operations such as underwater construction, maintenance, and repairs. They are commonly used in the oil and gas industry for inspecting and repairing pipelines, performing underwater welding, and maintaining offshore platforms. Work-class ROVs are capable of operating at great depths and can carry a substantial payload, making them suitable for heavy-duty applications and extended missions [36; 37; 35].