# Positioning Systems for Unmanned Underwater Vehicles: A Comprehensive Review
## 1 Introduction
### Autonomous Underwater Vehicles  
Autonomous Underwater Vehicles (AUVs) are unmanned robotic systems designed to carry out underwater tasks without requiring human control in real time. These vehicles are pre-programmed with specific instructions and can navigate independently in ocean environments. They are often equipped with various sensors, sonar systems, and cameras to collect data for scientific, environmental, rescue, and industrial purposes [15].  
AUVs play a key role in oceanographic research by helping scientists map the seafloor, study marine life, and analyze water properties like temperature and salinity [8; 17]. They are also used in environmental monitoring to detect pollution and study the impacts of climate change on marine ecosystems. In rescue applications, AUVs are used for tasks such as locating underwater wreckage, mapping hazardous or hard-to-reach underwater areas, and providing data to support rescue missions, while in archaeology, they help explore shipwacks and submerged ancient structures [18; 19].  
In the oil and gas industry, AUVs are vital for surveying the ocean floor to locate underwater resources and inspect pipelines [20]. Unlike Remotely Operated Vehicles (ROVs), which require a direct connection and real-time control by humans, AUVs operate autonomously, relying on advanced navigation systems such as GPS, inertial navigation, and acoustic communication to complete their missions even in deep, remote parts of the ocean [5].  
Autonomous Underwater Vehicles (AUVs) come in several types, each designed for specific tasks based on size, operational range, and mission requirements. One common type is man-portable AUVs (such as GAVIA [21]), which are small and lightweight, making them easy to transport and deploy by one or two individuals. These AUVs are typically used for shallow-water operations and are favored in coastal research, environmental monitoring, and search and rescue missions [22]. Their compact size and quick deployment capabilities allow for short-term, localized tasks where portability is essential, and they can operate in areas that may be difficult for larger vehicles to access [23].  
Another type is the lightweight AUV, which is slightly larger and capable of deeper operations than man-portable models. These AUVs are used for moderate-depth missions,carrying more advanced sensors and payloads for tasks such as underwater mapping, pipeline inspection, and rescue tasks. Lightweight AUVs offer a balance between portability and functionality, allowing them to gather more detailed data over longer distances than their smaller counterparts while still being relatively easy to handle [24].  
The largest and most capable category is heavy-weight AUVs, which are designed for deep-sea and long-duration missions. These robust vehicles are used in demanding fields like deep-sea mining, oil and gas exploration, and oceanographic research. Heavy-weight AUVs can operate at extreme depths and carry a wide range of sophisticated sensors, enabling detailed seafloor mapping, sub-surface imaging, and resource exploration. Their enhanced endurance and operational range make them suitable for missions that require extensive data collection over large areas, such as surveying remote regions of the ocean floor or monitoring deep-water ecosystems [25].  
These different types of AUVs allow for a wide range of applications, each suited to specific operational needs, from shallow-water environmental studies to deep-ocean exploration and industrial tasks.