# Positioning Systems for Unmanned Underwater Vehicles: A Comprehensive Review
## 1 Introduction
### Positioning Systems of UUVs
#### 2.4.1 Primary Positioning Systems  
1. Inertial Navigation Systems (INSs)  
Inertial Navigation Systems use accelerometers and gyroscopes to measure the vehicle's movement and orientation. By tracking changes in velocity and direction, INSs can calculate the vehicle's position relative to a known starting point. INSs are valuable for underwater navigation because they do not rely on external signals, making them effective in environments where GPS signals are unavailable. However, INSs can suffer from drift over time, requiring periodic updates or corrections [44; 45; 46]. 2. Long Baseline (LBL) Systems  
Long Baseline systems use a network of underwater transponders placed on the seabed. The UUV communicates with these transponders to determine its position based on the time it takes for acoustic signals to travel between the UUV and the transponders. LBLsystems provide high positional accuracy and are often used in environments where precise positioning is critical, such as deep-sea exploration and underwater construction [47; 48; 49]. Short Baseline (SBL) Systems  
Short Baseline systems are like LBL systems but use a smaller array of transponders located closer to the UUV. These systems are generally used for tasks that require less precision compared to LBL systems but are advantageous in shallower waters or areas with limited space for deploying long baseline transponders [50]. Ultra-Short Baseline (USBL) Systems  
Ultra-Short Baseline systems use a single, highly sensitive transducer mounted on the surface vessel to communicate with a transponder on the UUV. The USBL system calculates the UUV's position based on the angle and distance of the acoustic signals received. USBL systems are often used for real-time tracking and positioning of UUVs during operations and are useful for tasks requiring continuous monitoring and updates [48; 51]. Doppler Velocity Log (DVL)  
A Doppler Velocity Log measures the velocity of the UUV relative to the seafloor or water column using sonar technology. By tracking the Doppler shift in the sonar signals reflected off the seabed or particles in the water, the DVL provides information on the UUV's speed and direction. These data can be integrated with other positioning systems to improve overall navigation accuracy [52; 53]. Dead Reckoning (DR)  
Dead reckoning involves calculating the UUV's position based on its previous position, speed, and heading. This method can be used in conjunction with INSs and other systems to estimate the current position, especially when external positioning data are temporarily unavailable. It is often used to complement other systems and to provide additional navigation data [54; 55; 56]. Acoustic Modem Systems (AMSs)  
Acoustic modems can communicate between the UUV and surface vessels or other underwater stations using sound waves [57]. While primarily used for data transmission, these modems (some low-cost) can also assist in positioning by providing additional data points and allowing for communication with an external positioning system [58; 59]. More Positioning Systems and Navigation Aided Positioning Systems  
Optical Methods use cameras and laser systems to capture visual data for navigation and positioning. These methods rely on the visibility of landmarks, patterns, or features in the underwater environment. Optical methods are highly effective for tasks requiring high-resolution imagery and are often used in conjunction with other systems to provide visual confirmation and detailed mapping. However, they are limited by water clarity and lighting conditions and are typically less effective in murky or deep waters [60]. Magnetic Positioning systems use the Earth's magnetic field or local magnetic anomalies to determine a UUV's position. By measuring variations in the magnetic field, these systems can provide positional data even in GPS-denied environments [61]. Magnetic positioning is useful for underwater navigation in areas where other positioning systems may struggle.