# Positioning Systems for Unmanned Underwater Vehicles: A Comprehensive Review
## 1 Introduction
### Positioning Systems of UUVs
#### 2.4.1 Primary Positioning Systems
They are, however, expensive and may require additional calibration and integration with other positioning technologies [79]. * Towed Array Systems consist of a series of sensors or receivers deployed behind a moving vessel. These systems can detect acoustic signals or magnetic fields to determine the location and movement of UUVs. Towed arrays are useful for continuous monitoring and tracking but require the presence of a surface vessel for deployment [80]. * Geophysical Positioning methods use geological and geophysical data to determine a UUV's position based on underwater features such as magnetic anomalies or seafloortopography. These methods are valuable for scientific research and resource exploration but can be affected by the complexity of the underwater environment [81]. * Seismic Navigation employs seismic waves generated by controlled sources to map the seafloor and determine the position of UUVs based on the reflections and refractions of these waves. This method provides high-resolution imaging and is useful for detailed seabed surveys but requires specialized equipment and processing [82]. * Pinger and Hydrophone Systems use acoustic signals emitted by pingers and received by hydrophones to determine the position of UUVs. The time difference of arrival (TDOA) of the acoustic signals can be used to calculate the UUV's location. These systems are effective for tracking and localization but can be influenced by underwater noise and signal attenuation [83]. * Synthetic Aperture Radar (SAR) is used primarily for surface-based imaging but can be adapted for underwater applications by capturing reflections from the water surface. SAR provides detailed imaging and is useful for detecting surface and near-surface objects [84]. However, its use underwater is limited and typically involves surface support [85; 86]. * Biomimetic Navigation Systems are inspired by the navigation strategies of marine animals, such as fish and dolphins. These systems use principles observed in nature to develop advanced navigation and positioning techniques. They offer innovative approaches to underwater navigation but are still emerging in practical applications [87]. * Sonar Imaging Systems use sonar technology to create images of underwater environments by analyzing sound waves reflected from objects and the seafloor. These systems are essential for detecting and mapping underwater structures and features. They can be affected by water conditions and require substantial data processing [88]. * Laser Scanning uses laser beams to capture detailed 3D information about underwater environments. This technology provides high-resolution scans and is effective for creating accurate models of underwater features. However, its effectiveness is limited by water clarity and the range of the laser [89; 90]. * Gravity Gradiometry measures variations in the Earth's gravitational field to determine the location and movement of UUVs. This technique is useful for detecting geological structures and anomalies. It requires sophisticated equipment and can be influenced by external factors affecting gravitational measurements [91]. Each of these positioning systems presents distinct advantages, making them particularly well suited for specific underwater applications. In many cases, Unmanned Underwater Vehicles (UUVs) will employ a combination of these technologies to enhance the accuracy, reliability, and precision of their navigation and positioning in a variety of challenging underwater environments. Operating in difficult terrains such as murky waters, icy regions, or areas dense with algae presents significant challenges for UUVs. In such environments, the vehicles often face obstacles like reduced visibility, increased battery consumption due to colder temperatures, and the risk of thrusters becoming clogged by seaweed or debris. These factors can severely impact the vehicle's performance, making it essential for UUVs to rely on advanced sensor fusion, redundancy, and robust propulsion systems to maintain efficiency and reliability during missions.