# Positioning Systems for Unmanned Underwater Vehicles: A Comprehensive Review
## 6 Discussion
While these references offer important insights into system design or potential limitations, their lack of focus on measurable accuracy hinders the ability to assess their contributions in a review focused on precision and reliability. There are also a few studies that, while they do not provide specific accuracy figures, offer useful general observations. For example, [49] mentions accuracy concerns but does not provide specific numbers, instead focusing on component restrictions. Paper [21] discusses the drift rate of the INS as a key factor influencing long-term accuracy but, like many other studies, does not provide measurable data (accuracy). These observations, while valuable, fall short of providing the comprehensive information necessary for in-depth system evaluations. Across the studies, several common factors emerge that degrade accuracy in underwater positioning systems. The most frequently cited issues include compass calibration problems (references [38, 53]), signal-to-noise ratio adjustments (references [104, 105]), and depth-related degradation (references [103, 110]). In addition, the range is often cited as a limiting factor for accuracy, with studies like [107, 109] demonstrating that accuracy worsens as distance increases. Other notable degradation factors include nearby magnetic fields and sea states, both of which can significantly skew positioning results. This analysis of accuracy reveals both strengths and weaknesses in the current body of literature regarding underwater navigation systems. Several studies, including [15, 53, 107, 109], stand out for their detailed reporting of accuracy, offering precise metrics that are critical for evaluating system performance. However, many other studies focus primarily on qualitative discussions of accuracy degradation factors without providing numerical data, limiting their utility for direct comparison. Additionally, some references, such as [32, 99, 91], fail to provide any data regarding accuracy, highlighting a significant gap in the literature. This paper has explored various aspects of positioning systems used in Unmanned Underwater Vehicles (UUVs) and assessed their advantages and limitations based on a broad review of the literature. From the analysis of 110 papers, we selected 27 for in-depth study, focusing on the key positioning systems outlined in Section 2.4.1. Our findings underscore the significant role of Inertial Navigation Systems (INSs), which were frequently mentioned due to their ability to function effectively in GPS-denied environments, such as underwater. Despite the comprehensive data presented, several challenges emerged. Notably, many sources lacked updated information on maintenance costs, which is a crucial factor given its impact on operational efficiency and budgeting. Only a few papers addressed maintenance costs explicitly, highlighting a gap in the current research. The high costs and time-consuming nature of recalibration were identified as major concerns, underscoring the need for further investigation into cost-effective solutions. Our review identified that while some papers provided valuable insights into specific technologies, such as the combination of INS with LiDAR for enhanced camera quality and mapping precision, others offered limited details on practical applications or costs. Papers such as [110], although not providing much insight regarding its positioning system accuracy, stood out for their comprehensive coverage of range, depth suitability, overall cost, and maintenance, providing a useful benchmark for evaluating other sources. The study also highlighted the growing trend of integrating multiple positioning systems to improve localization, navigation, and stabilization. Notably, combinations of INSs with Doppler Velocity Logs (DVLs) and other technologies were frequently proposed to address various operational challenges.