# Positioning Systems for Unmanned Underwater Vehicles: A Comprehensive Review
## 6 Discussion  
In Section 3.4, we delve for the first time into personal commentary on the significance of specific aspects of Unmanned Underwater Vehicles. Out of the 109 papers we reviewed, we selected 27 for detailed analysis and categorization in our tables. The primary reason for this selection was the depth and relevance of these papers, despite some lacking specific information. Of the 27 papers chosen, 15 were conference papers, two were review papers, nine were journal articles, and one was a book. Additional review papers, such as [98], contributed to our understanding of the positioning systems used in terrain-aided navigation. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Reference**} & \multirow{2}{*}{**Accuracy**} & \multicolumn{2}{c}{**Reasons for Degradation**} & \multirow{2}{*}{**Error**} \\  & & & **of Accuracy** & \\ \hline
[15] & 0.1–5\% & Complex Terrain & \textless{}0.1 m \\ \hline
[103] & - & Low Depth and Distance & 0.25 m \\ \hline
[91] & - & - & - \\ \hline
[39] & - & - & - \\ \hline
[52] & 0.2\% & - & - \\ \hline
[104] & - & Signal-to-Noise Ratio Adjustment & - \\ \hline
[32] & - & - & - \\ \hline
[53] & 0.8-3.3\% & -Improper Calibration of Compass & 1.2 m and 8.3 m \\  & & -Nearby Magnetic Field & (2 test runs) \\ \hline
[107] & -3 mm & Increase in Range & 0.1 m at 10 m range \\ \hline
[29] & - & - & - \\ \hline
[110] & 0.01\({}^{\circ}\) & Extreme Depth & - \\ \hline
[93] & - & - & - \\ \hline
[105] &
\begin{tabular}{c} No Specific Number \\ is Provided \\ \end{tabular} & Signal-to-Noise Ratio Increase & - \\ \hline
[109] & 0.003 m–0.0089 m & - & - \\ \hline
[38] & - &
\begin{tabular}{c} Compass Calibration \\ Depth Sensor \\ \end{tabular} & - \\ \hline
[73] &
\begin{tabular}{c} No Specific Number \\ is Provided \\ \end{tabular} & - & - \\ \hline
[21] & - & The Drift Rate of the INS & - \\ \hline
[69] & - & - & - \\ \hline
[62] & - & - & - \\ \hline
[16] & - & - & - \\ \hline
[79] &
\begin{tabular}{c} Provides Table \\ of Accuracies \\ \end{tabular} & - & - \\ \hline
[27] & - & - & - \\ \hline
[86] & \(+/-15\%\) & Sea State & - \\ \hline
[59] & - & Component Restrictions & - \\ \hline
[49] &
\begin{tabular}{c} No Specific Number \\ is Provided \\ \end{tabular} & - & - \\ \hline
[20] & - & - & - \\ \hline
[87] & - & Air Pressure Change & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Accuracy of positioning systems. The book [8] offers significant insights into underwater technologies and their operation, highlighting the importance of replacing human offshore tasks with Unmanned Underwater Vehicles (UUVs) to reduce risk. A key focus is on the challenges we must address, with the book providing detailed discussions and insights on these specific challenges. While there are numerous papers on underwater environments and exploration, only a few provide up-to-date information on modern positioning systems. One of the main challenges we faced in writing our paper was the outdated information on positioning systems. For instance, the cost of electronics has decreased since 2014, the year the book was published. However, paper [110], despite being from 2008, provides valuable details about the construction and design of these systems. The methodology of this paper was informed by the two review papers [16; 49] that we utilized in Tables 2 and 3. As shown in Table 1, we categorized the papers based primarily on the key positioning systems discussed in Section 2.4.1. After categorizing them, we extracted the advantages and disadvantages presented in each paper. Some papers contributed additional insights by incorporating findings from their own research to further elaborate on the challenges, which were listed under the disadvantages column. One of the major drawbacks identified was the impact of water conditions and calibration issues. The Inertial Navigation System (INS) was the most frequently mentioned positioning system due to its widespread use and potential for improvement. The INS is capable of functioning in GPS-denied environments, such as underwater.