# Detecting and Mitigating Attacks on GPS Devices
## 6 Countermeasures and Their Comparison
### Countermeasures for GPS-Denied Environment--Alternate Positioning
#### 6.3.1 IMU-Based Approaches
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Positioning Method** & **Hardware Requirement** & **Advantages** & **Disadvantages** & **Level of Research Activity** \\ \hline \multirow{4}{*}{Visual Landmark} & \multirow{4}{*}{Camera} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{High accuracy has been demonstrated.} & \multirow{4}{*}{\(\bullet\)} & Some approaches require pre-placed physical objects & \multirow{4}{*}{\(\bullet\)} & Moderate—Research often coupled with IMU-based methods. \\  & & & & & & \\  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \multirow{4}{*}{Star Tracker} & \multirow{4}{*}{Star tracker} & \multirow{4}{*}{\(\bullet\)} & \multirow{4}{*}{Good performance in certain conditions.} & \multirow{4}{*}{\(\bullet\)} & Limited utility in daytime or when & \\  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \multirow{4}{*}{Alternate GNSS} & \multirow{4}{*}{GNSS receiver} & \multirow{4}{*}{\(\bullet\)} & Low-cost, small hardware, often the same hardware as the GPS receiver.} & \multirow{4}{*}{\(\bullet\)} & Vulnerable to spoofing and DoS attacks.} & \multirow{4}{*}{\(\bullet\)} & Low—Limited research on multi-GNSS performance.} \\  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \multirow{4}{*}{Mega LEO Constellation} & \multirow{4}{*}{RF receiver} & \multirow{4}{*}{\(\bullet\)} & High accuracy has been demonstrated & \multirow{4}{*}{\(\bullet\)} & Vulnerable to spoofing and DoS attacks.} & \multirow{4}{*}{\(\bullet\)} & Moderate—Growing research area focused on using commercial LEO constellations for navigation.} \\  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \multirow{4}{*}{LIDAR SLAM} & \multirow{4}{*}{LIDAR transceiver} & \multirow{4}{*}{\(\bullet\)} & Proven performance, has been in use for many years.} & \multirow{4}{*}{\(\bullet\)} & Expensive hardware. & \multirow{4}{*}{\(\bullet\)} & Moderate—Mature research area. Perceived \\  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \multirow{4}{*}{Visual SLAM} & \multirow{4}{*}{Camera} & \multirow{4}{*}{\(\bullet\)} & Proven performance, has been in use for many years.} & \multirow{4}{*}{\(\bullet\)} & Computationally expensive. & \multirow{4}{*}{\(\bullet\)} & High–Significant research in the space of visual-SLAM. Focus on multi-sensor fusion and IMU-hybrid methods. \\  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \cline{1-1}  & & & & & & \\ \hline \multirow{4}{*}{Generalized Vision} & \multirow{4}{*}{Camera} & \multirow{4}{*}{\(\bullet\)} & Good performance possible. Minimal additional hardware requirements.} & \multirow{4}{*}{\(\bullet\)} & Moderate—Less active research area compared with Visual-SLAM. \\ \cline{1-1}  & & & & & & Much research in the context of hybrid IMU approaches. \\ \hline \hline \end{tabular}
\end{table} 
Table 14: _Cont._extremely low errors these quantum devices produce, meaning that error accumulation would be insignificant; however, quantum accelerometers do not yet exist in a practical form for most applications. Quantum accelerometer technology faces many challenges, making the technology impractical for most platforms. Consequently, researchers are actively searching for methods to improve IMU performance. The authors of [112] have established that the key technical challenges facing quantum accelerometers are (1) a lower sample rate due to cold atom interrogation time and (2) a reduced dynamic range due to signal phase wrapping. Numerous papers in the literature focus on improving IMU performance for positioning and navigation. Two key trends in the literature propose IMU-based approaches: (1) IMU redundancy and (2) fusion of IMU with other onboard sensors into multi-fusion ensemble approaches.