# 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  
IMUs are packaged sensor suites that typically include accelerometers and gyroscopes for each axis of motion and sometimes contain magnetometer sensors. An accelerometer is an instrument that measures linear and angular acceleration, such as changes in speed or direction. Accelerometers can be mechanical, capacitive, or piezoelectric in nature. A gyroscope is a device used to measure or maintain orientation and angular velocity. A magnetometer is a device that measures magnetic field strength, allowing the UAS to always know magnetic north, similar in function to a compass. IMU sensors measure acceleration, orientation, angular rates, and magnetic and gravitational forces. These measurements can then be used to detect and track motion relative to the system's starting  
Figure 5: High-Level taxonomy of existing methods for positioning in GPS-denied environments. point, as in determining relative position and velocity [23]. IMU-based approaches are very mature and can provide highly accurate short-term positioning and navigation results; however, IMU accuracy decreases over time due to inherent error accumulation. These errors can take the form of biases, scale-factor errors, and misalignment errors, leading to significant inaccuracies after as little as one minute of use. IMU calibration procedures can significantly lower these errors; however, some level of error accumulation will generally linger, which will eventually yield inaccurate results. For example, magnetometer sensors often experience noisy measurements due to the strong currents in the electric motor circuits during flight, such as motors turning rotor blades in a quadcopter. Research in the literature provides recommendations for proper ground calibration to minimize this noise [111]. Another common approach is employing extended Kalman filters (EKFs) to dynamically weight IMU sensor measurements, which can further reduce accumulation errors and unwanted biases. \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Positioning Method** & **Hardware Requirement** & **Advantages** & **Disadvantages** & **Level of Research Activity** \\ \hline \multirow{6}{*}{GPS} & \multirow{6}{*}{GPS receiver} & \multirow{6}{*}{\(\bullet\)} & Small lightweight hardware. & \multirow{6}{*}{\(\bullet\)} & Vulnerable to spoofing and jamming attacks. & \multirow{6}{*}{\(\bullet\)} & Moderate—Some ongoing research on GPS performance. & \multirow{6}{*}{\(\bullet\)} & Moderate—Some ongoing research on GPS performance. & \multirow{6}{*}{\(\bullet\)} & \\  & & \(\bullet\) & Low-cost hardware. & & & & \\  & & \(\bullet\) & Proven performance. & & & & \\  & & & performance. & & & & \\ \hline \multirow{6}{*}{Receiver-based GPS Performance Improvement} & \multirow{6}{*}{RF hardware (e.g., antenna)} & \multirow{6}{*}{\(\bullet\)} & Demonstrated ability to recover GPS performance in the presence of degradation. & \multirow{6}{*}{\(\bullet\)} & Additional hardware or computational complexity. & \multirow{6}{*}{\(\bullet\)} & Moderate—Continuing antenna-based and signal processing-based methods for jamming and spoofing suppression/rejection. \\  & & & & & & & \\ \hline \multirow{6}{*}{IMU} & \multirow{6}{*}{IMU} & \multirow{6}{*}{\(\bullet\)} & Good performance for short flight times. & \multirow{6}{*}{\(\bullet\)} & Accuracy decreases over time due to error accumulations. & \multirow{6}{*}{\(\bullet\)} & High—Stand-alone IMU research is receiving moderate interest. & \multirow{6}{*}{\(\bullet\)} & High—Stand-alone IMU research is receiving moderate interest. & \multirow{6}{*}{\(\bullet\)} & \\  & & & & & & \\ \cline{1  
Quantum accelerometers are an exciting research area that has the potential to eventually provide very good IMU-based positioning and navigation, primarily due to the