## 2 Sensors
### Inertial Measurement Unit (IMU)  
An IMU works based on gyroscopic principles. A gyroscope is defined as a solid object rotating at a high angular velocity around an axis going through its body. A gyroscope is usually bundled with a mechanical instrument called Cardan suspension that allows it to freely rotate without applying noticeable force on the rotating body. If the internal frictions of a gyroscope are ignored, the gyroscope's rotation will last forever based on [PERSON]'s second law. The rotating body preserves its orientation in space as long as no external force applies; therefore, the rotating angles (Euler angles) could be measured by observing the Cardan suspension mechanism. Four main classes of IMU have been developed and employed in the MMS industry. The first class is mechanical IMUs, which are based on a gyroscope supported by the suspension mechanism as described above [42]. The second class is based on the Sagnac effect [43]. In 1923, French physicist [PERSON] observed an interesting phenomenon: the phase between two light beams traveling on opposite sides of a ring changes when an angular velocity applies. This phenomenon called the Sagnac effect is the basis for fiber optic gyroscope (FOG) IMUs [44]. To create a FOG IMU, two light beams are considered to travel in a medium such as a mirror-based path or optical fibers. Then the phase shift between two lights is measured. This shift directly relates to the angular velocity. Usually, three optical fiber loops are considered perpendicular to each other to measure independent rotation angles. The third class of IMUs is based on micro-electro-mechanical system (MEMS) technology [42; 45], which combines electrical and mechanical parts into very small-scale productions. MEMS IMU technologies enable significant savings in the space required in comparison to mechanical and FOG IMUs. MEMS IMUs need to be accurately calibrated for different temperatures, since their behaviors change due to the environmental changes. MEMS IMUs are noticeably less expensive than the aforementioned classes; therefore, many novel recent applications arise for them. Some examples of MEMS IMUs could be seen in works such as in [PERSON] et al. [46]. The fourth class of IMU works based on ring laser gyroscope (RLG), which is based on the Sagnac effect, similar to FOG IMUs [47]. In addition to a gyroscope, a modern IMU usually contains other components such as accelerometers, barometers, and magnetometers. An accelerometer measures the force applied on the sensor in a specific direction. Two main classes of accelerometers exist: closed loop [48; 49] and open loop [50]. In close-loop accelerometers, a mass body is placed on the sensor, and the movement of the mass body is observed. Therefore, closed loops are less expensive and easier to build. Open-loop accelerometers work on the concept of maintaining the position of the mass body by applying a specific amount of current as magnetic force; e.g., therefore, closed-loop accelerometers are more accurate and more expensive to build. A barometer is a sub-component of an IMU measuring air pressure that is used to estimate the approximate altitude of the IMU device. The altitude is required for magnetic shift estimation. There are different sensor principles to measure pressure. A comprehensive review could be seen in works such as [PERSON] and [PERSON] [51] or [PERSON] et al. [52]. A magnetometer is employed when absolute orientations of an IMU are required. The Lorentz force principle [53] is employed in MEMS magnetometers as the base law. There is a shift between geographical altitude and magnetic altitude that is compensated by employing a suitable magnetic model such as the world magnetic model [54]. Calibrating an IMU usually requires a precise rotating table whose orientations and forces are precisely controlled by electro-mechanical servo motors. The standard calibration process involves accessing very accurate readings of the table and subsequently adjusting IMU readings to their relevant reference values [55].