North finding is critical for tactical navigation and target orientation localization. It is used either in fixed static observation systems or in hand-held portable devices. The present application note focuses on the military domain, mainly about scouting operations.
Common north finding modules embed a 3-axis magnetometer. Such a sensor measures the angle between the pointing direction of the system and the magnetic field of the Earth. Magnetometers are cheap and small, however they show some weaknesses. First and foremost, this type of sensors measures the magnetic north, leading to an error to adjust. Secondly, its accuracy is greatly reduced when the sensor is surrounded by magnetic elements.
Advanced north finding modules are using a gyroscopic compass instead of a magnetometer. Gyroscopes measure angular velocity relative to its inertial position. A 2-axis gyroscope can find north, and a 3-axis gyroscope detects the Earth’s axis of rotation. There are many types of gyroscopes, based on different principles:
- Classic Mechanical – Spinning wheel mounted on a gimbal
- Optical – Fiber optic gyros (FOG) and Ring Laser Gyro (RLG), based on the Sagnac effect
- Vibrating – Coriolis effect – Hemispheric Resonating Gyros (HRG), Micromechanical Gyros (MEMS)
The gyroscopes measure true north and are not affected by external magnetic fields or surrounding metals but they are affected by Bias drift.