Applications in Navigation
Inertial guidance systems detect acceleration with respect to inertial space, and with those data it is possible to calculate the current velocity and position with respect to the velocity and position at the moment the acceleratometers started registering data.
For detecting rotation, gyroscopes and fiber optic ring interferometers are used. The operating principle of ring interferometers is called the Sagnac effect
A gyrocompass, employed for navigation of seagoing vessels, finds the geometric north. It does so, not by sensing the Earth's magnetic field, but by using inertial space as its reference. The outer casing of the gyrocompass device is held in such a way that it remains aligned with the local plumb line. When the gyroscope wheel inside the gyrocompass device is spun up, the way the gyroscope wheel is suspended causes the gyroscope wheel to gradually align its spinning axis with the Earth's axis. Alignment with the Earth's axis is the only direction for which the gyroscope's spinning axis can be stationary with respect to the Earth and not be required to change direction with respect to inertial space. After being spun up, a gyrocompass can reach the direction of alignment with the Earth's axis in as little as a quarter of an hour.
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