How does an inertial navigation system work?

The INS is initially provided with its position and velocity from another source (a human operator, a GPS satellite receiver, etc.) accompanied with the initial orientation and thereafter computes its own updated position and velocity by integrating information received from the motion sensors.

INS is self-contained and does not rely on external signals. This independence makes it highly reliable in environments where GPS signals might be weak or unavailable, such as underwater, underground, or in space.