Plain English Meaning

A system that figures out where the aircraft is, which way it is pointed, and how fast it is moving, purely by sensing its own movement. Once it knows where it started, it tracks every turn and acceleration to keep updating that picture, with no help from the ground or satellites.

Full Definition

A self-contained navigation system that determines an aircraft's position, velocity, attitude, and heading by sensing its own motion through space. It uses ring laser gyros (or fiber-optic gyros) and accelerometers to detect every change in direction and speed from a known starting point, then continuously calculates where the aircraft is without needing any signal from outside the .