Here is a brief excerpt from Honeywell….

An inertial navigation system (INS) calculates the location, orientation and velocity of a moving object without the need of GPS technology.
An INS device typically uses accelerometers and gyroscopes, meaning motion and rotation sensors, that communicate with a computer unit which then translates the data into actionable controls. This is your basic inertial navigation system, to which other features can be added. For example, inertial navigation systems can be upgraded with magnetic sensors and barometric altimeters.
INS devices operate on a dead reckoning system, which means that the initial position, velocity and orientation of the vehicle are provided by an external source, which can be a GPS satellite receiver or an operator. Equipped with this data, the INS can begin calculating position, velocity and other movement elements. As the vehicle continues to move, the INS device will keep calculating and updating, on its own, all motion elements via the information received from motion sensors.