Inertial Navigation System. An INS is a navigation aid that uses a computer with a Kalman filter, motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate via dead reckoning the position, orientation, and velocity (direction and speed of movement) of a moving object without the need for external references. It is used on vehicles such as ships, aircraft, submarines, guided missiles, and spacecraft. Other terms used to refer to inertial navigation systems or closely related devices include inertial guidance system, inertial instrument, inertial measurement units (IMU) and many other variations. Inertial Navigation is used where GPS is not accurate or not available.