Inertial navigation
Navigation by integrating a vehicle's own rotations and accelerations — no external reference needed.
Definition
Inertial navigation is the art of knowing where you are, and which way you are pointing, using nothing but instruments carried onboard the vehicle itself. Three orthogonal gyroscopes measure the vehicle's rotation rate about each axis; three orthogonal accelerometers measure its acceleration. A computer integrates the rotation rates to keep track of orientation, and integrates the accelerations twice to keep track of position. No radio, no satellite, no landmark is needed after the initial alignment.
The principle follows directly from τ = dL/dt: a gyroscope's spin axis does not change direction in the absence of external torque, so a spinning wheel on gimbals is a direction memory that survives any acceleration of its carrier. The first operational inertial navigation system flew on the V-2 rocket in 1944; the Apollo Guidance Computer's IMU flew six crews to the Moon; modern airliners use ring laser gyroscopes with no moving parts; and the MEMS gyroscope in every smartphone is the same concept reduced to a few cubic millimetres of silicon.