Schuler tuning is a design principle for inertial navigation systems that accounts for the curvature of the Earth. An inertial navigation system, used in submarines, ships, aircraft, and other vehicles to keep track of position, determines directions with respect to three axes pointing "north", "east", and "down". To detect the vehicle's orientation, the system contains an "inertial platform" mounted on gimbals, with gyroscopes that detect motion connected to a servo system to keep it pointing in a fixed orientation in space. However, the directions "north", "east" and "down" change as the vehicle moves on the curved surface of the Earth. Schuler tuning describes the conditions necessary for an inertial navigation system to keep the inertial platform always pointing "north", "east" and "down", so it gives correct directions on the near-spherical Earth. It is widely used in electronic control systems. Principle As first explained by German engineer Maximilian Schuler in a 1923 paper, a pendulum that has a period that equals the orbital period of a hypothetical satellite orbiting at the surface of Earth (about 84.4 minutes) will tend to remain pointing at the center of Earth when its support is suddenly displaced. Such a pendulum (sometimes called a Schuler pendulum) would have a length equal to the radius of Earth. Consider a simple gravity pendulum, whose length to its center of gravity equals the radius of Earth, suspended in a uniform gravitational field of the same strength as that experienced at Earth's surface. If suspended from the surface of Earth, the center of gravity of the pendulum bob would be at the center of Earth. If it is hanging motionless and its support is moved sideways, the bob tends to remain motionless, so the pendulum always points at the center of Earth. If such a pendulum were attached to the inertial platform of an inertial navigation system, the platform would remain level, facing "north", "east" and "down", as it was moved about on the surface of the Earth. The Schuler period can be derived from the classic formula for the period of a pendulum: where L is the mean radius of Earth in meters and g is the local acceleration of gravity in metres per second per second. An inertial navigation system is tuned by letting it sit motionless for one Schuler period. If its coordinates deviate too much during the period or it does not return to its original