Schuler tuning

Schuler tuning is a modification to the electronic control system used in 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 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 modifications necessary to an inertial navigation system to keep the inertial platform always pointing 'north', 'east' and 'down', so it gives correct directions on Earth.

Principle

As first explained by German engineer Maximilian Schuler in a 1923 paper,^[1] a pendulum whose period exactly equals the orbital period of a hypothetical satellite orbiting just above the surface of the Earth (about 84 minutes) will tend to remain pointing at the center of the Earth when its support is suddenly displaced. Such a pendulum would have a length equal to the radius of the Earth. Consider a simple gravity pendulum, whose length equals the radius of the Earth, suspended in a uniform gravitational field of the same strength as that experienced at the Earth's surface. If suspended from the surface of the Earth, the bob of the pendulum would be at the center of the 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 the 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.

Application

Obviously, a pendulum the length of the Earth's radius is impractical, so Schuler tuning doesn't use physical pendulums. Instead, the electronic control system of the inertial navigation system is modified to make the platform behave as if it were attached to a pendulum. The inertial platform is mounted on gimbals, and an electronic control system keeps it pointed in a constant direction with respect to the three axes. As the vehicle moves, the gyroscopes detect changes in orientation, and a feedback loop applies signals to torquers to rotate the platform on its gimbals to keep it pointed along the axes.

To implement Schuler tuning, the feedback loop is modified to tilt the platform as the vehicle moves in the north-south and east-west directions, to keep the platform facing 'down'.^[2] To do this, the torquers that rotate the platform are fed a signal proportional to the vehicle's north-south and east-west velocity. The turning rate of the torquers is equal to the velocity divided by the radius of the Earth R:

$\dot{\theta} = v/R$

So:

$\ddot{\theta} = a/R$

The acceleration a is a combination of the actual vehicle acceleration and the acceleration due to gravity acting on the tilting inertial platform. So this equation can be seen as a version of the equation for a simple gravity pendulum with a length equal to the radius of the Earth. The inertial platform acts as if it were attached to such a pendulum.

Schuler's time constant has other applications. Suppose a tunnel is dug from one end of the earth to the other end straight through the center of earth, a stone dropped in such a tunnel oscillates with Schuler's time constant. It can also be proved that the time is the same constant for a tunnel which is not through the center of earth also.

References

^ Schuler, M. (1923). "Die Störung von Pendel und Kreiselapparaten durch die Beschleunigung des Fahrzeuges". Physikalische Zeitschrift 24 (16).
^ King, A.D. (1998). "Inertial Navigation - Forty Years of Evolution". GEC Review 13 (3): p.141. http://www.imar-navigation.de/downloads/papers/inertial_navigation_introduction.pdf. Retrieved 2010-09-27.