Schuler tuning describes the fundamental functional conditions for a gyrocompass.
As first explained by Max Schuler in his classic 1923 paper, 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.
This is the basic principle of Schuler tuning that must
be included in the design of any inertial guidance system
that will be operated near the earth, such as in ships and aircraft.
Consider three hypothetical objects:
- A simple gravity pendulum, whose length equals the radius of the earth, suspended near an enormous flat plate such that it encounters a uniform gravitational field of the same strength as that experienced by a small pendulum near the earth's surface;
- A small body orbiting very near the surface of an airless perfect sphere the size and density of the earth;
- A small body falling down an airless tunnel running from the North Pole to the South Pole through the center of the earth.
Each of these will have the same period: about 84 minutes -- though the third body only if the density of the earth is assumed constant (in which case its movement is a harmonic oscillation).
Schuler tuning provides the inertial platform of a
navigation system with a feedback loop between its velocity
output and its stabilizing gyros such that it behaves as
though it were such a pendulum. This makes it remain vertical
as the vehicle moves from place to place on the surface of
the earth.
