Inertial Measurement Unit
The IMU was gimbaled on three axes. The innermost stable member, a 6 inch beryllium cube, had three gyroscopes and three accelerometers mounted in it. Feedback loops including the resolvers used signals from the gyroscopes to control motors at each axis. This servo system kept the stable member fixed with respect to inertial space. The IMU was derived from the guidance system developed by Draper for the Polaris missile.
Inertial guidance systems are not perfect and Apollo system drifted about one milliradian per hour. Thus it was necessary to "realign" the inertial platform periodically by sighting on stars.
Read more about this topic: Apollo PGNCS
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