This module holds most of the electronic hardware used by the Attitude Control System (ACS) for determining and changing TOPEX/Poseidon's orientation in space. The module is attached to the zenith side of the MMS.
The ACS determines spacecraft orientation through the use of star trackers, the fine sun sensor, gyros, magnetometers, and signals from the Earth Sensor Assembly Module. Once its orientation is known, the ACS can make adjustments with reaction wheels, magnetic torquer bars, or by firing the thrusters on the Propulsion Module.
Normally the on-board computer is the controller for these functions, but in certain safe-hold modes, they are performed by the electronics in the ACS.
