A Case Study in NASA-DoD - The Black Vault

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Appendix D
ATTITUDE CONTROL AND STABILIZATION SUBSYSTEM:
A COMPARISON OF AEM, STPSS, AND MMS
by
T. B. Garber
The function of the attitude control and stabilization system is
to provide the means of orienting the satellite in some specific attitude
and then to maintain that orientation with acceptable angle and
angular rate errors. In addition, the stabilization and control system
should also be able to provide the information necessary for afterthe-fact
attitude determination.
Table D-I presents the performance specification and the physical
characteristics of the attitude control systems that have been proposed
for three spacecraft, NASA's AEM and MMS, and the Air Force's STPSS.
In the case of the STPSS design, three different attitude control systems
can be incorporated into the spacecraft depending upon the level
of performance required.
Of the three spacecraft designs, that of the AEM is the most firm.
As can be seen from Table D-l, the performance requirements of the AEM
attitude control system are quite modest. The performance of the AEM
control system should, under normal conditions, exceed the specifications,
with pointing errors roughly one-half those shown.
Basically, the AEM spacecraft is inertially stabilized in roll
and yaw by virtue of the angular momentum of a wheel spinning about
)
the pitch axis, normal to the orbital plane. Control of the spacecraft
about the pitch axis is achieved by modulating the pitch wheel's angular
rate. Errors in the spacecraft's pitch and roll attitudes are detected
by a horizon scanner.
To remove the small roll and yaw errors that result from both external
and internal disturbances, electromagnets are used to generate
the necessary torques. A three-axis magnetometer provides the required
knowledge of the earth's magnetic field vector. In addition to damping
precessional and nucational spacecraft motion, the electromagnets also
provide the necessary torque to unload the pitch wheel (desaturation).