A Case Study in NASA-DoD - The Black Vault

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and three reaction wheels are added to the stabilization and control
system and the two earth sensors are removed. Also, with the addition
of the star trackers, a star catalog and the spacecraft's ephemeris
must be ground-supplied periodically and thus an on-board computer
becomes mandatory. Pointing accuracies of 0.05 deg per axis can be
expected from the precision STPSS design.
Unlike the AEK design, the three reaction wheels of the precision
STPSS have no momentum bias and are used only to provide reaction control
torques. The primary function of the cold gas reaction jet system
is to unload the wheels when they approach saturation. As in the case
of the low-cost STPSS design, electromagnetic torques and a magnetometer
could be added as a supplement to the cold gas system if secular disturbance
torques become a problem.
The final spacecraft design to be considered is MMS. The attitude
control system of this spacecraft is very similar to that of the precision
STPSS. The major difference is that the MMS uses electromagnetic
torques to unload the reaction wheels rather than a jet reaction system.
However, a hydrazine jet reaction system can be added as an option.
The pointing accuracy specification of the MMS is ±0.01 deg per
axis, which is better by a factor of five than that claimed for the
precision STPSS. Since the same model strap-down star tracker assembly
is proposed for both the MMS and the precision STPSS, the superior performance
projected for the MKS must result from either a better gyro
reference unit or more frequent stellar updates.
Considering the relatively modest STPSS attitude control performance
specifications, it is apparent that all five spacecraft designs
of Table D-1 are well within the state of the art. In all cases the
major components that have been selected, such as earth sensors, reaction
wheels, or star trackers, are developed items of equipment with
a history of previous spacecraft use. The AEM and the STPSS spinstabilized
configuration have the least complex attitude control systems,
while the precision STPSS and MMS vehicles have the most complex systems.