Page 185 - Intro to Space Sciences Spacecraft Applications

Introduction to Space Sciences and Spacecraft Applications
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Each of the subsystem discussions that follow begin with a review of
important mission, payload, and launch vehicle inputs that influence the
subsystems calculations.

Attitude Reference and Control (ARC) Subsystem

Depending on the mission, a spacecraft may have varying requirements
for pointing accuracies. At some times it may be free to tumble and turn,
but at others it may have to pinpoint a discrete location on the earth or
deep in space. To do this, the spacecraft must be able to determine its own
attitude with respect to some reference, and then to modify this attitude to
perform the desired mission. Reference devices include earth horizon sen-
sors, sun sensors, star trackers, or even magnetometers which measure the
flux lines of the geomagnetic field. Attitude control devices include tiny
thrusters, angular momentum storage wheels, gravity-gradient booms, and
electromagnetic torque devices.
A satellite must determine its attitude with respect to some reference
(earth, sun, stars, and/or other satellites) and control its attitude to perform
the desired mission. Figure 8-4 provides a simplified block diagram of a
typical ARC subsystem. The control logic or computer that accepts input
from attitude reference sensors is usually based in space but may include
ground-based assets in the control loop, directly or as a backup. The mass
and configuration of the ARC subsystem are determined by mission and
payload inputs that include the final orbital attitude, payload pointing
accuracy, and satellite estimated on-orbit mass. These factors influence
the types of disturbances that will most perturb the spacecraft body, a few
of which are summarized in Figure 8-5. For example, at geosynchronous
altitudes, the effects of solar radiation can impart unbalancing torques on
the spacecraft body, as shown in Figure 8-6.
It is necessary to select the type of stabilization method that is to be
considered. Widely used candidates include:

3-axis, zero momentum
3-axis, bias momentum
dual-spin
spin
gravity gradient

These stabilization techniques are listed in descending order according to
their precise pointing ability.