Page 223 - Satellite Communications, Fourth Edition

The Space Segment 203

directional antennas point in the proper directions. In the case of earth
environmental satellites, the earth-sensing instruments must cover the
required regions of the earth, which also requires attitude control. A
number of forces, referred to as disturbance torques, can alter the attitude,
some examples being the gravitational fields of the earth and the moon,
solar radiation, and meteorite impacts. Attitude control must not be con-
fused with station keeping, which is the term used for maintaining a
satellite in its correct orbital position, although the two are closely related.
To exercise attitude control, there must be available some measure of
a satellite’s orientation in space and of any tendency for this to shift. In
one method, infrared sensors, referred to as horizon detectors,are used
to detect the rim of the earth against the background of space. With the
use of four such sensors, one for each quadrant, the center of the earth
can be readily established as a reference point. Any shift in orientation
is detected by one or other of the sensors, and a corresponding control
signal is generated which activates a restoring torque.
Usually, the attitude-control process takes place aboard the satellite,
but it is also possible for control signals to be transmitted from earth,
based on attitude data obtained from the satellite. Also, where a shift
in attitude is desired, an attitude maneuver is executed. The control
signals needed to achieve this maneuver may be transmitted from an
earth station.
Controlling torques may be generated in a number of ways. Passive
attitude control refers to the use of mechanisms which stabilize the
satellite without putting a drain on the satellite’s energy supplies; at
most, infrequent use is made of these supplies, for example, when
thruster jets are impulsed to provide corrective torque. Examples of
passive attitude control are spin stabilization and gravity gradient sta-
bilization. The latter depends on the interaction of the satellite with the
gravitational field of the central body and has been used, for example,
with the Radio Astronomy Explorer-2 satellite, which was placed in
orbit around the moon (Wertz, 1984). For communications satellites, spin
stabilization is often used, and this is described in detail in Sec. 7.3.1.
The other form of attitude control is active control. With active atti-
tude control, there is no overall stabilizing torque present to resist the
disturbance torques. Instead, corrective torques are applied as required
in response to disturbance torques. Methods used to generate active
control torques include momentum wheels, electromagnetic coils, and
mass expulsion devices, such as gas jets and ion thrusters. The elec-
tromagnetic coil works on the principle that the earth’s magnetic field
exerts a torque on a current-carrying coil and that this torque can be
controlled through control of the current. However, the method is of use
only for satellites relatively close to the earth. The use of momentum
wheels is described in more detail in Sec. 7.3.2.