Page 229 - Satellite Communications, Fourth Edition

The Space Segment 209

When a momentum wheel is operated with zero momentum bias, it
is generally referred to as a reaction wheel. Reaction wheels are used
in three-axis stabilized systems. Here, as the name suggests, each axis
is stabilized by a reaction wheel, as shown in Fig. 7.8c. Reaction wheels
can also be combined with a momentum wheel to provide the control
needed (Chetty, 1991). Random and cyclic disturbance torques tend to
produce zero momentum on average. However, there will always be
some disturbance torques that cause a cumulative increase in wheel
momentum, and eventually at some point the wheel saturates. In effect,
it reaches its maximum allowable angular velocity and can no longer
take in any more momentum. Mass expulsion devices are then used to
unload the wheel, that is, remove momentum from it (in the same way
a brake removes energy from a moving vehicle). Of course, operation
of the mass expulsion devices consumes part of the satellite’s fuel
supply.

7.4 Station Keeping
In addition to having its attitude controlled, it is important that a geo-
stationary satellite be kept in its correct orbital slot. As described in Sec.
2.8.1, the equatorial ellipticity of the earth causes geostationary satel-
lites to drift slowly along the orbit, to one of two stable points, at 75°E
and 105°W. To counter this drift, an oppositely directed velocity com-
ponent is imparted to the satellite by means of jets, which are pulsed
once every 2 or 3 weeks. This results in the satellite drifting back
through its nominal station position, coming to a stop, and recom-
mencing the drift along the orbit until the jets are pulsed once again.
These maneuvers are termed east-west station-keeping maneuvers.
Satellites in the 6/4-GHz band must be kept within 0.1° of the desig-
nated longitude, and in the 14/12-GHz band, within 0.05°.
A satellite which is nominally geostationary also will drift in latitude,
the main perturbing forces being the gravitational pull of the sun and
the moon. These forces cause the inclination to change at a rate of about
0.85°/year. If left uncorrected, the drift would result in a cyclic change
in the inclination, going from 0° to 14.67° in 26.6 years (Spilker, 1977)
and back to zero, at which the cycle is repeated. To prevent the shift in
inclination from exceeding specified limits, jets may be pulsed at the
appropriate time to return the inclination to zero. Counteracting jets
must be pulsed when the inclination is at zero to halt the change in incli-
nation. These maneuvers are termed north-south station-keeping maneu-
vers, and they are much more expensive in fuel than are east-west
station-keeping maneuvers. The north-south station-keeping tolerances
are the same as those for east-west station keeping, 0.1° in the C band
and 0.05° in the Ku band.