Page 225 - Satellite Communications, Fourth Edition

The Space Segment 205

Figure 7.5 Spin stabilization in the geostationary orbit. The
spin axis lies along the pitch axis, parallel to the earth’s N-S axis.

overall effect is that the spin rate will decrease, and the direction of the
angular spin axis will change. Impulse-type thrusters, or jets, can be
used to increase the spin rate again and to shift the axis back to its cor-
rect N-S orientation. Nutation, which is a form of wobbling, can occur
as a result of the disturbance torques and/or from misalignment or
unbalance of the control jets. This nutation must be damped out by
means of energy absorbers known as nutation dampers.
Where an omnidirectional antenna is used (e.g., as shown for the
INTELSAT I and II satellites in Fig. 1.1), the antenna, which points
along the pitch axis, also rotates with the satellite. Where a directional
antenna is used, which is more common for communications satellites,
the antenna must be despun, giving rise to a dual-spin construction. An
electric motor drive is used for despinning the antenna subsystem.
Figure 7.6 shows the Hughes HS 376 satellite in more detail. The
antenna subsystem consists of a parabolic reflector and feed horns
mounted on the despun shelf, which also carries the communications
repeaters (transponders). The antenna feeds can therefore be connected
directly to the transponders without the need for radiofrequency (rf)
rotary joints, while the complete platform is despun. Of course, control
signals and power must be transferred to the despun section, and a
mechanical bearing must be provided. The complete assembly for this is
known as the bearing and power transfer assembly (BAPTA). Figure 7.7
shows a photograph of the internal structure of the HS 376.
Certain dual-spin spacecraft obtain spin stabilization from a spinning fly-
wheel rather than by spinning the satellite itself. These flywheels are
termed momentum wheels, and their average momentum is referred to as
momentum bias. Reaction wheels, described in the Sec. 7.3.2, operate at zero
momentum bias. In the Intelsat series of satellites, the INTELSAT-VI
series spacecraft are spin-stabilized, all the others being 3-axis stabilized
(body stabilized) through the use of momentum wheels.