Page 232 - Satellite Communications, Fourth Edition

212 Chapter Seven

fraction of the radiation falling on earth which is reflected, can be sig-
nificant for low-altitude earth-orbiting satellites, although it is negligi-
ble for geostationary satellites. Equipment in the satellite also generates
heat which has to be removed. The most important consideration is
that the satellite’s equipment should operate as nearly as possible in a
stable temperature environment. Various steps are taken to achieve
this. Thermal blankets and shields may be used to provide insulation.
Radiation mirrors are often used to remove heat from the communica-
tions payload. The mirrored thermal radiator for the Hughes HS 376
satellite can be seen in Fig. 7.1 and in Fig. 7.6. These mirrored drums
surround the communications equipment shelves in each case and pro-
vide good radiation paths for the generated heat to escape into the sur-
rounding space. One advantage of spinning satellites compared with
body-stabilized is that the spinning body provides an averaging of the
temperature extremes experienced from solar flux and the cold back-
ground of deep space.
In order to maintain constant temperature conditions, heaters may
be switched on (usually on command from ground) to make up for the
heat reduction which occurs when transponders are switched off. The
INTELSAT VI satellite used heaters to maintain propulsion thrusters
and line temperatures (Pilcher, 1982).

7.6 TT&C Subsystem
The TT&C subsystem performs several routine functions aboard the
spacecraft. The telemetry, or telemetering, function could be interpreted
as measurement at a distance. Specifically, it refers to the overall oper-
ation of generating an electrical signal proportional to the quantity
being measured and encoding and transmitting this to a distant station,
which for the satellite is one of the earth stations. Data which are trans-
mitted as telemetry signals include attitude information such as that
obtained from sun and earth sensors; environmental information such
as the magnetic field intensity and direction, the frequency of meteorite
impact, and so on; and spacecraft information such as temperatures,
power supply voltages, and stored-fuel pressure. Certain frequencies
have been designated by international agreement for satellite teleme-
try transmissions. During the transfer and drift orbital phases of the
satellite launch, a special channel is used along with an omnidirectional
antenna. Once the satellite is on station, one of the normal communi-
cations transponders may be used along with its directional antenna,
unless some emergency arises which makes it necessary to switch back
to the special channel used during the transfer orbit.
Telemetry and command may be thought of as complementary func-
tions. The telemetry subsystem transmits information about the satellite