Page 233 - Satellite Communications, Fourth Edition

The Space Segment 213

to the earth station, while the command subsystem receives command sig-
nals from the earth station, often in response to telemetered information.
The command subsystem demodulates and, if necessary, decodes the com-
mand signals and routes these to the appropriate equipment needed to exe-
cute the necessary action. Thus attitude changes may be made,
communication transponders switched in and out of circuits, antennas
redirected, and station-keeping maneuvers carried out on command. It is
clearly important to prevent unauthorized commands from being received
and decoded, and for this reason, the command signals are often encrypted.
Encrypt is derived from a Greek word kryptein, meaning to hide,and rep-
resents the process of concealing the command signals in a secure code.
This differs from the normal process of encoding which converts charac-
ters in the command signal into a code suitable for transmission.
Tracking of the satellite is accomplished by having the satellite trans-
mit beacon signals which are received at the TT&C earth stations.
Tracking is obviously important during the transfer and drift orbital
phases of the satellite launch. Once it is on station, the position of a geo-
stationary satellite will tend to be shifted as a result of the various dis-
turbing forces, as described previously. Therefore, it is necessary to be
able to track the satellite’s movement and send correction signals as
required. Tracking beacons may be transmitted in the telemetry chan-
nel, or by pilot carriers at frequencies in one of the main communica-
tions channels, or by special tracking antennas. Satellite range from the
ground station is also required from time to time. This can be determined
by measurement of the propagation delay of signals especially trans-
mitted for ranging purposes.
It is clear that the telemetry, tracking, and command functions are
complex operations which require special ground facilities in addition
to the TT&C subsystems aboard the satellite. Figure 7.11 shows in
block diagram form the TT&C facilities used by Canadian Telesat for
its satellites.

7.7 Transponders
A transponder is the series of interconnected units which forms a single
communications channel between the receive and transmit antennas in a
communications satellite. Some of the units utilized by a transponder in a
given channel may be common to a number of transponders. Thus, although
reference may be made to a specific transponder, this must be thought of
as an equipment channel rather than a single item of equipment.
Before describing in detail the various units of a transponder, the
overall frequency arrangement of a typical C-band communications
satellite will be examined briefly. The bandwidth allocated for C-band
service is 500 MHz, and this is divided into subbands, one for each