Page 219 - Satellite Communications, Fourth Edition

Chapter
7

The Space Segment

7.1 Introduction
A satellite communications system can be broadly divided into two
segments—a ground segment and a space segment. The space segment
will obviously include the satellites, but it also includes the ground
facilities needed to keep the satellites operational, these being referred
to as the tracking, telemetry, and command (TT&C) facilities. In many
networks it is common practice to employ a ground station solely for
the purpose of TT&C.
The equipment carried aboard the satellite also can be classified
according to function. The payload refers to the equipment used to pro-
vide the service for which the satellite has been launched. The bus refers
not only to the vehicle which carries the payload but also to the various
subsystems which provide the power, attitude control, orbital control,
thermal control, and command and telemetry functions required to serv-
ice the payload.
In a communications satellite, the equipment which provides the con-
necting link between the satellite’s transmit and receive antennas is
referred to as the transponder. The transponder forms one of the main
sections of the payload, the other being the antenna subsystems.
In this chapter the main characteristics of certain bus systems and
payloads are described.

7.2 The Power Supply
The primary electrical power for operating the electronic equipment is
obtained from solar cells. Individual cells can generate only small amounts
of power, and therefore, arrays of cells in series-parallel connection are
required. Figure 7.1 shows the solar cell panels for the HS 376 satellite
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