Page 259 - Satellite Communications, Fourth Edition

Chapter
8

The Earth Segment

8.1 Introduction
The earth segment of a satellite communications system consists of the
transmit and receive earth stations. The simplest of these are the home
TV receive-only (TVRO) systems, and the most complex are the terminal
stations used for international communications networks. Also included
in the earth segment are those stations which are on ships at sea, and
commercial and military land and aeronautical mobile stations.
As mentioned in Chap. 7, earth stations that are used for logistic sup-
port of satellites, such as providing the telemetry, tracking, and command
(TT&C) functions, are considered as part of the space segment.

8.2 Receive-Only Home TV Systems
Planned broadcasting directly to home TV receivers takes place in the
Ku (12-GHz) band. This service is known as direct broadcast satellite
(DBS) service. There is some variation in the frequency bands assigned
to different geographic regions. In the Americas, for example, the down-
link band is 12.2 to 12.7 GHz, as described in Sec. 1.4.
The comparatively large satellite receiving dishes [ranging in diame-
ter from about 1.83 m (6 ft) to about 3-m (10 ft) in some locations], which
may be seen in some “backyards” are used to receive downlink TV signals
at C band (4 GHz). Originally such downlink signals were never intended
for home reception but for network relay to commercial TV outlets (VHF
and UHF TV broadcast stations and cable TV “head-end” studios).
Equipment is now marketed for home reception of C-band signals, and
some manufacturers provide dual C-band/Ku-band equipment. A single
mesh type reflector may be used which focuses the signals into a dual feed-
horn, which has two separate outputs, one for the C-band signals and one
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