Page 66 - Communications Satellites Global Change Agents

42 IPPOLITO AND PELTON

FIG. 2.4. A satellite network with various types of ground earth stations.

modest, squinted omni-beam satellite antenna could receive a radio signal at this
great altitude and then pass the signal through a filter to an on-board transponder.
This transponder (the most important part of the communications subsystem)
used an element called a traveling wave tube (TWT) to translate the incoming sig-
nal into another frequency and then amplify it for retransmission towards the
earth's surface in a down link. This whole process (i.e., reception, on-board filter-
ing, frequency translation, amplification, and retransmission) happens in just a
few milliseconds. The return signal is sent back through the feed on the satellite's
transmitting antenna to earth-based receiving antenna stations. The following sat-
ellite network diagram shows how signals are relayed from user terminals on the
earth's surface to the satellite and back to another earth station in a modern net-
work. This diverse network is shown in Fig. 2.4.
The power on today's satellites can vary from 2 to nearly 15 kilowatts. Yet
back in the mid-1960s, the power on board these early experimental satellites was
limited (about 100 watts or the same wattage as a small TV set). Because the sat-
ellites were power limited and antennas on the satellites were small and unsophis-
ticated, the ground-based antennas had to be capable of transmitting at very high
power levels. They also had to transmit and receive signals in concentrated beams
so that they could receive faint signals and send the signal more effectively to the
satellite.
These constraints on the earth station performance characteristics required a
large aperture and high gain ground facilities to send the up-link signal with as
much power and focus as possible. (Note: The larger the antenna's surface, or ap-
erture size, the better it can focus a signal—much like a spot light—and this in-
creased focus is known as a higher gain antenna.)
The early satellite earth stations were in fact huge, very expensive, multiton
parabolic dishes some 30 meters in diameter. The one exception was a large horn
antenna built by AT&T in Maine to support their experimental work with Echo