Page 269 - Satellite Communications, Fourth Edition

The Earth Segment 249

links also must be avoided. Terrestrial microwave links do not operate
at Ku-band frequencies.
In the receive branch (the right-hand side of Fig. 8.6), the incoming
wideband signal is amplified in an LNA and passed to a divider network,
which separates out the individual microwave carriers. These are each
downconverted to an IF band and passed on to the multiplex block,
where the multiplexed signals are reformatted as required by the ter-
restrial network.
It should be noted that, in general, the signal traffic flow on the receive
side will differ from that on the transmit side. The incoming microwave
carriers will be different in number and in the amount of traffic carried,
and the multiplexed output will carry telephone circuits not necessar-
ily carried on the transmit side.
A number of different classes of earth stations are available, depend-
ing on the service requirements. Traffic can be broadly classified as
heavy route, medium route, and thin route. In a thin-route circuit, a
transponder channel (36 MHz) may be occupied by a number of single
carriers, each associated with its own voice circuit. This mode of oper-
ation is known as single carrier per channel (SCPC), a multiple-access
mode which is discussed further in Chap. 14. Antenna sizes range from
3.6 m (11.8 ft) for transportable stations up to 30 m (98.4 ft) for a main
terminal.
A medium-route circuit also provides multiple access, either on the
basis of frequency-division multiple access (FDMA) or time-division mul-
tiple access (TDMA), multiplexed baseband signals being carried in
either case. These access modes are also described in detail in Chap. 14.
Antenna sizes range from 30 m (89.4 ft) for a main station to 10 m (32.8 ft)
for a remote station.
In a 6/4-GHz heavy-route system, each satellite channel (bandwidth
36 MHz) is capable of carrying over 960 one-way voice circuits simulta-
neously or a single-color analog TV signal with associated audio (in some
systems two analog TV signals can be accommodated). Thus the transpon-
der channel for a heavy-route circuit carries one large-bandwidth signal,
which may be TV or multiplexed telephony. The antenna diameter for a
heavy-route circuit is at least 30 m (98.4 ft). For international operation
such antennas are designed to the INTELSAT specifications for a
Standard Aearth station (Intelsat, 1982). Figure 8.7 shows a photograph
of a 32-m (105-ft) Standard A earth station antenna.
It will be appreciated that for these large antennas, which may weigh
in the order of 250 tons, the foundations must be very strong and stable.
Such large diameters automatically mean very narrow beams, and
therefore, any movement which would deflect the beam unduly must be
avoided. Where snow and ice conditions are likely to be encountered,
built-in heaters are required. For the antenna shown in Fig. 8.7, deicing