Page 252 - Satellite Communications, Fourth Edition

232 Chapter Seven

Figure 7.27 Anik-E C-band transponder functional block diagram. (Courtesy of Telesat
Canada.)

a good illustration of the body-stabilized type of satellite, and Fig. 7.27
shows a typical C-band transponder set-up. This is seen to use solid-state
power amplifiers (SSPAs) which offer significant improvement in relia-
bility and weight saving over traveling-wave tube amplifiers. The anten-
nas are fed through a broadband feeder network (BFN) to illuminate the
large reflectors shown in Fig. 7.26. National, as distinct from regional,
coverage is provided at C band.
The TWTAs aboard a satellite also may be switched to provide redun-
dancy, as illustrated in Fig. 7.28. The scheme shown is termed a 4-for-2
redundancy, meaning that four channels are provided with two redun-
dant amplifiers. For example, examination of the table in Fig. 7.28 shows
that channel 1A has amplifier 2 as its primary amplifier, and amplifiers
1 and 3 can be switched in as backup amplifiers by ground command.

7.11 Advanced Tiros-N Spacecraft
Tiros is an acronym for Television and Infra-Red Observational Satellite.
As described in Chap. 1, Tiros is a polar-orbiting satellite, the primary mis-
sion of which is to gather and transmit earth environmental data down
to its earth stations. Although its payload differs fundamentally from the
communications-relay-type payload, much of the bus equipment is similar.
Table 1.7 lists the National Oceanographic and Atmospheric
Administration (NOAA) spacecraft used in the Advanced TIROS-N (ATN)