Page 222 - Satellite Communications, Fourth Edition

202 Chapter Seven

Figure 7.3 Satellite eclipse time as a function of the current day of the year. (Courtesy
of Spilker, 1977. Reprinted by permission of Prentice-Hall, Englewood Cliffs, NJ.)

capacity of cylindrical and solar-sail satellites, the cross-over point is esti-
mated to be about 2 kW, where the solar-sail type is more economical than
the cylindrical type (Hyndman, 1991).
As discussed in Sec. 3.6, the earth will eclipse a geostationary satel-
lite twice a year, during the spring and autumnal equinoxes. Daily
eclipses start approximately 23 days before and end approximately 23
days after the equinox for both the spring and autumnal equinoxes and
can last up to 72 min at the actual equinox days. Figure 7.3 shows the
graph relating eclipse period to the day of year. In order to maintain serv-
ice during an eclipse, storage batteries must be provided. Ni-Cd batteries
continue to be used, as shown in the Hughes HS 376 satellite, but devel-
opments in nickel-hydrogen (Ni-H ) batteries offer significant improve-
2
ment in power-weight ratio. Ni-H batteries are used in the Hughes HS
2
601 (e.g., the SATMEX-5 and Anik-F2 satellites, see Secs. 7.9 and 7.10)
and were introduced into the Intelsat series with INTELSAT VI (Pilcher,
1982) and INTELSAT VII (Lilly, 1990) satellites.

7.3 Attitude Control
The attitude of a satellite refers to its orientation in space. Much of the
equipment carried aboard a satellite is there for the purpose of control-
ling its attitude. Attitude control is necessary, for example, to ensure that