Page 60 - Communications Satellites Global Change Agents

36 IPPOLITO AND PELTON

FIG. 2.1. Schematic representation of a satellite communication link.

Thus, Clarke did not even attempt to patent the idea of geostationary (or more
generally geosynchronous) communications satellites for these reasons—some-
thing he admitted that he later regretted.
A geosynchronous satellite orbits the earth every 23 hours and 56 minutes to
maintain perfect sidereal relationship to the earth and the sun on an annualized ba-
sis. A satellite in geosynchronous position, however, can stray above or below the
equator in the north and south direction. A geostationary satellite not only re-
volves with the earth, but also maintains its position over the same exact spot
above the equator as well and by means of control instructions from an operating
center on earth. A perfectly geostationary satellite is not allowed to rise above or
fall below its equatorial position. In practice, all GEO satellites make these excur-
sions or "inclinations" above or below this imaginary plane, but in most cases the
deviations are so small that the satellite earth terminals do not have to be
repointed to accommodate these minor shifts. The amount of fuel needed to keep
the satellite from moving north or south off the equatorial plane is 10 times the
amount needed to keep the satellite in position from east to west, and thus tight
station-keeping in the equatorial orbit is a highly specialized and difficult task that
is hard to execute with perfection.
It was a separate but related research breakthrough—the invention of the tran-
sistor less than a decade later—that made possible not only a new generation of