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A Primer On Satellite Communications

In 1945 Arthur C. Clarke wrote an article
entitled "The Future of World
Communications" for the magazine
Wireless World. This article, which the
editors renamed "Extra-Terrestrial
Relays", was published in the October
issue. In it Clarke described the properties
of the geostationary orbit, a circular orbit
in the equatorial plane of the earth such
that a satellite appears to hover over a
fixed point on the equator. The period of
revolution is equal to the period of
rotation of the earth with respect to the
stars, or 23 hours 56 minutes 4.1 seconds,
and thus by Kepler's third law the orbital
radius is 42,164 km. Taking into account
the radius of the earth, the height of a
satellite above the equator is 35,786 km.
Clarke observed that only three satellites
would be required to provide
communications over the inhabited earth.
As a primary application of such
a satellite system, Clarke proposed that
satellites in geostationary orbit might
provide direct broadcast television service
similar to DBS systems like DirecTV -- a
remarkable idea at a time when television
was still in its infancy and it was not yet
known whether radio signals could
penetrate the ionosphere. He worked out a
simple link budget, assuming a downlink
frequency of 3 GHz, and estimated that the
required transmitter output power for
broadcast service to small parabolic
antenna receivers would be about 50 watts.
Electric power would be provided by
steam generators heated by solar mirrors,
but advances in technology might make it
possible to replace them by arrays of
photoelectric cells. Batteries would be
used to provide uninterrupted service
during eclipses, which occur in two
seasons centered about the equinoxes.
Clarke also estimated the mass ratio of a
multistage launch vehicle necessary to
deploy the satellite. However, he imagined
the geostationary satellites to be outposts
inhabited by astronauts to whom supplies
would be ferried up on a regular...