A Primer on Satellite Communications

Topics: Modulation, Phase-shift keying, Geosynchronous orbit Pages: 58 (12765 words) Published: June 11, 2007
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 basis,
much like the Mir space station and the
international space station now under
Twenty years later, in his book Voices
from the Sky, Clarke wrote a chapter
entitled "A Short Pre-History of Comsats,
Or: How I Lost a Billion Dollars in My
Spare Time". For he did not patent the
idea of a geostationary orbit and, believe it
or not, orbits can and have been patented.
(Recall the recent patent controversy
between Odyssey and ICO.) However,
despite the tongue-in-cheek subtitle, the
famous author would not have profited
from his idea for two reasons. First,
arguably, prior art existed in the literature.
In 1929 the Austrian engineer
H. Noordwig observed that a satellite at an
altitude of 35,786 km in the equatorial
plane would appear motionless when
viewed from earth (as cited by Bruno
Pattan in Satellite Systems: Principles and
Technologies). Second, had Clarke
obtained a patent in 1945, it would have
expired in 1962, 17 years after the concept
was first disclosed and two years before
the first geostationary satellite, Syncom III,
was successfully launched. Nevertheless,
Clarke can rightfully claim credit for the
first detailed technical exposition of
satellite communications with specific
reference to the geostationary orbit. His
vision was realized through the pioneering
efforts of such scientists as John Pierce of
the Bell Telephone Laboratories, head of
the Telstar program and co-inventor of the
traveling wave tube amplifier, and Harold
Rosen of the Hughes Aircraft Company,
who was the driving force behind the
Syncom program.
Since 1964, approximately 265
satellites have been launched into
geostationary orbit, of which
approximately 185 are operational.
Another 67 GEO satellites are presently on
order. The majority of these satellites have
been used for the traditional fixed satellite
service in C- and Ku-band, but also include
satellites in the direct broadcast service,
digital audio radio service,...
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