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A Primer on Satellite Communications

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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

construction.

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.

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