Exploring the Unknown - NASA's History Office

12
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THE HISTORY OF SATELLITE COMMUNICATIONS
Document I-1
The Space-Station: Its Radio Applications
Arthur C. Clarke
25 May 1945
[1] 1. The Space-station was originally conceived as a refueling depot for ships leaving
the Earth. As such it may fill an important though transient role in the conquest of space,
during the period when chemical fuels are employed. Other uses, some of them rather
fantastic, have been suggested for the space-station, notably by Hermann Noordung. 1
However, there is at least one purpose for which the station is ideally suited and indeed
has no practical alternative. This is the provision of world-wide ultra-high-frequency radio
services, including television.
2. In the following discussion the word “television” will be used exclusively but it
must be understood to cover all services using the u.h.f. spectrum and higher. It is probable
that television may be among the least important of these as technical developments
occur. Other examples are frequency modulation, facsimile (capable of transmitting
100,000 pages an hour 2 ), specialized scientific and business services, and navigational aids.
3. Owing to bandwidth considerations television is restricted to the frequency range
above 50-Mc/sec [megacycles per second], and there is no doubt that very much higher
frequencies will be used in the immediate future. The American Telephone and
Telegraph Company are [sic] already building an experimental network using frequencies
up to 12,000 megacycles. 3 Waves of such frequencies are transmitted along quasioptical
paths and accordingly receiver and transmitter must lie not far from the line of
sight. Although refraction increases the range, it is fair to say that the service radius for a
television station is under 50 miles. (The range of the London service was rather less than
this.) As long as radio continues to be used for communication, this limitation will remain, as it is
a fundamental and not a technical restriction.
4. Wide-band frequency-modulation, one of the most important of radio developments,
comes in the same category. FM can give much better quality and freedom from
interference than normal amplitude-modulation, and many hundreds of stations are
being planned for the post-war years in America alone. The technical requirements of FM
make it essential that only the direct signal be used, and ionospheric reflexions cannot be
employed. The range of the service is thus limited by the curvature of the Earth, precisely
as for television.
5. To provide services over a large area it is necessary to build numerous stations on
high ground or with radiators on towers several hundred feet high. These stations have to
be linked by landline or subsidiary radio circuits. Such a system is practicable in a small
country such as Britain, but even here the expense will be enormous. It is quite prohibitive
in the case of a large continent and it therefore seems likely that only highly populated
communities will be able to have television services.
6. An even more serious problem arises when an attempt is made to link television
systems in different parts of the globe. Theoretical studies 2 indicate that using a radio relay
system, repeater stations will be necessary at intervals of less than fifty miles. These will
take the form of towers several hundred feet high, carrying receivers, amplifiers and transmitters.
To link regions several thousand miles apart will thus cost many millions of
pounds, and the problem of trans-oceanic services remains insoluble.