Satellite Communication-An Overview
Problems and Programs
WILBUR L. PRITCHARD,
principal technical problems It
OMMUNICATION satellites have several important
certainly the availability
widths exceeding anything previously available for inter-
continental communications. Although overland transmission
highquality TV pictures by microwave radio relays or cable
has been possible for some years, trans-Atlantic TV transmis-
sion took place for the first time only after the
communication satellite had been put in orbit. Interconti-
nental relaying of TV programs, now commonplace,
exclusively by satellites.
Another, perhaps the most important
ability to cover the globe. In the future, it
likely that cables
much higher carrier frequencies, probably as high as
the optical region
the spectrum. If
channels or their equivalent could be transmitted from one
another without satellites. However, a cable still
two fixed ends and there must
a connection between
every pair of points to be in communication. Satellite systems
offer, in this respect, a flexibility that cannot now be dupli-
cated. Furthermore, this flexibility applies not only to fixed
points on earth, but also to moving terminals, such as ships at
sea, airplanes, and space vehicles.
With communication satellites, then, instant and reliable
contact can be established rapidly between any points on
earth, in addition to, and well beyond, the capabilities
able land lines, microwave line-of-sight relay systems, and
other techniques. Satellites are the elements
tions revolution analogous to that in transportation resulting
from the airplane.
of the whole idea
no question that the synchronous,
or more accurately, geostationary, satellite was first proposed
by Arthur C. Clarke in an article in
“Extraterrestrial Relays.” He recognized the potential for
rocket launches based on the German V2 work during the war
and also the conspicuous advantage
the geostationary orbit.
Prophetidy, his proposal was for the
for FM voice broadcast rather than for telephone service.
Interestingly enough, Clarke also foresaw the
Manuscript received April, 1976;revised September 7, 1976.
with Satellite Systems Engineering, Inc., Washington,
electric power generated by panels
solar cells. Implementa-
idea still had to wait for the Space Age (Sputnik-
1957) and solid-state technology.
Thirty-one years have passed since
prophecy, and there
are now 22 satellite communication programs with satellites
orbit or under active construction. There are another score of
programs with earth stations only using the satellites
The Early Years
Moon reflections for radar and communication purposes
were repeatedly demonstrated in the late forties and early
fifties. In July 1954, the
voice messages were transmitted
Navy over the earth-moon path. In 1956, a
Navy moon relay service was established between Washington,
DC, and Hawaii. This circuit operated until 1962, offering
reliable longdistance communication limited only by the
the moon at the transmitting and receiving
sites. Power used was 100 kW, with 26-m diameter antennas
at 430 MHz.
A metallized balloon
the correct size, launched by a
rocket and placed in orbit can be used as a scatterer of electro-
magnetic waves generated by an earth transmitter. Part of the
energy can be picked up by receiving stations at any point on
earth from which the balloon
visible, thus obtaining a pas-
sive communications satellite system.
Through the joint action
Bell Telephone Labs, NASA, and
JPL, the ECHO experiment was performed. Successful
communications across the
were first established in early
August 1960, between Goldstone, CA, and Holmdel, NJ, at
960 MHz and 2290 MHz. The ECHO “balloon,”
in an inclined orbit at 1500-km altitude, was visible to the
unaided human eye.
Later in the same month, the first trans-Atlantic transmis-
sion occurred between Holmdel, NJ, and a French receiving
station [I]. This project alerted the entire world to
prospect of the new medium
communications although the
specific method was never exploited commercially.
Although passive satellites have infinite capability for
multiple-access communications, they are gravely handicapped
by the inefficient use
transmitter power. In the ECHO
experiment, for instance, only one part
mitted power (10 kW)
returned to the receiving antenna.
Since the signal has
compete with the noise coming from
various sources, special low-noise receivers must be used.
Luckily, the invention of the maser in 1954 and its successive
development, permitted the construction of very low-noise
receivers (with temperatures in the neighborhood
which, used with horn-reflector receiving antennas having an
about 43 m2, made possible the transmission of
teletype, voice, and pictures).