September 1962 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
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This 1962
Radio-Electronics magazine article reports on the design, launch, and operation
of the world's first satellite capable of relaying live television programs -
Telstar 1. It's 2 hour and 40 minute orbital period in a 600-mile-high
elliptical orbit was inclined at about 45° to the equator. Telstar 1 enabled
real-time transcontinental TV, facsimile, telephone, and telegraph
communications. American Telephone & Telegraph (AT&T) and Bell Telephone
Laboratories (Bell Labs) designed and built the Telstar 1. Six ground stations
in the U.S. and in Europe tracked Telstar 1. The satellite went out
of service in November, just a couple months after this article, due to a
suspected transistor failure associated with its time in the
Van Allen radiation
belt. That was before Telstar 2 launched in May of 1963 (info not available
at the time of this article), so the transcontinental capability was temporarily
lost until then.
Telstar - Giant Step into the Future
By Larry Steckler, Associate Editor
Eighty-six years have passed since Alexander Graham Bell received his patent
for that marvelous invention, the telephone. And right now, 600 miles over our heads,
a man-made star, the Telstar satellite, is circling the earth once every 2 hours
and 40 minutes. It is the first experimental link in a forthcoming chain of satellites
which will one day give the world a net of transcontinental communications Alexander
Graham Bell probably never dreamed possible.
Now only a few short days since its successful launch on July 10, 1962, successful
transmission of TV programs from the US to Europe and from France and England to
the US have already been completed. Also successful has been a test of trans-Atlantic
satellite telephone calls. The TV programs were seen on TV screens across the US
and Europe. Telstar definitely works.
TV or any other wide-band communications channel is normally restricted to vhf
or higher. This is mandatory because of the required 3- to 6-mc bandwidth required.
At lower frequencies, this much space is just not available. As soon as we go to
vhf, however, we run into the "line-of-sight" problem. Radio signals at these frequencies
go straight through the ionosphere and are not reflected to earth as are lower-frequency
radio signals. This restricts TV to about a 50-mile range-the distance to the horizon
as seen from the top of a TV tower. By the same token - extreme bandwidth - standard
telephone cables are not satisfactory for TV either. The lines will not pass the
entire signal without noticeably injuring its quality. With Telstar in the sky,
we get a 600-mile-tall TV relay tower that has enough line of sight to cover a distance
measured in thousands of miles, making intercontinental TV a practicality.
![Block diagram of the Telstar broad-band communications circuit - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-2_small.jpg)
Block diagram of the Telstar broad-band communications circuit.
![Final adjustments are made in Bell Telephone Labs - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-3_small.jpg)
Final adjustments are made in Bell Telephone Labs.
![How satellite system works - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-4_small.jpg)
How satellite system works: Quad antenna on left picks up satellite
beacon. Command is sent turning on communications gear. Precision tracking antenna
picks up 4080-mc signal. As this antenna tracks, so does big horn. Horn transmits
to satellite and it relays data over horizon to Europe.
![Electron tube carried by Telstar - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-5_small.jpg)
Only electron tube carried by Telstar is this 1-foot-long traveling-wave
tube.
![Receiving end of communications link is this tremendous horn antenna - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-6_small.jpg)
Receiving end of communications link is this tremendous horn
antenna.
![Control room at the Andover ground stations keeps careful track of operation - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-7_small.jpg)
Control room at the Andover ground stations keeps careful track
of operation. Consoles (foreground) track Telstar. Monitors (right) evaluate signal
levels during transmit and receive tests to satellite.
![Ring connects to one of the antenna belts around satellite - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-8_small.jpg)
This ring connects to one of the antenna belts around satellite.
![Quad-spiral automatic tracking antenna first detects Telstar - RF Cafe](images3/telstar-satellite-radio-electronics-september-1962-9_small.jpg)
Quad-spiral automatic tracking antenna first detects Telstar
as it comes over horizon.
The particular frequency used was selected as the one that presented the fewest
problems in the construction of the satellite.
Telstar's basic capability is to handle either one TV channel, 600 one-way simultaneous
telephone conversations, or an equivalent bandwidth of facsimile or other radio-frequency
transmission over a wide-band FM channel.
The system sounds simple. Telstar receives radio signals beamed at it from a
station on the ground, amplifies them some 10 billion times, and immediately rebroadcasts
them on another frequency to a ground station on another continent. Sounds simple,
but it takes a heap of electronics packed into a comparatively tiny package to make
it all possible.
Overall, the satellite appears spherical. Actually it has 72 flat faces, like
facets cut on a diamond. It is 34 1/2 inches in diameter and weighs 170 pounds.
Two rows of antennas encircle Telstar's waist, like the equator encircles a globe
of the earth. One set is for picking up signals beamed at the satellite, the other
is for retransmitting these signals.
In the instrument package carefully tucked away in Telstar's center are 1 electron
tube, 1,064 transistors and 1,464 diodes. Also, all the associated circuitry that
goes with these components. Power is supplied by 3,600 solar cells distributed over
the satellite's outer skin. These cells convert sunlight into electricity at a rate
of 15 watts while Telstar is on the sunny side of the earth, but this is expected
to drop to around 11 watts by the end of one year in orbit.
The drop will be caused by radiation and tiny micrometeroroids reducing the efficiency
of the cells.
Their output would drop much more rapidly if they had not been protected with
a thin tough layer of synthetic sapphire.
The 15 watts delivered by the solar cells goes to charge 10 nickel-cadmium batteries.
These batteries supply the power to Telstar's electronic circuits to keep them working.
The Communications Network
Ground stations send signals to Telstar on 6390 mc. Telstar retransmits them
on a lower frequency - 4170 mc. While the bandwidth is 50 mc for Telstar and 100
mc for the ground stations, effective bandwidth is only 3 or 4 mc. The rest of the
space is taken up by tracking, telemetering and control signals.
Aboard the satellite, the incoming 6390-mc signal is mixed with the 6300-mc output
of a crystal oscillator to produce a 90-mc i.f. signal. This is done to put the
signal in a low enough frequency range so it can be easily handled by the transistor
circuits. Fourteen germanium diffused-base transistors amplify the i.f. signal about
1,000,000 times. An automatic gain control holds the amplification within preset
limits, so that no matter how strong a signal Telstar receives, its retransmitted
signal has a strength of about 2 1/4 watts.
The amplified i.f. signal is then mixed with the output of another crystal-controlled
oscillator to produce the 4170-mc signal the satellite transmits.
Just before transmission, the signal is fed to the only vacuum tube Telstar carries
for final amplification. This special foot-long traveling-wave tube amplifies the
broadband signal an additional 5,000 times. The traveling-wave tube has a double
purpose. Along with the communications signal, it transmits a single-frequency 4080-mc
signal. This low-power transmission, a mere 1/200 watt, acts as a beacon for the
tracking antennas and devices to home on.
Telemetering and Control
Telstar is an experimental prototype and as such carries a number of research
experiments. Among the 115 conditions checked are density and energies of free protons
and electrons, effects of radiation on semiconductors, temperature of the electronic
chassis, how much sunlight is hitting the satellites skin at various points, and
currents and voltages on many of the electronic components.
Getting these measurements to the ground station is the task of a separate 136-mc
transmitter that has a 1/4-watt output. It transmits constantly, and since it does,
acts as a secondary beacon to help the ground stations track the satellite.
When a radio command (over still another circuit) is sent to the satellite from
the ground, Telstar adds a group of coded telemetry signals to the 136-mc beacon.
It continues sending this combination signal until it is told to stop. In each 1-minute
period of transmission, everyone of the 115 measurements is sent once.
Because of the need for constantly transmitting the 136-mc carrier to act as
a beacon, an unusual modulation system is used to add the telemetry data. The telemetry
pulses (PCM) frequency-modulate (FM) a 3-kc subcarrier, plus or minus 225 cycles.
This signal is then used to amplitude-modulate (AM) the 136-mc carrier. This gives
a total complex transmitted signal called a PCM-FM-AM transmission.
If all of the electronic systems aboard Telstar were operated simultaneously,
they would draw current from the batteries faster than the solar cells can deliver
it to the batteries, and in a short time the satellite would be silent. So to conserve
power, the communications section of the satellite can be turned on and off by remote
commands from the ground stations. This requires still another radio channel.
Telstar's command system is made up of two identical channels of equipment so
if one should fail, the other will take over. There are two radio receivers set
to pick up 120-mc commands and two decoders that change these commands into usable
instructions. Of course, there are also the relays that actually do the work.
The control system can turn the traveling-wave tube on and off. Similarly it
can switch on and off the various measurements being made inside the satellite,
telemetry receivers and transmitters, and of course, the communications equipment.
The last piece of gear aboard Telstar is an automatic cutoff device that turns off
all satellite systems after two years. This makes the channels being used by Telstar
available for other satellites.
Down on the Ground
Nestled in the middle of a ring of mountains near Andover, Me., is the United
States' "Earth Station for Communicating by Satellites." It comprises two major
buildings - a huge dome housing the tremendous horn antenna and, a quarter mile
away, the control building where all the computers and major control equipment are
located. This Earth Station is tied in to the US telephone network and is this country's
end of the satellite link.
Inside the radome is the largest horn antenna ever built. Its opening has an
area of 3,600 square feet and is designed to scoop up the billionth of watt from
Telstar that is available to it. The entire antenna structure weighs about 340 tons,
has an overall length of 177 feet and carries along with it two houses full of transmitter
and receiver gear.
As Telstar rises above the horizon the ground station goes to work. A quad-helix
(spiral) common-tracking antenna picks up Telstar's 136-mc beacon and locks in on
the satellite. As it follows the satellite, the big horn acts as a slave and also
starts tracking Telstar. This is handled by a bank of computers in the control building.
Once the tracking antenna has locked in, a 120-mc control signal is sent to Telstar
to turn on its communications equipment and the 4080-mc precision tracking signal.
As soon as Telstar responds by starting transmission, another tracking antenna,
this one an 8-foot dish, tracks the satellite with even greater precision. Again
as this antenna tracks, so does the big horn.
The horn itself takes care of final tracking accuracy, for, should the horn be
off just a fraction to any side of the satellite signal, the 4080-mc tracking frequency
is propagated in a slightly different manner through the waveguide connected to
the throat of the horn. This produces an error signal that corrects the tracking.
The system is called the vernier auto-track.
While the tremendous size of the receiving antenna makes it extremely sensitive,
this alone is not enough. For even greater sensitivity, a maser receiver using a
synthetic ruby crystal cooled by liquid helium to -456°F is located in the throat
of the horn.
To add to the sensitivity of the maser, a special frequency-modulation feedback
circuit is also used. Developed in 1930, it acts as an automatic tuning device.
It tunes a narrow-band receiver to the particular frequency being transmitted by
Telstar at a particular instant, even though the signal can vary over a 25-mc wide
band of frequencies. By doing this, background noise is drastically reduced because,
instead of getting all the noise of a 25-mc wide channel, only the noise in the
particular narrow band being received is picked up.
In this manner, communications continue as long as Telstar is above the horizon.
Just before the satellite disappears over the horizon, a command signal is sent
to turn off the communications gear and conserve power.
In future years, when a series of these satellites are in orbit, multiple antennas
will be used at the ground station and before one satellite sets another will rise.
The transmitted signal will be switched automatically from one to the other as necessary
to ensure continuous communications.
The Telstar satellite was conceived, designed and built by the American Telephone
and Telegraph Company as was the ground station at Andover, Maine. The firm also
paid NASA for the cost of the rocket and launching from Cape Canaveral that put
the satellite into orbit. While trans-Atlantic TV is not entirely new, this is its
first practical application. On Feb. 8, 1928, John Baird sent the first TV picture
from England to the US on 45 meters. Also, during the last sunspot maximum, there
were reports of European TV signals crossing the Atlantic on 50 me (Radio-Electronics,
September 1958, page 52).
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