Telstar - Giant Step into the Future
September 1962 Radio-Electronics

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.

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.

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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