part 1

Satellite Communications

“_Our intensive research and development in the field of communications satellites have brought us to the point where we are now certain of the technical feasibility of transmitting messages to any part of the world by directing them to satellites.... The actual operation of such a system would provide a dramatic demonstration of our leadership in this area of space activity.... The direct benefits—economic, educational, and political—of this improved world-wide communication will be invaluable_.” —JOHN F. KENNEDY

Why Do We Bother With Satellite Communications?

That’s a good question to begin with. Why should we get involved in a vast, complicated program such as communication by means of man-made satellites? Is the end result really worth all the trouble that is involved? As you go further, you will see that nothing to do with satellite communications is as simple as it first seems. Even some of the easier questions have been answered only after long hours of perceptive thinking, ingenious experimenting, and shrewd deduction. They have required a lot of hard work, led to many frustrating difficulties, and cost quite a bit of money. But the answer still is yes. Despite all the difficulties, it is clear that the creation of a successful satellite communicating system is worth it.

There is a double reason for this. On the one hand, it is a technological target that is now clearly within our range. We must either reach it or let progress pass us by. Satellite communications is one field in which, as far as we know, American engineering and science have been well in the lead—so we have an even greater incentive to press on in this direction as hard and as fast as we can.

But perhaps more important than the prestige it would give our country is a second reason for our great interest in satellite communications: We need it. The world today is going through one of its great periods of change. This has caused many complications, and one of the most important is the need for much better communications between nations and peoples. By “communications” we mean all the various ways of sending information from one place to another: mail, telephone calls, business data, radio, television. The demand for these services—especially when we look ahead to the 1970’s and 1980’s—will be tremendous. Our international communications channels will be completely swamped unless some major improvements are made.

Fortunately, modern technology—given a boost by the world’s interest in rockets, missiles, and the exploration of space—has shown us one answer to this problem: the communications satellite. The conventional pathways for long-distance communication have led along the earth’s surface, under the oceans, and through the lower atmosphere. No one of these routes has yet provided all the capacity, speed, or quality we need. Present underseas cables have a limited capacity; surface travel by ship is too slow for anything but routine mail; short-wave radio is subject to distortion and noise, and the available frequencies are rapidly being used up. Although jet planes can span the oceans in a few hours with mail and such things as taped television shows, the big need will be to send information instantaneously. And the communications satellite offers us a very promising way to do this.

What a Communications Satellite Can Do

One of the attractive things about using a satellite is that it doesn’t require a revolutionary breakthrough in technical knowledge. It can employ a satisfactory means of communicating that is already available: the microwave radio relay. Today, this kind of transmission is used on a routine basis to send thousands of telephone calls and television programs across long distances. It gives high-quality performance and has a large message capacity. But there has always been one difficulty keeping us from using it for overseas communications: Extremely high frequency waves can travel almost unlimited distances, but they can go only in straight lines. This means that the curvature of the earth limits a microwave’s line-of-sight path to about 30 miles; so we must build a series of transmission relay towers spaced every 30 miles or so. Obviously, this isn’t possible when you send messages across an ocean. But, if we could find a way to send a signal high up into the sky and then bounce it from there back again to a far-off spot, we could send microwave messages great distances.

As long ago as 1945, Arthur C. Clarke, an English writer and scientist, proposed that a man-made satellite orbiting in space might be used to relay signals in this way. In 1945, of course, the very idea of getting a satellite out into space seemed utterly fantastic, and satellite communications could only be classified as science fiction. Ten years later, although Sputnik I had not yet been launched, artificial satellites were close to reality. At that time, John R. Pierce of Bell Telephone Laboratories made the first serious study of what would have to be done to build a working satellite communication system—assuming it ever became possible to put satellites into orbit. And Bell Laboratories has been interested in satellite communications ever since.

The Road to Successful Satellite Communications

With the first launching of a satellite into orbit by the Soviet Union in 1957, the real development work on satellite communications began. By 1960 Project Echo had proved that signals could be reflected off a man-made satellite and received several thousand miles away. And, in 1962, Project Telstar demonstrated to the whole world that an active repeater satellite could send telephone calls, data, and television across the ocean.

Bringing satellite communications almost to reality has required more than putting a man-made satellite into orbit around the earth. Just as important have been the invention and development of many remarkable new devices: the transistor, the solar cell, the traveling wave tube, the horn-reflector antenna, the waveguide, the solid-state maser, and the electronic computer—to mention only some of the more important. Without them it would still be impossible to find a tiny speeding object miles out in space, send signals to it, amplify them billions of times, and then return them to distant points on the earth.

Some of the new devices that help make satellite communications possible

_When you look back at it, we have seen remarkable progress in satellite communications—and work is still continuing at a fast pace. Some of the milestones have been these:_

OCTOBER 1945 Arthur C. Clarke publishes “Extra-Terrestrial Relays—Can Rocket Stations Give World-Wide Radio Coverage?” in _Wireless World_, suggesting the use of satellites for communications.

JANUARY 11, 1946 Project Diana of the U. S. Army Signal Corps bounces microwave radar signals off the moon and back to the earth, proving that relatively low power can transmit signals over very long distances.

APRIL 1955 John R. Pierce publishes “Orbital Radio Relays” in _Jet Propulsion_, pointing out the requirements for a satellite communications system.

JULY 29, 1958 Congress passes the National Aeronautics and Space Act, setting up the National Aeronautics and Space Administration (NASA), with satellite communications experimentation as one of its interests.

DECEMBER 18, 1958 Score, the first communications satellite, is launched by the U. S. Air Force. It is equipped with tape recorder units that transmit prerecorded messages back to the earth upon receipt of signals. On December 19 a Christmas greeting to the world recorded by President Eisenhower—the first message from a satellite to the earth—is transmitted. Score continues to transmit for 12 days before its batteries become too weak for further use.

NOVEMBER 23, 1959 Live voice transmission is accomplished from Bell Telephone Laboratories in Holmdel, New Jersey, via the moon to Jet Propulsion Laboratories in Goldstone, California. This is the first of 17 tests in Project Moonbounce, all using the moon as a reflector.

JULY 8, 1960 The Bell System proposes to the Federal Communications Commission a detailed plan for a world-wide communications system using active repeater satellites to provide telephone circuits and facilities for transmitting television to various parts of the world.

AUGUST 12, 1960 Echo I is launched into orbit by NASA. Project Echo carries on a large number of communications experiments and, most important, proves that it is practical to use a man-made satellite to reflect two-way telephone conversations across the United States. Echo also dramatizes the possibilities of satellites for communications. Since it is a 100-foot inflated balloon made from aluminum-coated Mylar, it is large enough to be seen by the naked eye. People throughout the world see Echo I sail on schedule across the sky in its 1000-mile-high circular orbit. Three years later, although it is now wrinkled and deflated, the balloon is still in orbit.

Project Echo provided valuable data for future work in satellite communications. It demonstrated that a passive satellite—that is, one that simply reflects the microwave signals it receives from an earth station back to another point—would work. Two-way conversations of good quality were sent between the Bell Laboratories Holmdel station and Jet Propulsion Laboratories in Goldstone, and successful transmission was made to other points in the United States and Europe. A scaled-up horn-reflector antenna proved itself. A method of receiving microwave signals that had been little used until then, known as frequency modulation with feedback (FMFB), performed very well. New types of low-noise amplifiers using solid-state masers gave excellent results. And tracking of the satellite by electronic computers, by radar, and by telescope proved to be extremely reliable.

OCTOBER 4, 1960 Courier I-B is launched by the Army Signal Corps into a 500- to 650-mile-high orbit. A sphere weighing 500 pounds and measuring 51 inches in diameter, the Courier satellite is powered by 20,000 solar cells and contains four receivers, four transmitters, and five tape recorders. It is designed to demonstrate the possibility of using active repeaters for delayed transmission of messages. Signals are received, stored on the tapes, and then retransmitted back to earth when the satellite has moved on. After 18 days in orbit, technical difficulties ended Courier’s ability to send signals, but it received and retransmitted 118 million words during its active life.

JANUARY 19, 1961 The American Telephone and Telegraph Company is authorized by the Federal Communications Commission to establish an experimental satellite communications link across the Atlantic. Two 170-pound satellites are to be launched by NASA but will be designed, built, and paid for by AT&T. This project is later given the name “Telstar.”

MAY 18, 1961 NASA selects the Radio Corporation of America to design and build the Relay satellite, which will be used to test the feasibility of transoceanic telephone, telegraph, and television communications.

AUGUST 11, 1961 NASA awards the Hughes Aircraft Corporation a contract to build Syncom, an experimental active satellite to be placed into a 22,300-mile-high orbit that will be synchronous with the rotation of the earth. (See page 37 for definitions of various kinds of satellite orbits.)

DECEMBER 20, 1961 The United Nations adopts a resolution on the peaceful uses of outer space that includes a request for world cooperation in developing a system of communications satellites. Both the United States and the Soviet Union sign the resolution.

FEBRUARY 7, 1962 President Kennedy asked Congress to pass a bill setting up a corporation to operate a satellite communications system. The proposed corporation would be owned jointly by the public at large and the country’s communications common carriers.

JULY 10, 1962 Project Telstar is successful. For the first time, voice communications and live television are transmitted across the Atlantic via a man-made satellite that picks up signals sent from one continent, amplifies them, and retransmits them to another continent. (On pages 21 to 33 we talk at further length about Project Telstar.)

AUGUST 31, 1962 President Kennedy signs the Communications Satellite Act, establishing a private corporation under government regulation—the Communications Satellite Corporation—which will plan, own, and operate a commercial satellite communications system.

DECEMBER 13, 1962 Relay I is launched by NASA. Similar in many ways to the Telstar satellite, it is an active repeater device that picks up telephone, television, and other electronic signals and retransmits them to a distant point. Relay also provides the first satellite communications link between North and South America. The satellite is a tapered cylinder 33 inches long weighing 172 pounds. A mast-like antenna at one end is used to receive and transmit a single television broadcast or 12 simultaneous two-way telephone conversations. Four whip antennas at the other end of the cylinder handle control, tracking, and telemetry—turning experiments on and off and sending information on the behavior of its components and on the amount of radiation it encounters in space. Relay is powered by nickel-cadmium storage batteries that are charged by more than 8,000 solar cells mounted on its eight sides. It contains two identical receiving, amplifying, and transmitting systems called transponders, each with an output of 10 watts.

Relay I is traveling in an orbit that ranges from 820 to 4,612 miles high, and circles the earth about every 185 minutes. Soon after it is launched, Relay’s telemetry reports trouble in the voltage regulator of one of the transponders, which causes excessive power drain. On January 3, 1963, the alternate transponder is switched on, and a successful series of tests—including live television broadcasts between the United States and Europe—begins.

JANUARY 4, 1963 The Telstar I satellite, which for almost two months could not be turned on to transmit communications signals, is reactivated by Bell Laboratories engineers. (The story of this ingenious electronic detective work is told in detail on pages 78 to 85.)

FEBRUARY 14, 1963 The first Syncom satellite is launched by NASA, but its communications systems do not operate. It is the first satellite to try for a synchronous path, revolving around the earth once every 24 hours and thus appearing to hover continuously over the same longitude. Syncom is a short cylinder 28 inches in diameter and 15½ inches long, and weighs 86 pounds. Like Telstar and Relay, it is powered by a combination of solar cells and nickel-cadmium batteries, but it is designed to handle only one two-way telephone conversation and cannot transmit television.

MAY 7,1963 The Telstar II satellite is launched for the Bell System by NASA. (See page 31.)

What About the Future?

As this is written (June 1963), second Relay and Syncom launchings are in the offing. And there are plans for more experimentation with passive satellites, including a new, more nearly rigid Echo balloon.

Further in the future, studies are going on of a proposed Intermediate Altitude Communications Satellite for military use in the 6,000- to 10,000-mile-high range (beyond that of Telstar and Relay) and Advanced Syncom, a synchronous satellite of increased capacity. Work is also continuing to acquire new technical knowledge that will be needed in the future—such as various methods of keeping satellites stabilized in space and new ways of supplying power, including improved solar cells and the use of radioisotopes.

The ultimate goal, of course, is a working commercial communications satellite system. Exactly when this will be a reality—and what form it will take—are questions whose answers still lie ahead of us.

Echo I 1000 miles 1000 miles Relay I 4612 Miles 820 miles Telstar I 3531 miles 592 miles Telstar II 6697 miles 604 miles

Project Telstar

In this section we will go into some detail about Project Telstar. We do this because much of what we learned from this project applies to the general field of satellite communications. The problems that were faced and solved are typical of the challenges that working engineers and scientists must meet today. And there is, of course, another reason to put this much emphasis on Telstar: The six case histories in Part II of our book were written by men who were involved with that project. Before reading their accounts it will be helpful for you to have some background information about it.

What Project Telstar Was Designed To Do

Even its most enthusiastic planners at Bell Telephone Laboratories never expected the sensation that Telstar caused. Although it was a deadly serious venture—one of the steps along the way toward putting together a workable satellite communications system—its success made it the inspiration, among other things, of cartoons, jokes, and a couple of popular songs. “Telstar” soon became a name recognized around the entire globe. Stories about Project Telstar appeared in newspapers in almost every language, in children’s books, in women’s fashion magazines.

What caused all this stir in the summer and fall of 1962? The answer—now that we look back on it—seems rather clear: For the first time, the whole world discovered that satellite communications was really possible—that peoples separated by oceans could now be united by live television. Space had become an adventure, not just for lonely astronauts, but for everyone right in his living room.

Project Telstar, of course, had more serious objectives:

—to prove that a broadband communications satellite could transmit telephone messages, data, and television;

—to test, under the stresses of an actual launch and the hazards of space, some of the electronic equipment that had been developed for satellite communications;

—to measure the radiation that a satellite would meet in space;

—to find out the best ways to track a moving satellite accurately;

—to provide a real-life test for the special satellite communications antennas and other ground station equipment.

To do its principal job—communications—the Telstar I satellite had to receive a signal from a ground station, amplify it, and then retransmit it on a different frequency back to other points on the earth. This signal had to be strong enough and good enough to be received and understood on the ground.

To do its secondary job—measure radiation and other conditions in space—the satellite had to be equipped with special testing devices and had to have a means of reporting facts about the environment it encountered in space and the effects of radiation on solar cells and transistors.

To let us know how well its equipment was working, the satellite had to record and transmit a large number of measurements—including such things as the temperature and pressure inside the satellite, its orientation with respect to the sun, the current and voltage in various parts of its electronic circuitry. Sending these measurements back to a ground station is called _telemetry_.

To help with tracking, the satellite had to have a continuous radio beacon signal that could be easily picked up on earth.

Finally, the satellite had to be able to control its equipment by means of signals from the ground. To keep the solar power plant from being overloaded, there had to be some way of “commanding” the satellite to turn itself on or off. As you will read later, this was the one part of the satellite that caused us the most headaches once Telstar I got into orbit.

The Telstar I Satellite—Outside

Telstar’s outer appearance is very familiar by now: a 34½-inch sphere with 72 flat facets, a double row of rectangular openings circling its center, and a short, oddly twisted antenna on one end. Of the 72 facets, 60 are used for the solar cells that are the satellite’s main power source. When Telstar is in sunlight, these cells convert solar energy into electrical power; at full capacity the 3600 solar cells will supply about 15 watts. As time goes by, this power slowly diminishes as the cells are gradually damaged by such hazards of space as radiation particles and micrometeorites. To reduce this damage, the satellite’s cells are covered with a thin layer of man-made sapphire.

Two bands of rectangular openings go around the center of the satellite. The smaller cavities, of which there are 72, are receiving antennas; the 48 larger ones are transmitting antennas. This arrangement allows the antennas to transmit and receive equally well in all directions—except directly along the satellite’s poles.

At one end of the satellite is an entirely separate receiving and transmitting antenna that takes care of all the signals needed for Telstar’s command, tracking and telemetry. The antenna is composed of four metal loops joined in the shape of a helix. It receives the important command signals from the ground that give orders to the satellite’s equipment. It sends reports back to the ground from the special radiation measuring devices and other sensors aboard the satellite, and it also transmits the continuous 136-megacycle radio beacon that can be picked up by ground equipment searching for Telstar.

Six of the satellite’s flat facets are used for special measuring devices. Two different radiation studies are made: finding out how much damage will be done to solar cells and transistors, and determining how many actual energetic particles—protons and electrons—are present in the part of space that Telstar passes through. These different jobs are done by special devices on various facets. One, for example, consists of seven identical silicon transistors, six having different thicknesses of shielding and one being left unshielded—the amount of damage done to each is recorded and reported back to earth. Devices on another facet measure the radiation damage to solar cells protected by various thicknesses of sapphire. For the second radiation experiment—particle counting—four different types of silicon diodes are used as particle detectors. These measure the energy deposited both by protons of three energy levels and by electrons as the satellite passes through belts of natural and man-produced radiation in space.

telemetry, command and beacon antenna solar cells for power supply solar cells to measure radiation damage receiving antenna transmitting antenna transistors used to measure radiation damage solar aspect cells mirror

beacon transmitter traveling-wave tube amplifier radiation measurement equipment nickel-cadmium cells (foamed)

Measuring devices mounted on the surface of the Telstar I satellite

There are two other special devices: Six single solar cells are spaced at regular intervals around the satellite; these “solar aspect” indicators report the quantity of sunlight hitting them—and thus tell the direction in which the satellite is pointing. Three highly polished metal mirrors are also placed on Telstar; flashes of sunlight reflected from them can be seen in a telescope. To give a precise indication of the satellite’s position, the data obtained from both the solar aspect cells and from the flashes off the mirrors are combined.

The Telstar I Satellite—Inside

Within the white aluminum-oxide outer shell of the satellite is crammed a complicated array of electronic equipment. Surprisingly, most of this gear has to do not with Telstar’s prime function—communications—but with its command and telemetry systems. The reason is that the satellite is an experimental device, not just a spectacular way to relay television programs. Altogether, the satellite’s various electronic circuits contain more than a thousand transistors and almost 1500 semiconductor diodes, plus a single electron tube—a traveling-wave tube used in the communications amplifier.

The satellite itself has a magnesium frame that is covered with aluminum panels. All its electronic components are inside a aluminum canister, 20 inches in diameter, attached to the interior frame by special nylon lacings that reduced vibration inside the canister during launch. When all the components and subassemblies had been carefully put in place and thoroughly tested, the canister was filled with a liquid foam called polyurethane. This material hardens into a very light and rigid solid, completely enveloping the equipment and protecting it from damage and vibration. After the canister was solidly foamed, metal domes were welded onto the ends, and it was enclosed in a many-layered blanket of aluminum-coated Mylar (the same material used in the Echo balloon). To keep its temperature properly controlled, shutters on the canister’s two ends are operated by bellows.

The satellite power system includes more than just solar cells. When operating at full capacity, the satellite’s equipment needs more energy than the 3600 solar cells can provide at one time. So Telstar also uses a storage battery made up of 19 rechargeable nickel-cadmium cells designed for this special purpose. These ensure that the satellite has a continuous and sufficient supply of power, even when all equipment is in operation or when the satellite is passing through the earth’s shadow.

Ground Stations for Satellite Communications

Project Telstar is actually an extension into space of microwave communications methods that have been thoroughly proved on the ground. For Project Echo and other early experiments in satellite communications, Bell Laboratories built a large antenna of the type known as a _horn-reflector_ in Holmdel, New Jersey. For Project Telstar, a similar but much larger antenna was designed. It was located in a relatively isolated spot at Andover, in the western part of Maine, where it would be close to Europe. The site is nicely protected by a surrounding ring of low hills—high enough to keep out interfering radio signals, but low enough not to block the satellite when it is near the horizon.

The giant Andover horn is a steel and aluminum structure 177 feet long and 94 feet high that weighs 380 tons. At one end is a giant opening of 3600 square feet; from there the horn tapers down to a cab in which the very sensitive receiver and powerful transmitting equipment is located. The entire antenna—horn, cab, and supporting framework—moves smoothly on tracks that allow it to rotate in a 360-degree circle around its vertical axis (changing _azimuth_). It also can swing about its horizontal axis from the horizon up to the zenith (changing _elevation_). Despite its size, the antenna can revolve steadily and precisely in a complete circle in just four minutes.

Signals are beamed to the satellite on a frequency of 6390 megacycles, using modified Bell System microwave equipment and a special traveling-wave tube with an output of 2 kilowatts. Signals come back on a 4170-megacycle frequency at a much lower power level—as small as a _trillionth_ of a watt. They are amplified by a ruby crystal maser that operates at the temperature of liquid helium—just a few degrees above absolute zero. The whole antenna structure and its associated equipment are enclosed in a huge “radome”—a bubble made from Dacron and synthetic rubber only a sixteenth of an inch thick but measuring 210 feet in diameter and 160 feet high. It is one of the largest air-supported structures ever erected.

The Andover ground station includes a lot more equipment—most of it having to do with tracking the satellite, computing its orbits, sending and receiving command and telemetry signals, and interconnecting the satellite with regular telephone and television land links. Most of this is located in a control building about a quarter mile from the giant radome.

A ground station very similar to the Andover installation has been built by the French National Center of Telecommunications Studies at Pleumeur-Bodou in Brittany. The British General Post Office has established a station at Goonhilly Downs in Cornwall, England, which uses a large, deep parabolic dish rather than a horn-reflector antenna. Both British and French stations participated in the first Telstar experiments. Satellite communications ground stations also have been set up in Fucino, Italy, and near Rio de Janeiro, Brazil, and others are under construction in West Germany and Japan.

The Satellite Goes Into Orbit

At 4:35 a.m. (Eastern Daylight Time) on July 10, 1962, a Thor-Delta rocket launched Telstar I into its orbit, almost exactly according to plan, from the National Aeronautics and Space Administration’s Cape Canaveral base. On Telstar’s sixth orbit around the earth—at 7:26 p.m.—the first transmission to and from the satellite took place. During this pass telephone calls, television, and photos were transmitted between Andover and Holmdel. Some of these signals were also picked up in Europe. On the next day, a taped television program was sent from France to the United States, and a live program came from England via Telstar. During the next four months, more than 400 transmissions were handled by Telstar—including 50 television demonstrations (both black-and-white and color), the sending of telephone calls and data in both directions, and the relaying of facsimile and telephotos.

In addition, the satellite performed more than 300 valuable technical tests. Almost all of them showed remarkably successful results. Radio transmission was as good as was expected. Telstar’s communications equipment worked exactly as it should, with no damage from the shock and vibration of the launch. Temperatures inside the satellite were kept under good control. The satellite was successfully stabilized—prevented from tumbling over and over—by being spun around its polar axis, with the spin rate gradually decreasing, as predicted, from its rate of 177.7 revolutions per minute just after launch. The solar cells worked almost exactly as expected. Much extremely valuable data about radiation in space was reported. The ground stations accurately traced the fast-moving satellite in almost routine fashion.

But it would be asking too much to have everything perfect. Telstar I unexpectedly met radiation in space estimated to be 100 times more potent than had been predicted. As a result, difficulties arose during November 1962 in some of the transistors in its command circuit—and on pages 78 to 85 we tell you what these problems were, how they were discovered, and what steps were taken to overcome them. Some time later the satellite again failed to respond to commands from the ground, and on February 21, 1963, it went silent.

The Second Telstar Satellite

On May 7, 1963, the Telstar II satellite was launched into an elliptical orbit almost twice as large as that of Telstar I, ranging from an apogee of 6697 miles to a perigee of 604 miles. The new satellite circles the earth once every 225 minutes. The higher altitude provides Telstar II with longer periods when it is visible at both Andover and ground stations in Europe, and keeps it out of the high-radiation regions of space for a greater part of the time. The satellite itself is much the same as Telstar I, except for a few minor changes that make its weight 175 rather than 170 pounds. Its radiation measuring devices have a greater range of sensitivity, and there are six new measurements to be reported back to earth. Telemetry can now be sent on both the microwave beacon and, as before, on the 136-megacycle beacon. To help prevent the kind of damage that occurred in the transistors of Telstar I’s command decoders, Telstar II uses a different type of transistor, in which the gases have been removed from the cap enclosures that surround the transistor elements. A simplified method of operation for the giant Andover horn antenna is now in operation, with the autotrack alone being used for precise tracking and pointing. Telstar II’s first successful television transmission took place on May 7, and a new series of technical tests, radiation measurements, and experiments in transoceanic communications has begun.

How the Telstar Satellite Works

A lot of facts and figures sometimes lead only to confusion, but these pages may help make things clearer. Here you can see—step by step—exactly what happens during a typical pass of the Telstar satellite over the Andover ground station:

1 _The satellite comes over the horizon._

2 _The command tracker, knowing from computer data the satellite’s approximate location, begins to search for its continuous 136-megacycle beacon. A quad-helix antenna (four long spirals) tracks the satellite to an accuracy of one degree._

3 _When the satellite is located, the command transmitter turns on the satellite’s transistor circuits and telemetry. The ground station then checks on the satellite’s operating condition, as reported by telemetry._

4 _The command transmitter then turns on the satellite’s traveling-wave tube, which starts the transmission of a 4080-megacycle beacon signal._

5 _The precision tracker—an eight-foot parabolic dish (known as a Cassegrainian antenna) mounted on a pylon—locates this beacon and tracks it to within one-fiftieth of a degree._

6 _The horn antenna’s autotrack mechanism, which is pointed by both the precision tracker and data from magnetic tapes, locates the satellite’s beacon signal._

7 _Now the horn antenna locks onto the satellite, with the autotrack continuing to make fine adjustments in pointing the horn._

8 _The equipment is now ready for communications signals to be sent from the two-kilowatt ground transmitter to the satellite._

9 _The satellite receives the signals and converts them down to a frequency of 90 megacycles; they are amplified in transistor circuits and converted up to a new frequency of 4170 megacycles._

10 _The signals are amplified again by the traveling-wave tube—for a total amplification of as much as ten billion times—to get a radiated power of 2¼ watts._

11 _The 4170-megacycle signals are now transmitted in all directions by the satellite’s equatorial antenna._

12 _These signals can be picked up at Andover or at any other ground station equipped with a suitable antenna that is within line of sight of the satellite._

13 _At Andover, the received signals are amplified by means of a solid-state maser and a frequency-modulation-with-feedback circuit._

14 _They can now be relayed via regular land lines to their destination._

15 _Near the end of a pass, the command tracker turns off the communications circuits and telemetry in the satellite._

16 _The satellite drops below the horizon._

Some Big Problems in Satellite Communications

We hope the last few pages haven’t given you a wrong impression of satellite communications. It is easy to assume, when we list the orderly, step-by-step progress from purely theoretical ideas to a working satellite such as Telstar, that everything has gone like clockwork. That isn’t the case at all—and in the rest of this book we are going to show you why it isn’t. Many problems had to be solved; many scientific and technological advances had to be made.

We touched on a number of the problems of satellite communications in our detailed account of Project Telstar. Most of them are not confined to that project—they are the sorts of questions that any complex advance in satellite communications will run into. We will list some of the more important ones here. Then, in Part II, we will talk about some general methods of solving scientific and technological problems. All this is a rather roundabout—but necessary—way of leading up to our main interest: the accounts by six Bell Laboratories engineers and scientists of their work to solve some typical problems in satellite communications.

The many complications of satellite communications can be divided into several groups. First of all, there are the problems involved in _fitting satellite communications into an already established world communications system_. There are, next, many problems, both small and large, in _designing the right kind of satellite_. There are the problems of _launching a satellite and getting it into the proper orbit_. There are the problems in _making sure it stays in the right orbit once it gets there_. And, finally there are the problems in _seeing that it continues to do its job reliably_.

In these five categories there are a lot of specific questions that must be answered to plan a working satellite communications system. A list of some of them follows. We haven’t attempted to cover everything, but these should give you some idea of the tasks and questions involved in planning an immense project like this.

General Problems of a Satellite Communications System _What jobs could a communications satellite do best?_ _Should it be used for television?_ _Should it carry telephone messages? How many?_ _Would it be more valuable for data transmission? Facsimile?_ _What parts of the world should be covered?_ _Can all the problems of international cooperation be solved?_ _Would a satellite that could broadcast directly to home receivers be possible?_ _What military uses could a communications satellite system serve?_ _Would a passive satellite—one that reflects signals without amplifying them—be worth developing?_ _Or should the emphasis be on active repeaters, which can receive, amplify, and retransmit signals?_ _What kind of technical standards should be set as the minimums?_ _How detrimental is time delay in sending communications to a satellite and back?_ _What kind of ground transmitters and receivers would be needed?_ _How powerful or sensitive should they be?_ _Where should they be located?_ _How many satellites would be needed?_ _How much would all this cost?_

Satellite Design Problems _How big should a satellite be?_ _What should it be made of?_ _What color should it be?_ _What kind of power supply should it use?_ _How powerful should its electronic equipment be?_ _What should be its message-handling capacity?_ _What are the best receiving and transmitting antennas to use?_ _What frequencies ought to be employed?_ _What kind of modulation should be used?_ _How should signals be amplified?_ _What kind of telemetry equipment will be needed?_ _How can radiation in space be measured?_

Launching and Tracking Problems _How big a rocket booster would be needed?_ _From what part of the world should a satellite be launched?_ _What kind of orbit should it go into? (See table below)_ _How far up should the satellite go?_ _How can a satellite be tracked once it is in orbit?_ _How do we predict future orbits of a satellite?_

Orientation and Control Problems _How can orders be given to a satellite while it is in orbit?_ _If it is to stay in a fixed attitude, how can it be kept there?_ _What can be done to keep a satellite properly stabilized?_ _Can optical measurements be made on a satellite?_ _What will be the effects of sunlight and gravity on its position in space?_

Problems of Reliability _How long will the satellite remain active?_ _What factors will affect its service life?_ _Should its equipment be made redundant?_ _What kinds of components will be most reliable in space?_ _What is the best way to test its equipment before the satellite is launched?_ _What effects will radiation in space have on the satellite?_ _How can its components be protected from these radiation effects?_ _How can it be insulated from extremes of temperature?_ _What can be done to protect the satellite from the shock and vibration of launching?_ _Will it be possible to repair an orbiting satellite?_ _Could a satellite be brought back from orbit to be repaired or salvaged?_

Types of Satellite Orbits

Circular Orbit—_an orbit whose altitude from the earth remains constant; it makes a circle that has the center of the earth as a center_.

Elliptical Orbit—_an orbit whose altitude from the earth varies from one extreme to another; it makes an ellipse with the center of the earth as one focus. The orbit’s lowest altitude is called the perigee, its highest altitude is called the_ apogee.

Equatorial Orbit—_an orbit in the plane of the earth’s equator_.

Polar Orbit—_an orbit in a plane formed by the North and South Poles_.

Synchronous Orbit—_an orbit whose period is 24 hours, the same as that of the earth revolving on its axis—so that the satellite’s and the earth’s angular velocities are the same. Although there are many possible kinds of synchronous orbits, each must have an average altitude above the earth’s surface of approximately 22,300 statute miles_.

Stationary Orbit—_an orbit that is circular, equatorial, and synchronous—so that the satellite will appear stationary from any point on the earth_.

Inclined Synchronous Orbit—_an orbit that is synchronous but not stationary, since it does not follow the plane of the equator. From a point on earth, it will appear to follow a figure_ eight _pattern about a line of constant longitude_.