Satellite Communications Physics

BY MEMBERS OF THE STAFF OF BELL TELEPHONE LABORATORIES
EDITOR: RONALD M. FOSTER, Jr.

How scientists and engineers use basic physical principles to solve some of the problems in communicating by means of man-made satellites

© 1963 Bell Telephone Laboratories, Incorporated

All rights reserved.
Permission to reproduce any material contained in this book must be obtained, in writing, from the publisher.

Library of Congress Catalog Card Number 63-21667

Printed in U.S.A.

Foreword

John R. Pierce
Executive Director, Research,
Communications Principles and Communications Systems
Bell Telephone Laboratories

When I first talked about the possibilities and advantages of communications satellites to the Princeton Section of the Institute of Radio Engineers on the evening of October 14, 1954, I was diligent in my analysis and enthusiastic in my presentation but, I must confess, a little skeptical as to whether or not anything would come of the idea.

Still, others and I at Bell Laboratories remained interested, and, after the launching of Sputnik I on November 3, 1957, and of Explorer I on January 31, 1958, we worked actively toward satellite communications experiments. This led to our work with Echo I (launched August 12, 1960) and finally to the launching on July 10, 1962, of Telstar I—that satellite which became, in the words of Queen Elizabeth, “the invisible focus of a million eyes.”

This work on communications satellites has been a grand exploration and opening up of a hitherto dark continent of science and technology. My courageous friends at Bell Laboratories encountered therein surprising difficulties and perplexing problems which I had never dreamed of, and these intrepid and indefatigable adventurers grappled with them and mastered them all.

Now you, who have in your own homes seen pictures transmitted across the ocean by satellite, can learn first hand from the men who worked on hard and varied technical problems just what these problems were and how they were solved. And, by reading you can find out what sort of knowledge, training, and habits you yourself will need if you wish some day to adventure into those undiscovered or unexplored fields of technology which will be new and exciting when Telstar has become old hat.

June 5, 1963

High School Science Teaching Aids Prepared by Bell Telephone Laboratories

Classroom units including books, motion pictures, and demonstration devices: Similarities in Wave Behavior Ferromagnetic Domains The Speech Chain

Bell System Science Experiments for the advanced student: From Sun to Sound Solar Energy Experiment Speech Synthesis

Information about these and other materials is available to teachers from local Telephone Company business offices

Contents

[Introduction] 8 part 1: [Satellite Communications] 10 Ronald M. Foster, Jr., Editor part 2: [Satellite Communications Case Histories] 38 1. Franz T. Geyling [How Do We Calculate a Satellite’s Orbit?] 41 2. Peter Hrycak [What Color Should a Satellite Be?] 48 3. Jeofry S. Courtney-pratt [How Can We Make Optical Measurements on a Satellite?] 53 4. Kenneth D. Smith [How Do We Keep Solar Cell Power Plants Working in Space?] 62 5. Peter D. Bricker [Would Time Delay Be a Problem in Using a Synchronous Satellite?] 70 6. E. Jared Reid [How Can We Repair an Orbiting Satellite?] 78 [Suggested Reading] 87

Introduction

Despite the title, this is not a physics textbook, and it will tell you only part of the fascinating story of satellite communications. However, we have tried to tell this story in a rather special way. Part I explains why we are so interested in communicating by means of man-made satellites, describes the important events in the progress of satellite communications (with special emphasis on Project Telstar), and points out some of the very knotty problems that had to be solved. Then, in Part II, we pick out six typical satellite communications problems and go into them more deeply.

These case histories are examples of the things scientists and engineers are constantly faced with. To narrate them we have called on six experts—Bell Telephone Laboratories engineers and scientists who actually have been working on the problems. The second half of our book is taken up by their accounts of their own personal experiences. We hope that reading them will give you an insight into what it is really like to be a scientist or engineer working in a laboratory on an important new venture into the future. We hope you will see that what they do is not all excitement and glamour. It involves hard work, ingenious thinking, and plugging away at tough problems. But this is what scientists and engineers enjoy—along with the excitement and glamour, of course.

Only a part of what our authors talk about can strictly be called “physics”—it is also engineering, chemistry, mathematics, and even psychology. But almost all their work is based—when you get down to fundamentals—on basic principles of classical physics taught in high school.

Now, a word of caution. In Part I, in talking about satellite communications in general, we have kept things at a level that should be understandable to almost everyone—even those who have never taken a high school science course. But we warn you this isn’t true of Part II. Our authors have tried to tell their stories as carefully and as logically as possible, but some readers may have trouble in following all they say. This we expect. We haven’t tried to sugar-coat or gloss over any essential details of the problems or their solutions. We don’t want you to think that solving them was any easier than it actually was. And, since this is not intended as a textbook, we may sometimes omit elementary material and go right to the heart of the matter.

When this book was written satellite communications was still a technological infant. It is growing and changing so swiftly that much of what we say may soon be out of date. That can’t be helped, of course, and we ask you—who may be reading it long afterward—to be tolerant. Our problems may well be forgotten when new, more sophisticated ones appear. But we are dealing here in methods, not in history; the ways in which these problems were attacked are just as lasting and important as the problems themselves.

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.

Curvature of the earth requires microwave towers to be about 30 miles apart

Microwaves sent via an orbiting satellite can travel vast 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

horn-reflector antenna

traveling-wave tube

transistor

solar cell

solid-state maser

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.

The orbits of four communications satellites vary in size and shape

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.

On July 10 and 11, 1962, people on two continents saw these scenes on television at the same time, with the aid of the Telstar I satellite

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.

The Telstar I satellite—outside

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

The Telstar I satellite—inside (looking at the electronics canister from the top down)

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

Measuring devices mounted on the surface of the Telstar I satellite

particle detectors used to count protons

transistors used to measure radiation damage

solar cells used to measure radiation damage and as solar aspect indicators

particle detector used to count electrons

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.

After all electrical tests had been made on the satellite’s components, the electronics canister was filled with liquid polyurethane foam, using this specially developed foam machine

The giant horn antenna at Andover, Maine

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.

The French radome looms over the Brittany countryside

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.

New gold-domed device on the Telstar II satellite can measure electrons in an energy range from 750 thousand to 2 million electron volts.

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.

part 2
Satellite Communications Case Histories

Six Typical Problems

The questions we listed in [Part I] cover a very broad area of science and technology. Their answers involve, more than anything else, physics, electrical engineering, and mechanical engineering. Some, however, also require that the men who work on them know chemistry, metallurgy, mathematics, and occasionally even biology, psychology, geography, and economics.

We obviously can’t show you how all the problems in [Part I] can be solved. Rather, we have picked six of them as examples. They are not necessarily the most important ones, but they seem to us to be typical of what engineers and scientists working in the satellite communications program actually have to do. These are the six problems we will be talking about at length:

• How do we calculate a satellite’s orbit?
• What color should a satellite be?
• How can we make optical measurements on a satellite?
• How do we keep solar cell power plants working in space?
• Would time delay be a problem in using a synchronous satellite?
• How can we repair an orbiting satellite?

As you can see, we have picked problems that offer a good deal of variety. Some of them have been satisfactorily solved; for others the solutions are not yet complete. Some deal with basic scientific research; others are much more concerned with the engineering applications of technical knowledge. Some were solved by careful, logical thinking; others were solved almost by accident. Some deal with a particular immediate task (in this case, Project Telstar); others are more concerned with general planning for satellite communications.

At the Foundation: Basic Physical Principles

Despite these many important differences, there is one common thread running through the solving of all the problems we have chosen. The men who have been working on them had to know some basic principles of classical physics—principles that most of them first learned in their high school physics classes. You can’t, for example, calculate a satellite’s orbit without knowing Newton’s Laws of Motion. You can’t make optical measurements on a satellite without knowing the law of reflection of light. You can’t decide what color a satellite should be without knowing the law of heat exchange.

To emphasize the importance of a solid grounding in basic physical principles, we have tried to have our problems touch on most of the general areas of physics: mechanics, heat, sound, light, electricity and magnetism, electronics, the properties of matter, atomic physics, physics of the solid state. But most of them, of course, are not limited to just one of these—they cross the lines of a number of areas. For instance, the problem of keeping solar cell power plants working in space involves laws of heat, mechanics, and atomic physics, as well as physics of the solid state. And, in studying the perception of time delay, we even branch out into experimental psychology.

Problem-Solving Techniques

When you start to solve a problem in science or engineering you can go about it in several ways. In some cases you have no choice: There may be only one practical method of doing the job. Other times, there may be several ways to attack the problem. You may try one, find it to be unfruitful, and then work on another approach. You will see both these methods of attack in the case histories we present in the next chapter.

Here are some of the techniques of scientific problem solving that we will be discussing:

• Applying basic principles directly. In answering the questions “How do we calculate a satellite’s orbit?” and “What color should a satellite be?” the successful procedure was to begin with basic known concepts and use them in a new field.
• Adapting known devices. To answer the question “How can we make optical measurements on a satellite?” the story was somewhat different: Devices that already had been developed—mirrors, telescopes, and cathode ray tubes—were utilized in a new and different way.
• Developing entirely new equipment. Another question—“How do we keep solar cell power plants working in space?”—deals with an entirely new area of investigation; it could only be answered by perfecting some entirely new techniques.
• Experimentation. Sometimes there is no definite answer to a problem. In the case of “Would time delay be a problem in using a synchronous satellite?” investigation is still going on. Our report tells of one set of experiments that helped add to our information about this problem—the final answer, if there is one, must come later. But, as with many problems, experimentation continues.
• “Detective work.” Our sixth problem is really a unique one—and, for want of a better way to describe it, we use this title. It tells how the problem of “How can we repair an orbiting satellite?”—something never even attempted before—was ingeniously solved by means of scientific deduction hundreds of miles away from the problem itself.

CASE HISTORY NO. 1
How Do We Calculate a Satellite’s Orbit?

Franz T. Geyling
Mechanics Specialist—Head, Analytic and Aerospace Mechanics Department

THE PROBLEM

Before you can do anything with a communications satellite you have to know where it is at every instant of its motion around the earth. In other words, you need to know its orbit quite accurately. When you know this, you can predict when a particular station on the ground will be able to see the satellite and communicate with it. You also can tell when two or more stations can see the satellite simultaneously and communicate with each other. And you can estimate how many satellites will be needed to provide a group of ground stations with enough working time to maintain a communications service. This last, after all, is the ultimate goal of all our efforts in the communications satellite field.

In determining a satellite’s orbit, we find that we must do three things:

• We must obtain information on the position—and perhaps also the velocity—of the satellite whenever possible by observing it with scientific instruments.
• We must use this information to determine the satellite’s orbit at the time of the observations.
• We must be able to tell how this orbit changes between the times when we observe it, because a satellite’s orbit does not remain constant with the passage of time.

In the case of the Echo I satellite (see [page 16]), we engaged in the first and third of these activities. We had many chances to follow the satellite with our radars, and we could speculate how its orbit was changing through the months. In the case of the Telstar I satellite, we engaged in all three kinds of activity. We shall take a look at these problems in the sequence in which we came across them, for both the Echo and Telstar satellites.

How We Track Satellites

We collect on the ground most of the information to calculate a satellite’s orbit, using optical instruments or radar equipment. Following a satellite through the sky is called tracking; in the early days after the first Sputniks, some of this tracking was done with the naked eye or with very simple telescopes by the Moonwatch teams. Many of you may have observed Echo I on a clear night without any kind of instrument.

Figure 1

satellite horizon elevation azimuth North East

If we use a telescope, we note the time of the observation and we usually take a photograph of the satellite. We locate the satellite in terms of the two angles shown in [Figure 1]. One of these is the elevation angle—the number of degrees a telescope must be tilted above the horizon to see the satellite. The second is the azimuth angle—the number of degrees between the plane in which we measure the elevation angle and the north direction. Of course, we can also point a radar antenna at the satellite in the same manner. The radar can receive a signal transmitted by the satellite, or else it can send a signal to the satellite and watch for the reflected waves that eventually return. In the latter case, the satellite must have sufficient surface area to produce an adequate reflected signal. These two kinds of precision tracking were both possible with Echo I. Radar can also do something that optical equipment usually can’t do: measure the distance out to the satellite.

The Basic Physics of Satellite Motion

Figure 2

earth north pole satellite θ = n · t

The Echo I satellite was launched into a circular orbit inclined at an angle to the plane of the earth’s equator. In [Figure 2] this equatorial plane intersects the plane of the satellite orbit along the line OPM. The point O represents the center of the earth, the point M is on the satellite orbit, and the Point P is on the equator. At any instant, the satellite may be located in its orbit by the angle θ, which is measured between the line OM and the line OQ, where the point Q is the satellite’s location. If the satellite moves in a circular orbit, as in this case, the angle θ is proportional to time. That is, we can write θ = nt. We call n the angular speed of the satellite; one way of measuring this is in degrees per second.

Thus, the satellite is whirling at a constant speed about the earth like a stone tied to a string. Let us examine the physics of this situation a little more closely with the help of [Figure 3]. If the satellite is moving with the velocity v, then we know that the centrifugal force acting on it is

where m is the mass of the satellite and r is its distance from the center of the earth. Obviously, no string ties the satellite to the earth, but the force of gravitational attraction between the earth and the satellite has the same effect. Newton’s law of mutual attraction tells us that this force is proportional to the product of the two masses divided by the square of the distance between their centers, or

where k is a constant that essentially represents the mass of the earth.[1] Newton’s law also tells us that this force will be pointing toward the center of the earth if the earth is spherical. When the satellite is in circular motion, the centrifugal force and the gravitational force must balance each other. Hence we have

and from this we can solve to find that the velocity of the satellite must be equal to

v = √

In the case of the Echo I satellite, which was designed to have a radial distance of r = 5000 miles, this velocity amounts to about 4.4 miles per second. The time for one revolution in orbit is obtained with the formula

T =

For the Echo satellite this time, T, turns out to be just about two hours.

Figure 3

Calculating the Orbit of Echo I

These basic physical principles of satellite motion can give us many useful answers. They tell us how fast we must move a precision tracker to follow the satellite through the sky, how much time a satellite will spend above the horizon, and how long will be the time from one chance of seeing it to the next. However, in the Echo project we were not merely concerned with planning our experiments from hour to hour; we also needed to know how the satellite would move for weeks and perhaps months in advance. When you study the motion of a satellite over such a length of time, you discover that its circular orbit will not remain the same as it was at launch. This fact had been observed on other satellites and was to be expected also with Echo.

In everything we have said so far it was assumed that the earth was a perfect sphere, which is the way a geographer’s globe presents it to us. In reality, the earth is somewhat flattened, with its diameter from the north pole to the south pole being somewhat shorter than its diameter at the equator. One way of looking at this is to visualize the earth as a sphere with some material added in the equatorial zone, which we may call equatorial bulge. This bulge causes Echo’s orbit to have a slow “wobble” about the earth’s polar axis, somewhat like that of a spinning top.

Another force that makes the satellite’s orbit shift slightly is the faint pressure caused by the light from the sun. Although this pressure is much too small for us to perceive without the help of very delicate instruments, it is enough to affect a satellite, which has nothing to support it in space and is exposed to solar pressure for a very long time. Since the Echo balloon is a plastic sphere, 100 feet in diameter, that weighs only a little more than 100 pounds, the light rays striking its surface are enough to cause a second “wobble” effect. This wobble centers about the line from the earth to the sun. Light pressure also forces the orbit to go slightly out of round from a perfect circle, and other gradual effects on the satellite’s orbit are caused by the gravitational attraction of the moon and the sun.

All these disturbances are ever-present and act simultaneously, and a satellite’s total response to them is very complicated. Fortunately, however, most of the changes take place at a very slow and uniform rate, and we can predict them fairly accurately.

Calculating the Orbit of Telstar I

Figure 4

In Project Telstar we had to calculate the satellite’s orbit from observations made by our precision trackers. This introduced a few problems in addition to the ones we encountered with Project Echo. In the first place, the orbit of the Telstar satellite is a elongated ellipse, as indicated in [Figure 4], rather than being almost circular, as in the case of Echo I. We mentioned earlier that a precision tracker can furnish data on a satellite’s elevation angle, E, and azimuth, A (see [Figure 1]). It can also give us a reading for ρ, the distance from the tracker to the satellite ([Figure 4]). If we know the position of the tracker on the earth, we can reduce the quantities A, E, and ρ to the angle θ and the distance r (measured from the center of the earth to the satellite). These two quantities locate the satellite in the plane of its orbit, but in order to describe its position completely we must also specify this orbital plane. In [Figure 5] the orbital plane is shown as a shaded surface, with θ and r being the same as before. You will recall that the line OM represents the intersection between this plane and the equatorial plane; we call the angle i between the two planes the inclination of the orbit. Finally, we have the angle Ω between the line OM and some line OA to the point A, which we can choose as any convenient spot in the equatorial plane. Now we have specified the orbital plane completely. The point A can be found from day to day by fixing its position relative to a certain star in the sky.

Figure 5

Figures [4] and [5] tell us something about the geometry of the satellite’s position in space, but for the complete story we must also give the time at which it can be found there. For this purpose, there are some astronomical laws that relate position on an elliptic orbit to time. Two of these are illustrated in [Figure 6]; in looking at this figure, you should imagine that you are standing off to one side of the orbital plane to get a good view of the entire orbit. The longest dimension of the ellipse, 2a, is called the major axis; this dimension is related to the satellite’s period—the time it takes to go once around the ellipse. More than three hundred years ago the astronomer Johannes Kepler observed that the period T, of an ellipse is

T = 2π√

where k again was (using Newton’s work) essentially the mass of the earth.

Figure 6

Instead of a complete revolution, we may only be concerned with part of one orbit. Let’s say that this part lies between the two positions P₁ and P₂ that the satellite occupies at the two times t₁ and t₂ (see [Figure 6]). Then another of Kepler’s laws says that the ratio between the time difference t₂ - t₁ and the period T equals the ratio between the sector of the ellipse OP₁P₂ and the area of the entire ellipse.

Now let us see how we can use the quantities r, θ, i, and Ω as well as Kepler’s two time laws to determine the motion of the satellite in space. Suppose that we have made observations of the Telstar at two times t₁ and t₂ and that we have measured its distance along lines ρ₁ and ρ₂ in [Figure 7]. In other words, we know that at these two times the satellite was at the points P₁ and P₂. Since three points determine a plane, we know in this case that P₁, P₂, and O define the satellite orbital plane. Knowing this, we can now calculate the angles θ₁ and θ₂, the distances r₁ and r₂, and the angles i and Ω. (The detailed formulas for this are derived from analytic geometry.)

Figure 7

However, we still do not know the length and the width of the particular ellipse the satellite is following and how this orbit is oriented within the orbital plane. Let us imagine again that we can stand off to one side of the orbit and take a good look at it; [Figure 8] shows us what we would see. There are the two points P₂ and P₁ at which we have observed the satellite. We know the positions of these points relative to each other and in relation to the center of the earth, because we have already calculated r₂, r₁, θ₂, and θ₁, But any number of ellipses could be made to pass through these two points. Some might be very large, others might be so narrow that they would intersect the earth and thus be impossible. However, only one of these ellipses will satisfy the time difference that we observed between P₁ and P₂. In other words, the shape and period of this particular ellipse must be such that it will cause the satellite to pass through P₁ and P₂ in exactly the time interval t₂ - t₁. If we work out our time formulas, we will convince ourselves that there is only one such ellipse. When we have found it, we have determined the orbit of the Telstar satellite from the two observed positions and times.

Figure 8

Figure 9

In principle then, we could keep track of the Telstar satellite by making a pair of observations P₁ and P₂, and then predicting ahead a short segment of an orbit that is the ellipse we have computed. After a while we must verify this ellipse with two more observations P₃ and P₄, predict ahead over another segment, and verify again with P₅ and P₆ (see [Figure 9]), The reason we have to keep taking new measurements is that the elliptic orbit does not remain the same. As we discussed in connection with Echo I, the orbital plane will “wobble” about the earth because of the equatorial bulge. We also know that the orbit’s major axis will revolve within the orbital plane. As we have seen before, these effects are small and can be represented by appropriate mathematical formulas. If we calculate them, we will see the connection between one pair of observations and a later one, and eventually we can increase the time interval between successive pairs of observations. There are also mathematical formulas that we can use to predict the position of the satellite for many revolutions in its elliptic orbit.

In order to predict orbits successfully, we must also realize that the measurements we obtain from a precision tracker, such as the angles A and E and the distance ρ, are always subject to small inaccuracies. Thus it is not really possible to take just two measurements like P₁ and P₂ and determine a satisfactory orbit from them. In reality, our tracker takes many readings, and these are averaged to give adequate information about the orbit. Therefore, the picture we have in mind is not quite like [Figure 7], but rather like [Figure 10]. Here the trackers have established a series of points that are somewhat scattered, and by taking averages we can calculate an orbit that passes through them in a smooth fashion.

The trackers we have mentioned so far have given us azimuth and elevation angles and also the distance to the satellite at every instant. Sometimes we must use simpler instruments that do not yield all this information. They might, for instance, only give us the two angles. The mathematics of calculating an orbit from such measurements is somewhat different, but the process is fundamentally the same as we have discussed here.

When you do these calculations for the Telstar satellite from one day to the next—and especially if you have more than one satellite to keep track of—the amount of work will become quite large. Nowadays our calculations are done for us on electronic computers, which both receive information from the tracking instruments automatically through Teletype or DataPhone channels and send back information concerning future positions of the satellite to the ground stations. There are still quite a few problems to be solved, and we are presently working on ways of making all this equipment perform the orbit predictions for the Telstar satellites automatically and efficiently.

Figure 10

Franz T. Geyling was born in Tientsin, China, and received a B.S. in 1950, an M.S. in 1951, and a Ph.D. in 1954 from Stanford University. He joined Bell Telephone Laboratories in 1954, and has been engaged in celestial mechanics studies of rockets and satellites, as well as stress analysis of submarine cables.

CASE HISTORY NO. 2
What Color Should a Satellite Be?

Peter Hrycak
Mechanical Engineer—Member of Staff, Electron Device Laboratory

THE PROBLEM

It is important for a satellite to stay at the proper temperature while it is orbiting in space. The instruments aboard it must continue to operate properly, and one way of insuring this is to keep them from being exposed to extreme heat or cold. We can, of course, regulate a satellite’s temperature somewhat with various kinds of devices, and we can see that one of its ends does not point towards the sun for too long. But in designing the Telstar satellite we also wanted to control temperature in an easier way: by covering the satellite’s external surface with material with the best properties—including the right color—for maintaining its over-all temperature at the right level.

The Radiation of Heat

A satellite’s temperature is determined by the balance between the heat that enters the satellite and the heat that leaves it. This means that we must be concerned with how heat is transferred. Heat can be transferred in three ways: by conduction, when two bodies are in direct contact and their molecules collide; by convection, which utilizes the movement of warm currents in a fluid; and by radiation, in which heat energy travels as electromagnetic waves at the speed of light. With a satellite, we are concerned only with the last of these, since the only way energy can be gained or lost in space is by radiation.

In the transfer of heat by radiation, the surface of the heated body—such as a satellite—is very important. All energy gained must be absorbed at the surface; all energy leaving must be emitted at the surface. So the physical properties of this surface control how energy is absorbed and how it is emitted. The origin of the radiant energy is vitally important; most surfaces, for instance, will behave differently when exposed to solar radiation from the sun’s temperature of 10,000° Fahrenheit than when exposed to radiation from nearby objects at room temperature.

Absorptivity and Emissivity

The physical property of a material that controls the way it absorbs radiant energy is called its absorptivity, and the property that controls its emission of energy is its emissivity. For absorptivity we use the symbol α; for emissivity we use the symbol ε.

When radiant energy reaches a surface, only a certain part of it is absorbed; the rest is either reflected, just as light rays are reflected, or else passes right through it. The absorptivity, α, of a substance tells us what percentage of radiant energy it will absorb. A perfect absorber, or black body, would absorb all the radiant energy that reached it. If such an ideal substance existed (which it doesn’t) we would say it had an α of 1. The actual absorptivities of real substances are indicated by numbers between 0 and 1: The α of black velvet cloth, for example, is about 0.97; that of a polished silver mirror is about 0.08 for solar radiation (absorptivity for most polished metals for room temperature radiation is even lower).

We measure emissivity, ε, in very much the same way. A hypothetical black body would emit all the energy it possibly could and have an ε of 1; the emissivities of real substances are indicated by numbers between 0 and 1. For any given frequency (or color) of light, a substance’s absorptivity and emissivity are equal; however, the total spectrum of frequencies of the energy absorbed is usually different from that of the energy emitted.

The ratio between emissivity and absorptivity, α/ε, is very important, as we shall see later. If this ratio is greater than 1, it means that a substance absorbs heat faster than it emits it, and thus tends to become warmer. If the ratio is less than 1, the reverse is true—the surface emits radiant energy at a faster rate than it absorbs it, and tends to become cooler.