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Title: Satellite Communications Physics
Author: Various
Language: English
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*** Start of this LibraryBlog Digital Book "Satellite Communications Physics" ***
[Illustration: Satellite Communications Physics]
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
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
[Illustration: Title image]
“_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.
[Illustration: _Curvature of the earth requires microwave towers to
be about 30 miles apart_]
[Illustration: _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
[Illustration: horn-reflector antenna]
[Illustration: traveling-wave tube]
[Illustration: transistor]
[Illustration: solar cell]
[Illustration: 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.
[Illustration: {uncaptioned}]
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.
[Illustration: {uncaptioned}]
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.
[Illustration: {uncaptioned}]
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.
[Illustration: {uncaptioned}]
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.)
[Illustration: {uncaptioned}]
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.
[Illustration: {uncaptioned}]
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.
[Illustration: {uncaptioned}]
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.
[Illustration: _The orbits of four communications satellites vary in
size and shape_]
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.
[Illustration: _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.
[Illustration: _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
[Illustration: _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
[Illustration: particle detectors used to count protons]
[Illustration: transistors used to measure radiation damage]
[Illustration: solar cells used to measure radiation damage and as
solar aspect indicators]
[Illustration: 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.
[Illustration: _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_]
[Illustration: _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.
[Illustration: _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.
[Illustration: _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:
[Illustration: {Telstar satellite at work}]
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
[Illustration: Title image]
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.
[Illustration: _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
[Illustration: _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
(_mv_²)/(_r_),
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
(_km_)/(_r_²),
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
(_km_)/(_r_²) = (_mv_²)/(_r_)
and from this we can solve to find that the velocity of the satellite
must be equal to
_v_ = √((_k_)/(_r_)).
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_ = (2_πr_)/(_v_).
For the Echo satellite this time, _T_, turns out to be just about two
hours.
[Illustration: _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
[Illustration: _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.
[Illustration: _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, 2_a_, 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√((_a_³)/(_k_)),
where _k_ again was (using Newton’s work) essentially the mass of the
earth.
[Illustration: _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 _t_₁ and _t_₂ (see
_Figure 6_). Then another of Kepler’s laws says that the ratio between
the time difference _t_₂ - _t_₁ 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 _t_₁ and _t_₂ 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 _r_₁ and _r_₂, and the angles _i_ and Ω. (The detailed
formulas for this are derived from analytic geometry.)
[Illustration: _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 _r_₂, _r_₁, 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 _t_₂ - _t_₁. 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.
[Illustration: _Figure 8_]
[Illustration: _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.
[Illustration: _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.
How We Measure the Radiation of Heat
This is one of the fundamental relationships of modern physics:
_Q__body = ε_A_σ_T_⁴.
It was discovered experimentally by Josef Stefan in 1879, and verified
theoretically by Ludwig Boltzmann; it is known as the _Stefan-Boltzmann
Law_. This formula tells us the amount of radiant energy, Q_body, that
will be emitted by a body having the surface area _A_ when it is at the
temperature _T_. Temperature, here, is measured in degrees Rankine (°R),
or Fahrenheit temperature above absolute zero (to calculate degrees
Rankine, add 460 to the temperature in degrees Fahrenheit). The
expression ε_A_ is used to show that only a certain fraction of the
energy that would leave a perfect black body of area _A_ will actually
leave a real body of the same size; the size of this fraction is
determined by the body’s emissivity. The symbol σ is a quantity we call
the _Stefan-Boltzmann constant_.
We can also calculate the heat from the sun that will be absorbed by a
body. If we let _S_ be the total amount of solar energy that would be
absorbed by a perfect black body, α_S_ will be the amount that is
actually absorbed by a body with an absorptivity of α for solar
radiation. If our body is a spherical satellite, the sun’s rays will
only strike it from a single direction. Thus only an area equivalent to
the sphere’s cross-section (largest inscribed circle) will receive
energy at any one time. Since, _as shown in the sketch_, this area (_a_
= π_r_²) is one-fourth that of the sphere’s total surface area (_A_ =
4π_r_²), we know that the radiant energy from the sun that is absorbed
will be
_Q__sun = (_A_)/(4)α_S_.
A man-made satellite’s position relative to the earth is very like that
of the earth in relation to the sun; the earth, after all, is itself a
satellite of the sun. And during most of its useful life a satellite
will be in _thermal equilibrium_—it will be losing just as much heat
energy by its own radiation into space as it will be gaining from other
sources, primarily the sun. Since the total amount of energy it absorbs
is equal to the amount of energy it emits, _Q__body is equal to _Q__sun.
This means that we have the equality
ε_A_σ_T_⁴ = (_A_)/(4)α_S_.
Now, if we solve this for temperature, we will get
_T_ = ((α)/(ε) × (_S_)/(4σ))(^¼).
[Illustration: _Although the total surface area, A, of a sphere is
4πr², light rays from the sun only strike half the surface at any
one time. This area, a, is effectively equal to the sphere’s
cross-section, πr²._]
This equation is well known in astronomy, and has been used for more
than 80 years to calculate the temperatures of various objects in the
sky. Today, we still find it useful for measuring the surface
temperatures of man-made satellites such as Telstar. Since both _S_ and
σ are known constants (in this case, we use the quantities _S_ = 445 and
σ = 0.173 × 10⁻⁸), you can see that temperature is dependent on the α/ε
ratio.
Finding the Right Surface for Telstar
[Illustration: _Cutaway view of the inside of the Telstar I
satellite, showing the electronics canister covered with its
protective blanket of many layers of Mylar. To control temperature,
shutters automatically open all the way if the canister gets hotter
than 80°F, close completely if it goes down to 50°F._]
shutter (closed)
electrical heat transfer
insulation blanket
heat transfer by radiation
shutter (open)
solar cells
electronic chassis
heat transfer by circulation
thermal control mechanism
nylon lacing
microwave antennas
frame
In designing the Telstar satellite, both its internal and external
temperatures had to be controlled. The electronics canister inside the
satellite operates best if it stays at approximately room temperature of
65 to 75°F. This much heat is supplied in the canister by dissipation of
electrical energy from the solar cells. The container is well insulated
to keep its temperature relatively stable, and it has shutters that open
automatically if it begins to get overheated (_see above_). The
operating characteristics of the solar cells on Telstar’s surface also
had to be considered; they work better at rather cool temperatures. So
we decided to keep the satellite’s skin at an average temperature of
about 0°F, although temperatures actually will range quite a bit above
and below the average as the satellite moves from sun to shadow.
Now, using this average temperature of 0°F (converted to 460°R) as _T_
in our formula, we can solve for α/ε. We find that this gives us a ratio
of approximately 0.7 for the satellite’s surface. However, this presents
a problem. Almost 40 per cent of Telstar’s surface is taken up by its
power plant of 3600 sapphire-covered solar cells. These cells,
unfortunately, have a relatively high α/ε ratio—their α is 0.8 and their
ε is 0.54, for an α/ε of 1.5. This means that the portion of the surface
not used by either solar cells or antenna openings must, in order to
give us an over-all average of about 0.7, have a very low α/ε ratio—less
than 0.3.
To get this sort of ratio, we had to select carefully the material for
the outer surface of the Telstar satellite. There were many kinds of
surfaces that might have been used; they could have been metal or
non-metal, rough or smooth, shiny or dull. And they could have been any
color from black to white. However, to get a 0.3 ratio we needed
something with a relatively high emissivity for the low-frequency
electromagnetic radiation that the satellite emits and a rather low
absorptivity for the high-frequency radiation coming from the sun. High
emissivity meant that we should use a nonmetal surface rather than
polished metal, since the emissivity of nonmetals is quite high at the
temperatures in which we were interested, while that of polished metals
is relatively low. And, to get low absorptivity, we decided that the
color of these surface areas should be very close to a pure white.
[Illustration: _Partially molten aluminum oxide particles being
sprayed onto aluminum outer surface panels._]
There were several substances that met our requirements. After testing a
number of them, we decided to use aluminum panels coated with a thin
layer of aluminum oxide (Al₂O₃). This coating is very pure, hard, and
stable, and we left it rough to minimize changes due to meteoroid
abrasion. Its α/ε ratio is 0.24. The aluminum oxide coating can be
applied by means of the plasma jet process—particles of aluminum oxide
are heated to a partially molten state, mixed with gases, and then
sprayed onto the cleaned, pre-coated aluminum panels (_see illustration
above_).
Using this carefully selected outer surface has helped solve the
temperature-control problem. Since Telstar has been in orbit its
internal and skin temperatures have kept well within the ranges we
wanted them to. Thus you can see how some basic formulas from classical
physics helped us choose the right material for the satellite’s
surface—and even what color it should be. The blue-and-white checkered
appearance that Telstar I finally took on was no accident—it was the
result of carefully combining various colors and materials in just the
right amounts to obtain the temperature balance we needed.
Peter Hrycak _was born in Przemysl, Western Ukraine, and received a
B.S. in 1954, an M.S. in 1955, and a Ph.D. in 1960 from the University
of Minnesota. He joined Bell Telephone Laboratories in 1960, and has
worked on low temperature refrigeration problems and thermal design
and thermal testing of the Telstar satellite._
CASE HISTORY NO. 3
How Can We Make Optical Measurements on a Satellite?
Jeofry S. Courtney-Pratt
_Physicist—Head, Mechanics Research Department_
THE PROBLEM
_Since the first Telstar satellite went into orbit, we have tried to
trace its path through space precisely. But we also have had to keep a
constant check on the position, or attitude, that the satellite takes as
it travels. We are particularly interested in the direction of the spin
axis about which it revolves, and we also want to know its spin rate,
which is the number revolutions the satellite makes each minute.
Although these might seem relatively simple jobs, they actually turned
out to be rather complicated. And only at virtually the last minute,
just before the satellite’s design was finally set, did we think of a
new way of using reflected flashes of sunlight to report on its spin
axis and spin rate._
Why We Want to Know About the Spin Axis
When the satellite was injected into its orbit, it was spin-stabilized
to keep it from tumbling over and over, much as a rifle bullet is
stabilized by being spun about its longitudinal axis. The Telstar
satellite is roughly spherical, and it was designed to spin with the
helical antenna end as its north pole and the antenna bands as its
equator. On July 10, 1962, the satellite was given an initial spin of
177.7 revolutions a minute. As we expected, this rate is decreasing
gradually; after two years it will only be spinning one tenth as fast.
The most important reason for keeping a close watch on the satellite’s
spin axis is to make sure that microwave signals are sent and received
steadily. It isn’t possible to build an antenna that radiates at exactly
the same power in all directions. Telstar’s antennas work very well, but
they operate better in the direction of the satellite’s equator than
they do towards its poles. This means that if the spin axis is
constantly changing transmission will fade in and out—even at times
passing through “null” where no transmission at all is possible. No
single fixed orientation is perfect for the spin axis, but we decided
that the best average position would be to keep it always perpendicular
to the plane of the earth’s orbit. We tried to make sure that the spin
axis would not vary by more than five degrees from this direction at any
time—although it probably could depart as much as 15 or 20 degrees
without doing serious harm.
A second reason for being careful about the satellite’s spin axis is the
problem of heat balance. If one end of the satellite points constantly
at the sun and the other end does not, the end near the sun will get
much too hot and the other will get much too cold. Therefore, we tried
to fix the spin axis so that it stayed perpendicular to a line drawn
from the satellite to the sun.
We also wanted to get a continuing report on the effects of the magnetic
field of the earth at high altitudes. We knew these would cause the spin
axis to change with time, or _precess_, but we couldn’t be exactly sure
what these changes would be.
Since the orientation of Telstar’s spin axis was so important we
installed a _torque coil_ in the satellite. This is a coil of wire in
which, upon a signal from the ground, an electric current can be made to
flow. The current produces a magnetic field that interacts with the
earth’s magnetic field to change the position of the satellite’s spin
axis. However, we could not be sure that this device would work
properly—and this is another reason why we wanted to keep track of the
exact position of the spin axis.
Ways of Measuring the Spin Axis
One of the devices built into Telstar is a set of six _solar aspect
cells_ spaced at regular intervals around the satellite. These give a
fairly accurate indication of the angle between the spin axis of the
satellite and a line joining the satellite and the sun. When sunlight
strikes these solar cells, they produce electric currents, and the value
of the current from each cell is sent back to the ground via telemetry.
Three of the cells are in the satellite’s northern hemisphere; three are
in the southern hemisphere. If Telstar’s north pole were pointing to the
sun, for example, the three northern cells would record large, equal
currents; those in the southern hemisphere would show zero current. But
if the spin axis were perpendicular to the satellite-sun line (as we
want it to be) all six cells would report equal, average-sized currents,
which would fluctuate as the satellite spun around. The solar cells were
carefully calibrated before Telstar was launched, and we estimate that
they can tell us the angle between the satellite’s spin axis and the
satellite-sun line to within one or two degrees.
However, this one angle is not enough to locate the spin axis exactly.
As you can see in _Diagram 1_, there are many possible positions for the
spin axis OP that have the same angle θ with the satellite-sun line OS.
These positions all would lie on the surface of an imaginary cone OPP′
that has OS as its axis and 2θ as its vertex angle. We need to have a
second measurement to find a single position for the spin axis. As late
as November 1961 we had not found a satisfactory way to make such a
second measurement. Then Donald Gibble of Bell Telephone Laboratories
suggested that we observe the reflections of sunlight from mirrors
fitted onto the satellite[2].
Only when a satellite is in the right position can you see the
reflection of sunlight from a plane surface on its body. _Diagram 2_
shows how flashes of reflected light are observed. The light of the sun,
S, is reflected from a plane surface, R, on the satellite to our
observing station, T, on the earth. If we imagine the line ORB drawn
perpendicular to R, we know, from the law of reflection, that the angle
of incidence, _i_, made by the sunlight to this line will be equal to
the angle of reflection, _i_′, between the reflected light and the same
line. The law of reflection also tells us that the sun, the line ORB,
and the observing station all must now lie in the same plane. And, since
we can calculate where the satellite is in its orbit at this exact
moment, we can locate line ORB. But what about the spin axis? We know
where on the satellite our reflector R is located, so we know ahead of
time what the angle θ′ between ORB and the spin axis, OP, will be. We
call it the _flash angle_. Thus we can tell that the spin axis will be
somewhere on the surface of an imaginary cone OPP″ that has ORB as its
axis and 2θ′ as its vertex angle[3].
[Illustration: 1.
_Solar aspect cells on the satellite report via telemetry the amount
of sunlight they receive; from these data we can calculate the angle
θ between the satellite’s spin axis, OP, and the satellite-sun line,
OS. This means that OP can be anywhere on the surface of cone
OPP′._]
[Illustration: 2.
_When sunlight is reflected to observing station T on the earth, we
know that the angle of incidence i must be equal to the angle of
reflection i′, and, if ORB is a line perpendicular to the reflector
R, we know that the sun, the observer, and line ORB must all lie in
one plane. Since we also know the position of the satellite in its
orbit and the distance from it to the earth, we can locate line ORB
precisely. The reflector R is set at an angle θ′ of 68° from the
spin axis OP. This tells us that the spin axis must lie on the cone
OPP″, which is now precisely determined by its axis ORB and its
vertex angle 2θ′, equal to 136°._]
[Illustration: 3.
_Cones OPP′ and OPP″ intersect along the two lines OP and OQ, so
these are the only possible spin axis locations. From our general
knowledge of the situation (or from any third measurement of glint
time), OQ can be ruled out, and we conclude that only OP can be the
true spin axis._]
In _Diagram 3_ we have combined our two measurements of the satellite’s
spin axis. You can see that the two cones will intersect along two
straight lines, OP and OQ; these are thus the only possible positions
that will satisfy both our measurements. Actually, of course, only one
of these lines is the true location of the spin axis. And it is usually
obvious which one it is, when we consider all our other data about the
satellite’s position.
Using this technique, if we measure the exact times when we see flashes
of reflected sunlight from Telstar, we can combine that information with
data from our six solar aspect cells and get a good plot of the position
of the satellite’s spin axis.
In theory, this looked like a very promising idea. But finding a
satisfactory way to put it into practice was something else again. Our
first thought was simply to make use of the light reflected from the
sapphire covers on the satellite’s solar cells. However, these covers
have a low coefficient of reflection and do not form a completely flat
surface. This means that the light reflected from them is very much
reduced in intensity and spreads out too much to give us the precise
readings we want. On the other hand, if we attached a plane mirror with
a high reflection coefficient to the satellite, we thought we could pick
up the minute flashes of reflected light from a distance of as much as a
few thousand miles. So we decided to press ahead with this scheme and
install one or more reflectors on the satellite.
By the time we started work on the mirrors, the final design of Telstar
I was almost complete; this meant that we had to squeeze our mirrors
aboard it as best we could. The most stringent physical requirement in
designing them was weight; they could not add more than half a pound to
Telstar’s total load. Nor could they project more than one-eighth inch
from the satellite’s surface, or they might interfere with the radiation
pattern for the main antenna. We also decided to make the mirrors out of
highly polished metal, since any other possible material might break too
easily. And the mirrors had to be as flat as possible, so the beam of
reflected sunlight would not diverge by more than one degree.
Thus we had to design mirrors that would be very thin, very shiny, very
flat, very light, and almost unbreakable. After much experimenting, we
solved this rather tricky problem. The mirrors we added onto Telstar I,
as you can see in _the illustration below_, were machined from aluminum
alloy sheet, carefully polished by hand with abrasive papers, and buffed
on a cloth wheel. Finally, we evaporated a thin layer of pure aluminum
onto their surfaces to improve their reflection coefficients and make
them resistant to corrosion. The three mirrors were fastened to the
surface of the satellite with small screws, which had to be tightened
and shimmed very carefully so that the mirrors stayed as flat as
possible.
Locating the Mirrors on the Satellite
As we mentioned above, the flash angle θ′ between the satellite’s spin
axis and a line perpendicular to the mirror is very important in our
calculations. We made detailed studies of the various flash angles that
would be possible during the first 60 days after launch. We plotted the
times when the satellite would be above the horizon while our Crawford’s
Hill, New Jersey, observing station was in darkness, and we made
allowance for satellite orbits that might deviate slightly from the
planned one. These calculations told us that the best flash angle for
the mirror would be 68 degrees—which is the angle made by the first
facets above Telstar’s equatorial antenna band. So we located a flat
mirror on one of these facets. Because one of the solar aspect cells was
already installed in the center of this facet, we were forced to cut a
circular hole out of the center of the mirror.
But we knew that one mirror could not do the whole job. After Telstar I
had been in orbit more than 30 days, the 68-degree mirror would only be
in position to give infrequent flashes, and one at about 95 degrees
would be more useful. This presented two problems. First, no facet on
the satellite makes a 95-degree angle with the spin axis. However, we
could use one of the facets just below the equatorial antenna, which
makes a 112-degree angle, and groove or _facet_ the mirror so that its
reflecting faces became narrow strips slanted 17 degrees away from the
base at the angle of 95 degrees (112 - 17 = 95). Our second problem was
space—since there was not enough room left on any of the 112-degree
facets to mount a second large mirror, we substituted two smaller
mirrors and mounted them 120 degrees apart. This arrangement lets us
know from which mirror we see flashes—the plane mirror gives one flash
for each revolution of the satellite; the faceted mirrors give two
flashes for each revolution of the satellite.
[Illustration: _Sketches of three reflecting mirrors and their
locations on the Telstar satellite. The upper plane mirror is set at
68° to the spin axis; the lower ones are faceted to give reflecting
surfaces at 95°. Two of the satellite’s six solar aspect cells can
be seen within the circular cut-outs in the mirrors._]
How We Record Flashes from the Mirrors
Now we had finally found a satisfactory way to reflect a train of tiny
flashes—much too faint to be seen by the naked eye—from Telstar as it
passed across the sky during the night. But our main aim was to record
the exact times when these flash bursts occurred. With this information,
we could, using the method we described above, tell very accurately both
the satellite’s spin axis and its rate of spin. We do not have space to
describe the many problems that had to be solved in setting up the
equipment to record the flashes. Let us merely outline the procedure
that we finally devised:
1. _To pick up the satellite’s flashes we use a 12-inch-aperture
photoelectric telescope mounted on a radar trailer (shown in
illustration below). It is pointed by means of prediction drive tapes
produced by an electronic computer; these are based on data from
previous passes._
2. _On clear, dark nights when the satellite is at relatively short
range, we can see it with an auxiliary finder telescope, and then adjust
the large telescope precisely. Or, if the satellite’s high-frequency
beacon has been turned on, the Holmdel microwave antenna can
automatically point our large telescope._
3. _When flashes of light are picked up by the telescope, they fall
directly onto the cathode of a photomultiplier tube. They are then
filtered out from the random light in the night sky and amplified._
[Illustration: _Twelve-inch telescope and electronics box mounted on
a radar antenna pedestal at Crawford’s Hill. Three-inch sighting
telescope mounted on top has since been replaced by six-inch
telescope._]
4. _Rather than make a continuous recording of the output—one night this
would have produced a record twelve miles long for us to pore over—we
use an electronic trigger. This is the time base of an oscilloscope,
whose sawtooth output is set in operation only if a signal of four volts
or more is received (photo below)._
5. _A pen recorder makes a continuous line on a revolving drum, with a
heated stylus connected to a galvanometer marking a permanent record on
heat-sensitive paper. Any signal output from the oscilloscope is picked
up by the galvanometer and causes the pen to make a sawtoothed mark;
when the paper is unrolled from the drum, these marks are clearly
visible as notches in a series of otherwise straight lines._
6. _A synchronous timer marks the chart every ten seconds, and we are
able to time individual pulses with a precision of one tenth of a
second. Because the beginning and end of a train of pulses are not
always distinct, we can only determine the center of a burst of
flashes—which we use as our most important time indication—to within two
seconds. However, this is accurate enough, for a change of only one
degree in the orientation of the satellite’s spin axis would change the
time of the flash burst center by about half a minute (see below)._
7. _We use a second oscilloscope to check on whether the signals we
receive are genuine flashes or just accidental stray light. This
oscilloscope has a long-persistence screen, which we use as a temporary
memory. The pulses traced on its cathode ray tube are automatically
photographed by a 35-mm camera while they persist on the screen. We can
then examine the photograph to see if the pulses are genuine, which we
ascertain from (a) their shape and size and (b) the intervals between
successive pulses. Looking at the photographic record also confirms
whether we are observing flashes from the 68° plane mirror or the 95°
faceted mirrors. We can calculate the satellite’s spin rate by measuring
the intervals between individual flashes._
[Illustration: _General view of amplifying, monitoring, and
recording gear that picks up glints of sunlight at the Crawford’s
Hill observation station._]
[Illustration: _Enlarged portion of a typical pen record of flashes
of sunlight from Telstar mirrors, showing a burst of 21 glints from
the 68° mirror recorded at 03:40:58 Greenwich Mean Time on August 9,
1962. Synchronizing vibration mark seven lines above the recorded
burst indicates the time 02:59:00. Measuring the horizontal distance
between consecutive sawtooth marks tells us that the spin rate is
between 163 and 164 revolutions per minute. (Precise measurements of
the oscilloscope traces fixed the exact spin rate at the time of
this burst at 163.64 revolutions per minute.)_]
Results
Telstar I was launched on July 10, 1962. That evening, beginning on the
satellite’s seventh pass, we were able to detect trains of flashes from
the mirrors. We assumed that, since Telstar had been launched almost
exactly according to plan, its spin axis would be perpendicular to the
plane of the earth’s orbit, and we calculated when we should see the
flashes. And, each time, we actually saw them within two minutes of the
times we had predicted—so we knew that the spin axis was almost exactly
where it should be.
Our measurements have continued whenever the weather and other
conditions permitted. Combining readings from the bursts of flashes and
telemetry data from the solar aspect cells, we have accurately plotted
Telstar’s spin axis; it has continued to precess very much as we
predicted it would. We have also seen what happens to the spin axis when
the satellite’s torque coil is turned on. And, by measuring the
intervals between flashes, we have made very precise measurements of the
spin rate, which is gradually decreasing mostly according to schedule.
However, the plot is showing some small unexplained variations of spin
decay rate, and a study of them will, we hope, throw light on some of
the variations of the earth’s magnetic field.
For future communications work, particularly with satellites at longer
ranges, it would seem to be preferable to use stiffer, flatter mirrors
and to make them from beryllium rather than aluminum alloy. More
accurate tracking means, more observatory sites, and more powerful
telescopes will also be needed. But for this first experimental use our
little mirrors have worked very well.
Jeofry S. Courtney-Pratt _was born in Hobart, Tasmania, Australia, and
received a Bachelor of Engineering degree from the University of
Tasmania in 1942 and a Ph.D. from Cambridge University in 1949. He was
also awarded an Sc.D. by Cambridge in 1958. He joined Bell Telephone
Laboratories in 1958, and has done research in high-speed photography,
optics, optical masers, the properties of materials, and the physics
of the contact of solids._
CASE HISTORY NO. 4
How Do We Keep Solar Cell Power Plants Working in Space?
Kenneth D. Smith
_Electronics Engineer—Member of Staff, Semiconductor Device Laboratory_
THE PROBLEM
_Before we learned about the Van Allen belts, we expected that the solar
cells used to power satellites would last for many years in space. We
thought they would be damaged only by cosmic rays, micrometeorites, and
occasional bursts of particles from the sun. But when the solar plants
of some American satellites went out of action after only a few weeks in
orbit, we realized that in the future solar cell power units would need
better protection from radiation damage. We had learned that
satellites—and particularly medium altitude communications
satellites—must spend a lot of time in regions where they will be struck
by thousands or even millions of high-speed radiation particles each
second._
_This fact forced us to change almost all our thinking about solar power
plants for satellites. To make sure they would last for several years,
we had to design new types of solar cells and devise new ways of
mounting them. We also had to revise our estimates of how much power we
could expect to get from our cells._
_If a communications satellite is to go into regular commercial service,
it must continue working for several years in space. The Telstar
satellite, however, was designed as an experimental project, and we
decided that two years would be a reasonable lifetime to plan for. When
Project Telstar began, our problem was to develop solar cells that would
operate in an environment subject to strong radiation effects—and keep
on operating there for two years._
Organizing the Work
Our work on the solar cells for Telstar I began in October, 1960. With
just a little more than a year to go before the satellite had to be
ready, there was no time to lose. So we decided to break down the
over-all problem into three parts:
—Finding out how radiation would affect various kinds of solar cells;
—Making experimental cells and, when the best had been picked,
determining the best ways to make them in the large quantities we
would need; and
—Developing ways to mount the cells on the Telstar satellite so that
they would withstand the stresses of being launched, the effects of
radiation particles, and extreme changes in temperature.
A different group of people began work simultaneously on each of these
three parts of the problem, with each of them going ahead under the
assumption that the others would be successful. Each group had to find
the answers to many very interesting questions, but since our space is
limited we can only discuss some of them here. Before doing so, however,
we must say something about what a solar cell is and how it works.
Technical Background on Solar Cells
There are two ways of making a silicon solar cell. In one, the body of
the cell is what we call _n-type_ silicon—that is, pure silicon that has
been doped with a small number of impurity atoms of an element such as
phosphorus or arsenic (from group V of the periodic table). This kind of
semiconductor[4] conducts electricity by means of a supply of
free-to-move electrons (negative charges) caused by the presence of
these impurity atoms. To make a workable solar cell from n-type silicon,
a thin surface layer of p-type silicon is formed by diffusing atoms of a
material from group III of the periodic table—usually boron—into the
silicon. Metallic contacts then are made to these two regions. This kind
of cell is known as a _p-on-n cell_.
The second type of solar cell is just the reverse. It begins with a body
of p-type silicon (with impurity atoms from a group III element) and
conducts electricity by means of “holes”—vacant sites where electrons
might be but are not. These holes act as free-to-move positive charges.
We can make a solar cell from this material by diffusing a layer of
n-type impurity, such as phosphorus, into it. We call this an _n-on-p
cell_ (see the _figure below_).
[Illustration: Construction of a silicon solar cell of the n-on-p
type (thickness of n-layer greatly exaggerated).]
titanium-silver evaporated contact with solder dip finish
antireflection coating
contact gridding for lower series resistance
15 mil wafer, p-type
0.4 micron front layer, n-type
The key to the operation of either type of solar cell is the junction
between the regions of n-type and p-type material—what we call the _p-n
junction_. In an actual n-on-p cell this junction is only about twenty
millionths of an inch below the surface, since that is the thickness of
the n-layer. At this point, where the hole-rich p-region meets the
electron-rich n-region, there is a permanent, built-in electric field.
As shown in the figure below, the n-layer has many free electrons
(indicated by minus signs) and a few holes (circled pluses), while the
p-region has many holes and a few electrons. When the cell is in
equilibrium, thermal agitation causes some holes to diffuse into the
p-region. We call these stray holes and electrons _minority carriers_
(the circled pluses and minuses in the figure). Thus, the n-layer has a
slight positive charge and the p-body has a slight negative charge; this
results in a difference in potential across the junction, which in
silicon amounts to about seven-tenths of a volt.
[Illustration: _Schematic diagram of an n-on-p solar cell. In the
n-layer, minuses represent free electrons, circled pluses are
minority-carrier holes; in the p-type body, pluses represent holes,
circled minuses are minority-carrier electrons._]
Sunlight is made up of individual corpuscles of energy called _photons_.
When these photons are absorbed in or near a cell’s p-n junction, they
liberate both a free-to-move negative charge and a free-to-move positive
charge—this is called generating a _hole-electron pair_. The electric
field across the p-n junction causes the holes to flow to the p-side and
the electrons to the n-side of the barrier. This flow tends to make the
p-side positive and the n-side negative, so that, when a load is
connected between them, a useful external voltage (amounting to about
six-tenths of a volt) is produced, and electric current will flow. Thus,
we have converted light energy into electrical energy.
Only part of the energy in light can be used to generate an electrical
output, since a good deal of the light striking a cell is absorbed as
heat or is reflected from its surface. The percentage of solar energy
that can be converted into usable electric power is called the cell’s
_conversion factor_ or _efficiency_. Although this can theoretically be
as high as 22%, the best cells we have made in the laboratory have
conversion factors of only about 15%, and the better commercial cells
have efficiencies of 12% or more.
Although both p-on-n and n-on-p cells were made in early laboratory
studies, the p-on-n cells gave a somewhat higher output. As a result,
all the American commercial solar cells up to 1960 were of this type,
and they were used on all satellites before Telstar I. (Russian
satellites, we believe, have used n-on-p cells from the beginning.)
The U.S. Army Signal Corps Research and Development Laboratory, however,
decided to make both p-on-n and n-on-p cells and compare their
performance. This laboratory work led to a surprising discovery: The
n-on-p cells were several times as resistant to energetic particle
radiation as were comparable p-on-n cells. These results were announced
in 1960, and confirmed by our measurements and those of other
laboratories. The timing was very fortunate, since we had just learned
of the greatly increased radiation hazards presented by the Van Allen
belts.
Finding Out About Radiation Damage
Now, having given you a very brief account of how a solar cell works,
let us return to our three-part problem. The first objective was to
study all the aspects of radiation damage. To do this, we had to find
out how much radiation the Telstar satellite would encounter; we needed
to estimate the concentration of high-energy particles—both electrons
and protons—at various altitudes and locations. Several government
agencies are now carrying on research in this important area, but at the
time of the Telstar I launch we did not know exactly how much radiation
the satellite would run into. And the high-altitude nuclear explosion of
July 9, 1962 (the day before Telstar I went into orbit) may have
increased the quantity of high-energy electrons injected into its path.
We also wanted to find out whether electrons and protons would do the
same damage to solar cells. Several kinds of cells were exposed at Bell
Laboratories and at various university research laboratories to a wide
range of radiation dosages. The experiments showed, generally, that the
damage effects of electrons and protons should be about the same.
Although protons are 1840 times as massive as electrons, there are a
great many more electrons in the Van Allen belts, so that an unprotected
solar cell would be much more likely to be injured by electrons than by
protons.
In fact, we found that the Van Allen belt protons have so much energy
that they can go through transparent shielding material as much as
several centimeters thick and still damage a solar cell. Thus, to screen
our cells from protons we would need very thick transparent cover
plates, and this added weight would be intolerable. So we decided to use
no proton shielding at all.
With electrons, the situation is different; they are much lighter and
have much less energy. Also, if their energy is reduced below a certain
level (about 180 thousand electron volts) electrons will not be able to
knock silicon atoms out of position, and thus cannot harm a solar cell.
We experimented with a number of different kinds and thicknesses of
cover plates, and found that transparent material with a mass of 0.3
gram per square centimeter would slow down electrons enough to make them
no problem.
Another radiation study helped us take advantage of the fact that solar
cells respond differently to light of different wave lengths. If the
surface layer of a cell is extremely thin, it will absorb blue, green,
and yellow light well, but may be much less sensitive to the deeply
penetrating red and infrared waves. We experimented with n-on-p cells
having very shallow p-n junctions, exposing them to an extremely strong
radiation dosage. The cells still responded very well to blue and green
light, even though most of their response to infrared and red light was
lost. These findings convinced us that we should work to make our new
cells as blue-green sensitive as possible, since they were going to be
exposed to heavy radiation.
Designing and Making the Best Solar Cells
After it was discovered that the n-on-p cell was more resistant to
radiation, we decided to make an all-out effort to develop an n-on-p
cell that could be manufactured in quantity for our new satellite. Since
we didn’t know whether we could solve this problem in time to meet the
Telstar I launch date, we “hedged” by designing the new n-on-p cells to
be the same physical size (one by two centimeters) as conventional
p-on-n cells. Thus, if the n-on-p project hit a snag, we probably could
use regular p-on-n cells.
As you can imagine, making a solar cell to fit the very high
requirements we had set for the Telstar satellite is not an easy job—and
making these cells by the thousands is even more of a task. During
October, November, and December of 1960 we carried on a crash program in
which we made hundreds of experimental cells in our laboratories, using
a variety of materials and many different manufacturing techniques.
We perfected a phosphorus diffusion process to develop the very thin
n-layer (about one forty-thousandth of an inch thick) that we needed for
our special blue-sensitive n-on-p cells. We also had to devise an
entirely new way to attach the metallic contacts to the highly polished
surfaces of our cells, using a combination of titanium and silver.
Some tricky manufacturing problems also had to be solved once the
Western Electric Company began to make the large quantity of cells
needed for the Telstar program. For example, during the diffusion of the
n-layer of the cell, the silicon slice is surrounded by phosphorus
pentoxide vapor, which covers the entire slice with an “n-skin.” This
skin must be removed from the bottom of the cell by etching or grit
blasting before the p-contact is applied. Another difficult problem
occurred when we decided to give our cells an anti-reflection coating.
Because polished silicon has a refractive index near 4 and space has an
index of 1, silicon will reflect about 34% of visible light from the
sun. However, if we apply an anti-reflection layer onto the silicon this
percentage of reflection can be considerably decreased. We found that
the best substance for this purpose was a layer of silicon monoxide only
three-millionths of an inch thick. But it was only after quite a bit of
trouble—and scrapping several thousand cells—that we were able to get
this coating to adhere properly in the right thickness.
Mounting the Cells on the Satellite
The third part of our problem had to do with finding the best ways to
mount and protect the cells on the Telstar satellite itself. Since a
satellite’s solar power plant usually has several thousand cells, we
find it best to mount the cells in groups, or modules. These can be
pretested as a unit after individual interconnections have been made.
For Telstar I, we decided to mount the 3600 solar cells in 12-cell
modules like those shown in the _figure below_.
[Illustration: _The satellite uses 300 modules of twelve solar
cells, in groups of six or three modules per facet._]
[Illustration: _Lengthwise diagram of a solar cell module, showing
how individual cells were fixed in place._]
Each of the cells has a top contact along one edge and a bottom contact
all over its base, so we were able to assemble the 12-cell groups like
shingles, with the bottom edge of one cell covering the top edge of the
next, leaving only the active area of each cell exposed. But this meant
that each module would be over four inches long and only 14 thousandths
of an inch thick—far too weak to withstand stress and vibration. To
support the cells, we decided to mount them on a metallized ceramic
base. But this presented a problem: If the cells were soldered directly
to the base, the different thermal expansion rates of the silicon and
the ceramic would cause the structure to break during the cycles of
extreme changes in temperature that Telstar would pass through. We
remedied this by connecting each cell to the ceramic support by a thin
U-shaped strip of silver (_see above_). Since silver has a much higher
thermal expansion coefficient than silicon, we added tiny sandwiches of
Nilvar or Invar (36% nickel, 64% iron) where the cells were attached.
With this mounting method, the cell modules withstood thermal and
mechanical shocks much more severe than those they would undergo in
actual use. In one test, for instance, an entire cell module with its
cover plates was first dipped in hot water, then plunged into liquid
nitrogen at a temperature of -195° Centigrade. In orbit, the temperature
range for the satellite was not expected to be more than from +80° to
-100°C, with a rate of change of no more than three degrees a minute.
Finally, we needed to find the right kind of transparent protective
cover for the Telstar solar cells, both to keep micrometeorites from
damaging the sensitive and very thin diffused layer and to slow down the
incoming electrons to nondestructive energy levels. For micrometeorite
protection, only a thin layer of hard transparent substance was needed;
for electron protection, the cover plates should have a mass of no less
than 0.3 gram per square centimeter (as we explained above). And there
were two other important considerations: The material we used should not
be darkened or discolored by prolonged exposure to ultraviolet
radiation, and it should have good thermal conductance, so that some of
the heat absorbed by the solar cells could be conducted out to the cover
plates and re-radiated. All these requirements led us to the choice of
clear, man-made sapphire. Although sapphire is more expensive and
difficult to make than the equivalent quartz or glass, it only has to be
30 mils (three hundreds of an inch) thick. Twice this thickness would be
required if quartz or glass were used.
We have had space to describe only a few of the things involved in
designing a solar cell power plant that would work unattended out in
space. We have not mentioned a good many of the tough problems that had
to be worked on. But we are glad to report that we could find answers to
almost all our questions. And the most significant answer is shown in
_the figure below_, where you can see how the Telstar I solar power
plant slowly diminished in power almost exactly as we predicted it
would.
[Illustration: _Very gradual decay due to radiation effects of the
Telstar I solar cell plant in the first months after the satellite
went into orbit; it was extremely close to the predicted rate (solid
line)._]
Kenneth D. Smith _was born in Galesburg, Illinois, and received a B.A.
from Pomona College in 1928 and an M.A. from Dartmouth College in
1930. He joined Bell Telephone Laboratories in 1930, and has worked on
the development of proximity fuzes, radar bombing systems, broadband
microwave radio systems, and various semiconductor devices, including
radiation-resistant solar cells for the Telstar satellite._
CASE HISTORY NO. 5
Would Time Delay Be a Problem in Using a Synchronous Satellite?
Peter D. Bricker
_Psychologist—Member of Staff, Behavioral Research Laboratory_
THE PROBLEM
_One of the satellite communications systems that has been proposed
would make use of stationary synchronous satellites. These would be
precisely located above the earth’s equator in orbits 22,300 miles high,
where they would circle the earth once every 24 hours, and thus appear
to remain stationary over a point on the earth. There are several
advantages to this type of system—the most important being that we would
need only three satellites for communications between almost all the
inhabited regions of the earth._
_On the other hand, there are several problems in establishing a
synchronous system. Just getting the satellites into exactly the right
places and keeping them in position is a formidable one. We also have
something of a mystery to contend with, because of the tremendous
distances that would be involved. Although we can communicate at speeds
close to that of light—186,000 miles per second—we cannot go any faster
than that. You might think that 186,000 miles a second was fast enough
for us, and most of the time it is. However, if you send signals 22,300
miles up into the sky, transmit them back to earth, perhaps send them up
again to a second satellite, and finally bring them 22,300 miles back
down to earth, even the speed of light may not be fast enough. The delay
will be only about a second or so, but it may—for some kinds of
communications—be long enough to cause trouble. How much trouble, we
don’t yet know._
_For one-way signals such as television, a transmission delay of about
one second obviously makes little or no difference. But for two-way
conversations on the telephone, where there is rapid back-and-forth
talking, even this tiny amount of time delay may be a problem. And then
again, it may not be. There have been a lot of experiments to find out
something about this delay problem, and these have given us a lot of
different answers. Work is still going on, and there is still much to
find out. In this chapter, we tell you about one small, early
experiment. Its results were not conclusive, but they should give you
one example of how to set up and carry out a typical experimental study
on human behavior._
[Illustration: _Typical circular satellite orbit: r is distance from
center of earth to satellite; R is radius of earth._]
How a Synchronous Satellite Would Work
For our purposes, we will not be concerned with all the problems of
launching a synchronous satellite into its proper orbit. But you may be
curious why we know that this orbit must be 22,300 miles high. It can be
calculated by using two basic formulas from elementary physics.
From Newton’s Law of Gravitation we know that the velocity, _v_, of a
satellite moving in a circular orbit[5] will be
_v_ = √((_gR_²)/(_r_)),
where _R_ is the radius of the earth, _r_ is the distance from the
center of the earth to the satellite, and _g_ is the acceleration due to
gravity (_see diagram above_).
We also know that this velocity must be
_v_ = (2π_r_)/(_T_),
since the distance the satellite travels to complete an orbit is 2π_r_,
and _T_ is the time of one complete revolution. Thus we have the
equality
√((_gR_²)/(_r_)) = (2π_r_)/(_T_),
and, solving for _r_, we get
_r_ = (_gR_²_T_²)/(4π²)(^⅓)
Since we are interested in a synchronous satellite, _T_ in this case
will be 24 hours. We can now find _r_ (using _g_ = 32 feet per second
per second and _R_ = 3960 miles), and then obtain the distance _r_ -
_R_, which will be 22,300 miles. By using our previous formulas, we also
can find the velocity of a satellite moving in this orbit, which will
turn out to be _v_ = 6870 miles per hour.
[Illustration: _One possible method of using synchronous satellites.
Signals from New York (N) to Paris (P) would go via satellite S₁;
signals from New York to Calcutta (C) would go via satellites S₁ and
S₂._]
_The illustration above_ gives a rough idea of how a synchronous
satellite system might be set up. Three communications satellites, _S_₁,
_S_₂, and _S_₃, are above the equator in fixed positions equal distances
apart and 22,300 miles up. Located in this manner, they would cover the
major part of the earth’s surface. From a point directly beneath it, the
distance would be 22,300 miles to a satellite; from other points the
slant range would be greater. Signals sent from, say, New York (point N)
to Paris (point P) would be reflected via satellite _S_₁. In doing this,
they would travel a total distance of about 46,000 miles. Because we
can’t send signals any faster than the speed of light (186,000 miles per
second), it would take at least a quarter of a second for a signal to go
this far. For communicating a much greater distance, say from New York
to Calcutta (point C), the signal path would use two satellites, _S_₁
and _S_₂. In this case, the total distance traveled by a signal would be
more than 90,000 miles, and the one-way time delay would be about half a
second.
The Effects of Time Delay
Delays of a quarter- or half-second have different effects on various
kinds of communications. However, we are concerned here only with what
they might do to telephone conversations. Time delay will affect
conversations in two ways. One of these—pure delay—depends on the nature
of speech and the way people use it to converse; the other—echo—has to
do with the nature of the world’s telephone systems.
The first effect can be illustrated by an example. Suppose that George
in Paris is talking to me in New York. He says, “Do you want to go?” and
I answer “Yes” immediately upon hearing the word “go.” But that word
didn’t arrive in New York until a quarter of a second after George said
it, and my reply was delayed another quarter-second, so George hears my
instantaneous reply a half-second late. Under some circumstances, he
might interpret this delay to mean that I was less than enthusiastic
about going. We don’t know exactly what response times people expect in
conversation, or how much variation in such intervals they can tolerate.
But it has been assumed that delays of a half-second or more would make
a noticeable and perhaps disturbing difference. A little later on, I
will describe an experiment dealing with this first effect. But first we
must briefly discuss the second effect of delay on telephone
conversation, to show why we decided to try to isolate the first effect
and study it separately.
The Echo Problem
All the world’s telephones are individually connected to the rest of the
system by what we call _two-wire local loops_. Speech travels in both
directions on the same wires over these local parts of the circuit. In
other parts of the system, where speech travels farther and must be
amplified, it is carried over four-wire circuits. These consist of two
pairs of wires, one for transmission in each direction. At the junctions
where the two-wire and four-wire parts of the telephone system meet,
specially designed transformers, called _hybrid coils_, are used.
It is impossible to have these junctions between two-wire and four-wire
circuits always in perfect balance, so part of the speech that reaches a
local loop will be reflected back along the path on which it arrived.
Unless a circuit has been specially treated, this reflected speech will
get all the way back to where it started, and the talker will hear an
echo of his own voice. When the circuit is short enough, the echo is
heard almost instantaneously, and is not bothersome. But when the echo
is delayed by a twentieth of a second or more, it can become extremely
annoying, and even temporarily destroy one’s ability to speak
coherently.
Telephone engineers have long been aware that this echo effect was
present on their long-distance circuits, and they have not let it go
unchecked. Devices known as _echo suppressors_ are installed on circuits
that have more than a critical amount of delay. They are placed in a
four-wire part of the circuit, where there is one-way transmission over
each pair. Since incoming and outgoing sounds are using separate paths,
an echo suppressor can attenuate or shut off the return path when speech
is coming in on the other path.
Unfortunately, echo suppressors have effects of their own on
transmission. They may, for example, cut off some speech that should be
getting through, because they can’t distinguish it from echo. Echo
suppressors can be made more sophisticated, but whether they can be made
to operate more successfully than present ones is not clear. And the
problem of adapting them to the long delays of synchronous satellite
circuits will require a great deal of research and development effort.
Experimenting With Pure Delay
Although we don’t know how good echo suppressors can get, we do know
that a long circuit with the best possible suppressors could never be
_better_ than a circuit of the same length that had no echos. This
brings us back to the problem of how serious the effect of delay alone
is on conversations. If the delay in a synchronous satellite system,
even without any echo, made conversation all but impossible, there would
be little point in developing echo suppressors for such satellites.
This question looked like one that we could answer, at least in part, by
experimenting with special four-wire circuits that had delay but no
echo. The strategy we adopted, then, was to do some experiments on pure
delay while other people at Bell Telephone Laboratories began to attack
the problem of testing and improving echo suppressors. In the pages that
follow, I will describe one of our experiments on the pure delay
problem. More elaborate ones have been performed since, and there will
be more to come.
It should now be clear how this sort of experiment might be helpful to
the development of a synchronous satellite communications system. If it
showed pretty convincingly that conversation was extremely difficult
with a pure echo-free delay of about a second, synchronous satellites
for two-way conversations would be less practical. On the other hand, if
the experiment were to show that some conversation, at least, could be
carried on without too much difficulty, our results would be less
decisive. We would know only that _echo-free_ delayed circuits might
_sometimes_ be all right. But we would not know how bad they were under
a variety of conditions or how closely they resembled a real circuit
with echoes and echo suppressors. In either case, the experiment would
have no bearing on the use of synchronous satellites for one-way
purposes, such as television.
Designing the Experiment
[Illustration: _Recording equipment used in the experiment._]
delay = (θ)/(360) ρ, where ρ is the period of rotation in seconds
“Can people converse over an echo-free, four-wire circuit that has delay
like that of a synchronous satellite?”—that is one way of putting the
question we isolated to study. The next problem was to find a way of
setting up this question in the form of an experiment whose results
might be interpreted as a meaningful answer. The problem of apparatus,
fortunately, was fairly simple. Two telephone sets in separate rooms
were connected by four wires, with one pair going directly from each
transmitter to the other receiver, so that no echo would go back the way
it came. This gave us the “echo-free, four-wire circuit” we wanted. To
simulate the satellite, I inserted a magnetic delay device (_see
sketch_) between one of the transmitters and the other receiver. This
had a revolving drum on which speech could be recorded and then played
back a short interval later. By moving the playback head, I could
produce any amount of delay up to two satellite bounces’ worth. At this
point, the equipment was ready, but there were still two major problems:
(1) how to get people to converse over the circuit in a natural way, and
(2) what to observe and measure that would give us an answer to our
question.
When you think about the first problem, you soon realize that it is hard
to say exactly what a “natural” conversation is. But even if we can’t
describe it, we can try to find examples of it. I experimented with
several ways of making people talk that could be recognized as
_un_natural: word games, list-checking, shape description and
recognition, and a system of rewards for spurts of talk. None of these
schemes seemed to generate the real conversational interplay we wanted.
Finally, I noticed that my coworkers often got involved in vigorous
conversations on political and social issues at the lunch table. So I
circulated a questionnaire to help me pick pairs of people who might
enter into lively discussions on one or more topics. I arranged seven
conversations of this sort, and they became the basic material of my
study. Only one of these lacked sufficient spirit to yield good data,
and six out of seven is a pretty good percentage when you try to study
human behavior in such a free situation. Note that the conversers
expressed ideas that came from _within_ them at the time—not from any
external materials—and that they usually felt rather strongly about what
they were saying to the other fellow, who disagreed and therefore needed
some convincing. Of course, these conversations do not represent the
whole range of possible conversations; they are only a small sample of
one type. This doesn’t limit the truth of the particular result we got,
but it does limit how far we may generalize from these results.
What Should We Observe?
You might think we could solve this problem simply by asking the
conversers their opinions. We found out long ago, however, that the
opinions you get are affected by a lot of things: how you ask the
question, the attitude of the respondent, and his unrelated experiences
outside the experiment. So we usually try a more subtle approach. In
this case, my basic observation was of what psychologists call _escape
behavior_. The conversers were told that they would start talking over a
normal circuit, and that delay would be introduced at some point. (The
delay was inserted in such a way that an abrupt change could not be
noticed.) All the conversers had pushbuttons for signaling the
experimenter. If they thought they noticed a delay, they were told that
it would be removed if they pushed the button. Thus they could always
escape from this possibly unpleasant condition.
My reasoning was this: If the conversers found it very difficult to talk
with delay in the circuit, they would surely push the button soon after
the delay was introduced. On the other hand, any time when they
continued to converse without pushing the button—while delay was in the
circuit—was obviously also a time when the delay did _not_ make
conversation impossible. So we had at least one measurable quantity—the
time taken to detect delay—which we could interpret as an answer to our
question. Note that we could tell if people pushed the button “just to
be on the safe side” by seeing how often they did this when there
actually was no delay in the circuit.
There are just a few more necessary details before we discuss the
results:
—I recorded the conversations for later analysis, but the conversers
knew that the recordings would be held in confidence.
—The amount of delay used was 1.2 seconds, which represents the total
round-trip delay for a circuit using two satellites, including the
delay in typical end connections on the ground. We used this much
delay because our preliminary tests indicated that it would be more
likely to produce an effect than would the 0.6 second delay in a
one-satellite link.
—The entire 1.2 seconds of delay was put into one of the lines, since we
had discovered that, where there is no echo, conversers cannot tell
the difference between a delay of 2_t_ seconds in one line and a delay
of _t_ seconds in each of two lines. I did this for the sake of
convenience, so that I could introduce delay in the quiet line while
the other one was active.
—After someone detected delay, I removed it immediately and then waited
at least a minute before putting it back in.
—Altogether, I collected about two hours of conversation and introduced
delay 22 times.
The Results and What They Mean
Now we could answer the question, “How long _does_ it take people to
detect 1.2 seconds of delay?” As you can see from _the table opposite_,
the times ranged all the way from 20 seconds to over 10 minutes, and, in
two cases delay was not detected at all. The results in the table are
also shown in _the histogram on the next page_, which depicts how
broadly the detection times were distributed. To me, one of the most
interesting things is that even people who were able to detect delay
quickly sometimes did _not_ detect it for a couple of minutes. For
example, the pair K/G had two times under a minute, one of 143 seconds,
and one of 421 seconds. I interpret their two short times to mean that
they knew what to look for, since they made no incorrect responses while
delay was not present. However, their long times seem to mean that they
sometimes didn’t notice delay for quite a while. Incidentally, only two
responses were made during the total of about 40 minutes when I did not
introduce delay, and these “false alarms” were by two of the fastest
pairs at true detection—F/K and S/H.
Length of Time Before Seven Pairs of Talkers Could Detect 1.2 Seconds of
Delay
PAIRS OF TALKERS NUMBER OF SECONDS BEFORE TALKERS DETECTED DELAY
G/H 161
224
107
F/K 87
65
43
220
false alarm
A/L 618
95
367
F/T no detection after 954 seconds
S/H 227
false alarm
90
75
83
K/G 38
421
20
143
S/W 257
229
no detection after 260 seconds
[Illustration: _Data from table have been arranged in this histogram
to show wide range of delay detection times that were recorded in
the experiment. Each block represents a single delay detection,
identified by initials of conversers._]
number of cases
6
S/H
5
S/H
4
S/H S/W
3
K/G A/L S/H
2
K/G F/K K/G F/K
1
F/K F/K G/H G/H S/W A/L K/G A/L
0
0 50 100 150 200 250 300 350 400 450 500 550 600 650
detection time (seconds)
Now, to answer our question about whether people can converse over our
circuit, we can say something like this: We have found some cases (of a
certain type of conversation) where people _can_ use a circuit with 1.2
seconds of echo-free delay for the amounts of time listed in our table.
But there are two important things to remember about our results:
—_The findings are a_ non-negative _answer to the original question,
“Can people converse over a four-wire, echo-free circuit that has
delay like that of a synchronous satellite?”_
—_The experiment applies_ only _to four-wire, echo-free circuits, and
does not help with the problem of improving echo suppressors or with
that of finding out how good synchronous satellite circuits with the
best possible echo suppressors would be if they were used to
interconnect the world’s telephones._
Since we didn’t get a “no” answer to our question, we have been
encouraged to do more experiments with other subjects and other types of
conversations. We also have begun to look for more than a yes-or-no
answer; we now want to find out how serious various amounts of pure
delay would be. Some of my colleagues have been working on this problem
by furnishing special four-wire telephones to a group of people, so that
echo-free delay can be inserted in the line when one member of this test
group calls another member. Their experiment has confirmed our finding
that conversation with a round-trip pure delay of 1.2 seconds is not
impossible, but it has also shown that the degradation of conversation
that results is not trivial.
Recently an international committee on commercial telephone standards
set the maximum permissible echo-free delay (round-trip) at 0.7 seconds.
However, the search for a more precise evaluation is still going on.
Peter D. Bricker _was born in Scranton, Pennsylvania, and received an
A.B. from Bucknell University in 1950 and an M.A. in 1952 and a Ph.D.
in 1954 from the Johns Hopkins University. He joined Bell Telephone
Laboratories in 1954, and has been engaged in psychological studies of
telephone color preferences, pushbutton set designs, voice
identification, and transmission quality evaluation._
CASE HISTORY NO. 6
How Can We Repair an Orbiting Satellite?
E. Jared Reid
_Electrical Engineer—Member of Staff, Satellite Design Department_
THE PROBLEM
_It is hard enough to fix a new piece of scientific equipment when it
goes out of order in the laboratory. And when the equipment is sailing
around the earth a couple of thousand miles up in the sky a repair job
ought to be impossible. But during the last two months of 1962 we found
this not to be true at all. That was when the Telstar I satellite began
to misbehave and eventually would not obey the commands we sent it from
the ground. This presented us with a nice little problem: We had to find
out exactly what was wrong with the satellite and then—the really tough
job—devise a way to cure the trouble. We were able, finally, to do both
these things, after a combination of logical deduction, trial-and-error
experimentation, and laboratory testing—plus a certain amount of plain
good luck. Our “cure,” unfortunately, turned out to be only a temporary
one, for our patient had a relapse some weeks later. However, the story
of how we went about doing our never-before-attempted task should give
you an idea of the things you have to improvise in the laboratory when
an experiment doesn’t work out exactly as you had planned._
The Telstar Command Circuit
As shown on pages 32 and 33, the operation of the Telstar I satellite
was controlled by orders sent from the ground on a frequency of 123
megacycles. Fifteen different commands could be given to the satellite,
each a coded signal made up of a series of ones and zeros. The signals,
as you can see in the _table below_, turn on or off the radiation
experiments, the telemetry, the communications equipment, and the
orientation coil. These are important functions, and we wanted them to
be going on when they were needed. But we did not want them to be
operating continuously.
Command was the only Telstar I function that we felt had to be
“redundant,” so two duplicate chains of components were provided. As you
can see in _the block diagram on the next page_, the satellite has two
radio receivers in parallel, so that one can operate if the other fails.
There are also two command decoders, which take the pulse-coded signals
from the receivers and translate their zeros and ones into usable
instructions. In the command switching control, these instructions
operate nine relays that turn on or off the power to all the electronic
circuits except the command receiving chain, which operates
continuously.
Telstar I’s telemetry unit reported back 112 measurements every minute
over the 136-megacycle frequency. These told both what the satellite
encountered in space and the condition of the satellite’s own
components—as indicated by a variety of different sensors. The telemetry
also gave a check on whether the commands sent to the satellite were
actually obeyed.
The Fifteen Telstar Commands
COMMAND FUNCTION
A Turns on traveling-wave tube filament voltage
B Turns on traveling-wave tube helix and collector voltages;
energizes all transistor circuits associated with
communications experiments
AA Turns off traveling-wave tube helix, collector, and filament
voltages; de-energizes transistor circuits
C Turns on traveling-wave tube by applying anode voltage
CC Turns off traveling-wave tube anode voltage
D Turns on telemetry and energizes radiation experiment circuits
DD Turns off telemetry and de-energizes radiation experiment
circuits
E Turns on current in orientation torque coil
EE Turns off current in orientation torque coil
F Connects telemetry encoder No. 1 to circuit
FF Connects telemetry encoder No. 2 to circuit
SS Performs duties of AA, CC, DD, EE, and FF; de-energizes
136-mc beacon transmitter and removes all load from storage
battery
S Connects storage battery back into the circuit and energizes
the 136-mc beacon transmitter
T1 Turns off command receiver and decoder No. 2 for 15 seconds,
so that command receiver and decoder No. 1 can be tested
T2 Turns off command receiver and decoder No. 1 so that No. 2
can be tested
[Illustration: _This block diagram of Telstar I’s command circuit
shows the redundant receiver and decoder chains, No. 1 and No. 2,
which are fed the command signals from the ground that are picked up
by the VHF antenna. Decoded instructions go to the command switching
control, which operates relays to turn equipment on or off. Reports
on the operation of this control are sent back by telemetry to the
ground station along with information from the satellite’s
sensors._]
VHF antenna
diplexer
command receiver No. 1
command decoder No. 1
T1 command
command receiver No. 2
command decoder No. 2
T2 command
command switching control (operates relays)
S-SS relay
D-DD relay
other relays
136-mc beacon transmitter
SS open
S close
telemetry
DD open
SS open
D close
sensors (radiation detectors, particle counters, solar aspect cells,
thermistors, etc.)
power supply
solar cell power plant
storage battery power plant
SS open
S close
through other relays to communications circuits and orientation torque
coil
What Went Wrong With Telstar I
During Telstar I’s first two months in orbit, the only indication of
trouble cropped up in one of the command operations. Telemetry told us
that the satellite was no longer executing the T2 command. This meant
that we could not temporarily disconnect command chain No. 1 to check
the performance of chain No. 2. Then, a short while later, No. 2 began
to give intermittent operation. Finally it failed completely. At the
time, we didn’t know why this had happened, but, since the satellite’s
other command chain still seemed to be operating normally, we were not
very worried.
However, in the middle of November 1962 command chain No. 1 also began
to be intermittent. We would send a command but get no response from the
satellite; only after we repeated it a few times would the satellite
finally do what it had been told to do. Now there was something to be
concerned about. And, if chain No. 1 should fail, we had to make sure
that Telstar would be left in a favorable operating condition. We didn’t
want the satellite’s communications equipment to be left on without our
being able to turn it off—this would keep a continuous drain on the
power supply.
As we feared it would, the other command circuit went out of commission
on November 23rd. However, when this happened, the communications
circuits had been turned off, although the command chains themselves and
the telemetry remained on. This meant that we could still try to send
commands, the condition of the satellite could still be monitored by
telemetry, and the solar cells could still supply useful power. But,
since we could not turn the communications equipment on, Telstar I could
no longer be used for transatlantic television or any of the experiments
we had been carrying on successfully since July.
Looking for the Trouble Spot
At this point a number of Bell Telephone Laboratories engineers began to
analyze Telstar’s troubles. As you can imagine, we had a rather
difficult problem. We obviously could neither go up and look at Telstar
nor bring it down for an overhaul on the ground. We could only send
different commands to the satellite and watch the telemetry data to see
what, if anything, happened.
After checking the satellite’s other equipment, we were happy to find
that everything except the command chains was in good condition. So we
decided the trouble had to be one of five possibilities:
—_excessive electronic “noise” in the satellite, which had blocked the
command receivers;_
—_extreme temperature variations, which had caused a joint to expand,
contract, and finally break;_
—_a loose connection;_
—_slow aging of an electronic component;_
—_deterioration of a component from excessive radiation bombardment._
We could quickly narrow this list down. Telemetry indicated that the
receiver was not being blocked by noise. Reports from the
temperature-measuring thermistors told us that inside the satellite the
temperature was 75 degrees Fahrenheit, just as it should be. A loose
connection was very unlikely, because every one had been made by expert
wiremen, examined by trained inspectors, and then completely
encapsulated in polyurethane foam. Aging also seemed very improbable,
since all the components had been individually tested and selected for
the highest reliability and longest life.
The Villain: Radiation
This left only radiation damage. We had other good reasons to suspect
this, too. As far back as October 1961, scientists at Bell Labs and
Brookhaven National Laboratories had made an important discovery about
the effect of radiation on a transistor. They found that, when radiation
penetrates the outer shell of a transistor and ionizes the gases inside,
electrically charged particles (ions) tend to collect on the surface and
change the transistor’s electric properties. This effect is particularly
noticeable when a transistor is operating under reverse bias voltage. We
knew that some of the 37 transistors used in each Telstar I decoder
circuit were operating under continuous reverse bias and that they also
had less metal shielding than did those semiconductors in the telemetry
and receiver circuits.
Telstar’s radiation detectors had been telling us that the concentration
of high-energy electrons near the inner edge of the Van Allen belt was
greater than we had expected. We now know this may have come as a result
of man-made high-altitude nuclear explosions, one of which took place
the day before Telstar I was launched. But we had had no reason to
anticipate this extra radiation—it was more than one hundred times the
predicted level—when we tested the transistors to be used in Telstar I.
So we were not too surprised that they were more susceptible to
radiation damage than we had thought they would be.
All this seemed to give us a theoretical explanation for the trouble.
So, after November 23rd, we began looking for laboratory evidence to
confirm our radiation theory. First, two engineers traveled down to
Johannesburg, South Africa. At this time the highest point of Telstar’s
orbit—when it passes through the least Van Allen belt radiation—was over
the southern hemisphere, and it seemed a good idea to command the
satellite when it was under the condition of lowest radiation and see if
anything would happen. However, everything the engineers tried proved
fruitless.
[Illustration: _A duplicate of one of Telstar I’s command decoders,
containing 37 transistors and 191 diodes, being placed in a small
elevator that will lower it into a gamma radiation chamber._]
At our Murray Hill, New Jersey, laboratories we worked on a different
approach. We exposed transistors like those used in the decoders to
large doses of radiation. We also exposed entire spare decoder units to
accelerated radiation to find out where their weakest points were (_see
illustration_). And then we built and tested decoders using
radiation-resistant transistors to see if they worked better. After a
week of intensive laboratory work, we had some pretty good evidence. The
tests of individual transistors definitely showed that heavy radiation
would cause them to deteriorate. Testing of the complete decoders also
led to some failures, and, when we analyzed them, they turned out to be
the kind that would be caused by faulty transistors. We also discovered
that the most sensitive part of a decoder circuit was the zero gate,
which recognizes the zeros in the one-and-zero code that commands the
satellite.
[Illustration: _A typical command signal sent to Telstar I. It
consists of seven pulses: a three-unit “start” pulse and a binary
code made up of three two-unit “one” pulses and three one-unit
“zero” pulses._]
Fooling the Decoder
Now we thought we knew the guilty component, but the hardest job still
lay ahead of us. We had to do something to the commands so that they
would bypass this troublesome zero gate. Each of the fifteen satellite
commands is a binary code of seven pulses, as illustrated in _the
diagram above_. The first—the _start pulse_—is three units wide. Then
follow six more pulses of which three are two units wide (_one pulses_)
and three are one unit wide (_zero pulses_). The arrangement of this
group of six ones and zeros determines the particular command.
Each time a one pulse arrives at the decoder, a _one gate_ counts the
pulse and stores a one in its memory. A _zero gate_ counts the zero
pulses, but does not store anything. So, if the zero gate is blocked,
the decoder will not count the zeros in any of the coded commands and
thus cannot decode them properly.
[Illustration: _The special “notched one” pulse that was invented to
fool Telstar I command decoder._]
What could be done about this? The answer seemed to be to devise a new
type of pulse—a pulse that would be enough like a one so that it would
pass through the one gate and advance the counter, but, at the same
time, be enough unlike a one so that the one gate would not store it in
its memory. This led to the invention of the special long pulse with a
dip or notch in it that is shown in _the diagram above_. When we tested
it in the laboratory on one of the duplicate decoders we had exposed to
radiation, this new notched pulse worked as we hoped it would. It passed
through the one gate and advanced the counter, but was not stored as a
one in the one gate’s memory. Thus it fooled the decoder by doing just
what a zero is supposed to do, even though it had gone through the one
gate rather than the zero gate.
[Illustration: _Magnetic tapes of special codes using notched ones
in place of zeros being prepared for use._]
But the real test was yet to come. Special modified signals for two of
the fifteen Telstar commands, using our new notched ones in place of the
usual zeros, were put on magnetic tape (_see photograph_). Then, on
December 20th, when Telstar made its 1492nd pass over Andover, Maine, a
group of tired engineers huddled about the mass of equipment they had
assembled. Finally, on the third try, the notched pulses were
successful; Telstar’s telemetry flashed back the word that the proper
relay had operated upon command.
Removing the Ionization
We now wanted to get Telstar to do something that had seemed to work in
the laboratory. The transistors most affected by radiation were those
operating under continuous reverse bias, to whose surfaces unwanted ions
were attracted. If we removed the voltage from these transistors, we
felt that the ionization layer would be dissipated, and they would act
normally again. Our plan was to prepare a complete taped program of all
fifteen commands, and carefully disconnect Telstar’s storage battery
(using command SS). Then, when the satellite went into eclipse, there
would be no power available from the solar cells either, and—if our
calculation was right—the complete lack of voltage ought to restore the
transistors to working order. This was a hazardous procedure, for if
something went wrong we might have a completely silent satellite on our
hands.
As it turned out, an accident did happen—but one of a different and much
more fortunate kind. On December 27th Telstar misinterpreted our “trick”
commands and disconnected its own battery before we asked it to. Then,
as the satellite went into the earth’s shadow, we held our breath while
all its power stopped and the telemetry went silent. But, as we had
hoped, a rest period with all power removed from the deteriorated
transistors apparently made them work almost normally once again. On
January 1, 1963, we were able to disconnect the battery in regular
fashion—that is, using the one-and-zero code. After this was done, and
all power had been removed, both decoders again would operate when given
normal commands (actually, the first one restored to duty was decoder
No. 2, which had gone out of order first, back in August).
Back to Normal—For a Time
For more than a month Telstar I behaved as it should, and our
communications experiments, including television broadcasts, were
resumed on January 3rd. During this time we used both normal commands
and our special notched-pulse modified commands. Whenever normal
commands became intermittent we used the modified commands to disconnect
the battery for several eclipses.
Our good fortune, however, did not last. Continued exposure to radiation
apparently led to further damage to Telstar I’s transistors. By February
14th, disconnecting the storage battery no longer returned the decoder
to normal, and we could operate only with our modified commands. And, on
the 21st, the satellite apparently misinterpreted a command,
disconnected its storage battery, and went silent. Since then, none of
our modified commands has been able to bring back its voice. There is
still a possibility that Telstar I may recover if it remains out of the
high-radiation part of space for a long enough period—but as time goes
by this appears less likely.
However, our work was not in vain. Because we pinpointed the effects of
radiation on the transistors in Telstar I, this problem was counteracted
on the Telstar II satellite launched on May 7, 1963 (see page 31). To
avoid the worst of the radiation effects, the second Telstar is in a
considerably larger orbit, which causes it to spend less time in the
heaviest high-energy Van Allen belt regions. It carries new radiation
detectors with much greater measuring capacity. And in one of Telstar
II’s command decoders we are using a new type of transistor, which we
hope will not be affected nearly as much by radiation as were the ones
in Telstar I’s ill-fated decoders.
E. Jared Reid _was born in Hartford, Connecticut, and received a B.S.
from Trinity College in 1956, a B.E.E. from Rensselaer Polytechnic
Institute in 1957, and an M.E.E. from New York University in 1959. He
joined Bell Telephone Laboratories in 1957, and has worked on the
design and testing of the Time Assignment Speech Interpolation (TASI)
system for the transatlantic cable, as well as on transistor circuits
for the Telstar satellite._
A Final Note to the Reader
_Now, having read Part II of_ Satellite Communications Physics, _you
should have an idea how we predict the orbit of an artificial satellite
and how we find out where it points while passing a thousand miles above
our heads. You can see how we pick the best material to cover its
surface with and how we protect its solar cells from the hazards of
space. And you have watched the steps we would take when our satellite
stops working properly._
_It would, we admit, take a little more experience to solve problems
like these on your own—and to deal with all the other complications of
satellite communications. But we hope our brief glimpses into the
laboratory have shown what this experience might be like. Our six case
histories have only scratched the surface, but they should give you a
good idea of the fascinating work that goes into practical science and
engineering. They should show that something like Project Telstar
doesn’t succeed only because of far-sighted, imaginative thinking—nor
only because of ingenious engineering. It draws upon the best of both of
these._
_Along the way, we hope you have noticed some important
guideposts—things like Newton’s law of gravitation, the law of
reflection of light, the Stefan-Boltzmann law. They typify the basic
principles of physics that engineers and scientists, whatever they do,
must always keep in mind. No matter how exotic or up-to-the-minute the
application, the ground rules of physics must be followed. If we have
convinced you of this, we have done what we set out to do!_
Suggested Reading
If you would like to read further about satellite communications in
general or get some information about the case histories in Part II, you
may be interested in using the following reading list. The references
under each of the subheadings are listed chronologically; they include
books, reports, technical papers, and magazine articles. As you can see,
some of these ought to be understandable by almost anyone, but others
are quite technical in nature.
For further background in the basic physical principles that are
discussed in Part II, you may refer to many good high school and college
physics texts. An increasing number of useful physics books—both
originals and reprints—are now being published in paperback form.
Satellite Communications
Arthur C. Clarke, “Extra-Terrestrial Relays—Can Rocket Stations Give
World-Wide Radio Coverage?,” _Wireless World_, October 1945, page
305.
John R. Pierce, “Orbital Radio Relays,” _Jet Propulsion_, April 1955,
page 153.
John R. Pierce and Rudolf Kompfner, “Transoceanic Communication by Means
of Satellites,” _Proceedings of the I.R.E._, March 1959, page 372.
John R. Pierce, “Exotic Radio Communications,” _Bell Laboratories
Record_, September 1959, page 323.
Steven M. Spencer, “Dial ‘S’ for Satellite,” _The Saturday Evening
Post_, January 14, 1960, page 13.
Space Electronics Issue, _Proceedings of the I.R.E._, April 1960.
William Meckling, “Economic Potential of Communication Satellites,”
_Science_, June 16, 1961, page 1885.
Special Issue on Project Echo, _Bell System Technical Journal_, July
1961.
C. C. Cutler, “Radio Communication by Means of Satellites,” _Planetary
and Space Science Journal_, July 1961, page 254.
W. C. Jakes, Jr., “Project Echo,” _Bell Laboratories Record_, September
1961, page 306.
John R. Pierce, “Communication Satellites,” _Scientific American_,
October 1961, page 90.
United States Senate, Committee on Aeronautical and Space Sciences,
_Communication Satellites: Technical, Economic, and International
Developments_ (staff report), U. S. Government Printing Office,
Washington, 1962.
L. J. Carter, editor, _Communications Satellites_, Academic Press, New
York and London, 1962.
“Situation Report on Communications Satellites,” _Interavia_, June 1962,
page 749.
Leonard Jaffe, “Communications by Satellite,” _International Science and
Technology_, August 1962, page 44.
“Communicating by Satellite,” _Business Week_, October 27, 1962, page
86.
Project Telstar
Rowe Findley, “Telephone a Star,” _National Geographic_, May 1962, page
638.
Louis Solomon, _Telstar_, McGraw-Hill Book Company, New York, 1962.
Special Telstar Issue, _Bell Laboratories Record_, April 1963.
Special Telstar Issue, _Bell System Technical Journal_, July 1963.
Satellite Communications Case Histories
_1. How Do We Calculate a Satellite’s Orbit?_
Mario Iona, “Satellite Orbits,” _The Physics Teacher_, May 1963, page
55.
A. J. Claus et al., “Orbit Determination and Prediction and Computer
Programs,” _Bell System Technical Journal_, July 1963, page 1357.
_2. What Color Should a Satellite Be?_
P. T. Haury, “Thermal Design of the Electronics Canister,” _Bell
Laboratories Record_, April 1963, page 161.
J. W. West, “Space Hardware Aspects of the Satellite,” _Bell
Laboratories Record_, April 1963, page 167.
Peter Hrycak, et al., “The Spacecraft Structure and Thermal Design
Considerations,” _Bell System Technical Journal_, July 1963, page
973.
_3. How Do We Make Optical Measurements on a Satellite?_
W. C. Jakes, Jr., “Participation of the Holmdel Station in Project
Telstar,” _Bell System Technical Journal_, July 1963, page 1421.
_4. How Do We Keep Solar Cell Power Plants Working in Space?_
D. M. Chapin et al., “The Bell Solar Battery,” _Bell Laboratories
Record_, July 1955, page 241.
G. R. Frost, _From Sun to Sound_, Bell Telephone Laboratories, New York,
1961.[6]
F. M. Smits, K. D. Smith, and W. L. Brown, “Solar Cells for
Communications Satellites in the Van Allen Belt,” _Journal of the
British I.R.E._, August 1961, page 161.
D. M. Chapin, _Energy from the Sun_, Bell Telephone Laboratories, New
York, 1962.[6]
R. E. D. Anderson et al., “The Satellite Power System,” _Bell
Laboratories Record_, April 1963, page 142.
K. D. Smith et al., “The Solar Cells and Their Mounting,” _Bell System
Technical Journal_, July 1963, page 1765.
_5. Would Time Delay Be a Problem in Using a Synchronous Satellite?_
G. M. Phillips, “Echo and Its Effect on the Telephone User,” _Bell
Laboratories Record_, August 1954, page 281.
W. A. van Bergeijk, J. R. Pierce, and E. E. David, Jr., _Waves and the
Ear_, Anchor Books (Science Study Series paperback), Doubleday &
Company, New York, 1960.
R. P. Haviland, “The Synchronous Satellite,” in _Communications
Satellites_, L. J. Carter, editor, Academic Press, New York and
London, 1962, page 113.
_6. How Do We Repair an Orbiting Satellite?_
D. S. Peck et al., “Surface Effects of Radiation on Transistors,” _Bell
System Technical Journal_, January 1963, page 95.
“Fixing Up Telstar,” _Time_, January 18, 1963, page 48.
E. P. Moore and W. J. Maybach, “Satellite Command and Telemetry
Systems,” _Bell Laboratories Record_, April 1963, page 156.
J. S. Mayo et al., “The Command System Malfunction of the Telstar
Satellite,” _Bell System Technical Journal_, July 1963, page 1631.
_Note_: The _Bell Laboratories Record_ is published by Bell Telephone
Laboratories, Incorporated, 463 West Street, New York 14, New York.
_The Bell System Technical Journal_ is published by the American
Telephone and Telegraph Company, 195 Broadway, New York 7, New York.
The Editor
Ronald M. Foster, Jr., _was born in Plainfield, New Jersey, and received
an A.B. degree from Harvard College in 1948. He joined Bell Telephone
Laboratories in 1956, and is a member of the Educational Aids Department
of the Public Relations and Publication Division. He is engaged in
development of material for the Bell System Aid to High School Science
Program._
Footnotes
[1]This is obtained from _k_ = _gR_², where _g_ is the acceleration due
to gravity and _R_ is the radius of the earth. (Here, we can use _k_
= 96,500 miles³ per second².)
[2]Donald R. Herriott of Bell Labs had suggested using plane reflectors
on satellites as long ago as 1957—although his idea was that this
would increase their visibility, rather than aid in determining
their attitude.
[3]This method was developed by D. W. Hill of Bell Telephone
Laboratories.
[4]We will not attempt to go into all the details of semiconductor
physics here. If you would like to know more about how solar cells
work, refer to the Suggested Reading on page 88.
[5]See pages 42 and 43.
[6]Published as part of the Bell System Aid to High School Science
Program.
[Illustration: Wraparound cover image]
Transcriber’s Notes
—Silently corrected a few typos.
—Modified some image references to reflect the pageless flowable eBook
format.
—Retained publication information from the printed edition: this eBook
is public-domain in the country of publication.
—In the text versions only, text in italics is delimited by
_underscores_.
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