Chapter III: Part 3

Text size

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_.

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.

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._

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.

_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

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

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

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.

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.

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.

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.

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!_

Comments

Satellite Communications PhysicsChapter III: Part 3

0%34 min left in chapter