Fixing Up Telstar

Lobbing a working communications satellite out to the fringes of space was a signal achievement. The task of repairing it in orbit seemed wildly improbable. But when Bell Telephone Laboratories' Telstar sullenly ignored a command after four months of faithful performance, Bell's electronic doctors wasted no time. While unresponsive Telstar circled the earth in silence, they spent six frantic weeks in their labs concocting a cure for its ailment.

Practical experience with Relay, the communications satellite built by RCA, was no help. Relay, which went into orbit on Dec. 13, refused to work properly right from the start; a defective part let most of the power leak out of its storage battery. But Relay still obeyed commands. It was told to throw switches to isolate the faulty transponder (transmitter-receiver). Solar cells were able to recharge the satellite's battery. Last week Relay was in operation, using its spare transponder.

Diagnosis. Telstar, which had no spare transponder, was in much more serious trouble. But its case was not hopeless. Its radio beacon was transmitting normally; so were the host of instruments that report by telemetry on its internal condition. They showed that Telstar's solar cells were generating plenty of electricity. Its temperature was normal, and no intruder, such as a meteorite, had damaged its delicate nervous system. Apparently the only trouble was in the command decoders. Telstar was ablebodied, but without working decoders it could not hear and obey commands.

One clue to Telstar's trouble was the curious manner in which the satellite had quit. Several times before total failure it had reluctantly obeyed a command only after it had been repeated for many minutes. This suggested that the passage of a signal through one of the ailing decoders tended to cure it in much the same way that exercise helps some human ills.

But what disease of space is cured by electrical exercise? The Bell men were aware that transistors behave strangely after they are bathed in powerful radiation. Ions (electrically charged particles) released by the radiation are attracted to electrically charged parts of the transistors. There they form a surface layer that prevents the transistors from working properly. Something of this sort may have happened to Telstar. It was built to resist an expected level of radiation in space, but just before it was launched, the U.S. exploded a powerful nuclear test bomb above the atmosphere near Johnston Island (TIME, July 20). Eminent scientists had dismissed the suggestion that the test would create much high-level radiation, but their forecast was wildly wrong. Telstar's instruments reported long-lasting radiation 100 times as strong as had been expected.

Zero Gate. To find out whether space radiation was the guilty party, Bell engineers hooked up a command decoder just like Telstar's and exposed it to gamma rays in a shielded chamber. It went out of action quickly, and the engineers traced the trouble to a single transistor called the "zero gate" designed to react to short pulses--coded zeros--in command signals. With the zero-counting transistor blocked by ions, the decoder could receive no zeros, and a binary code, which consists only of zeros and ones, is meaningless if deprived of half its vocabulary.

Theory was reinforced by practice when the engineers sent a long string of commands to the experimental decoder in the radiation chamber. Passage of the command signal through the decoder reduced the voltage on the zero-gate transistor. Its clustered ions dissipated, and the gate counted zeros once more.

This was the treatment that had doctored Telstar in the early stage of its radiation sickness, but stronger medicine was needed now. The electronic medicine men figured that one way to open the blocked zero gate would be to shut off power from Telstar's storage battery. When Telstar went into the earth's shadow, its solar cells would stop generating electricity. Telstar's sick transistors would lose all voltage. Their unwanted ions would drift away, allowing the circuitry to revive.

Ailing Transistors. But how to throw a switch on an orbiting satellite that cannot hear commands? This knotty problem was solved by devising a new code language. The short zero pulses in the coder were replaced by longer pulses with notches (a sort of hesitation) in their centers. These special pulses were designed to fool the decoder's "one gate," which advances a counter when it receives a long pulse (meaning a "one") and also stores the one in a memory system. The notched pulse, the engineers figured, would make the one gate advance the counter, but would not make it store a one in its memory. This is just what a short zero pulse is supposed to do when it passes through the zero gate.

Engineer Henry Mann put commands in the special new code, recorded them on magnetic tape and took them to Maine. When Telstar's orbit brought it near the ground station at Andover, Mann flashed a notched-pulse order telling Telstar to open a particular switch. The satellite doctors waited anxiously. Then a light glowed and meaningful numbers appeared in a punched tape. Telstar's telemetry system was reporting that the order had been understood and obeyed.

Next planned step was to turn off the storage battery so that ailing transistors could be rested and freed of ionization. This was a risky move; if it went wrong it might permanently silence Telstar's beacon and telemetry, turning it into a space derelict. While the engineers were bracing themselves to take the risk, Telstar decided the problem for them. Its decoder misunderstood a minor test command, reading it as an order to disconnect its battery.

When Telstar passed into the shadow of the earth, its solar cells went out of action too. With no power of any kind, the telemetry and beacon died. According to theory, the troublemaking ions were now clearing away from the transistors, but no one could be sure. For all the engineers knew, their notched code might have killed Telstar.

But the satellite was only resting. When it came into the sunlight again, its solar cells awoke and energized its circuits. A command went up from Andover in normal code language, and the decoder understood and obeyed. After the voltage-removal treatment had been repeated several times, the satellite recovered completely from its radiation sickness. Smoothly and precisely, it obeyed normal commands and transmitted TV pictures as crisply as it had when it was young in space.

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