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.