"In 1939 the Nazis were using U-boats to control the seas. The
Allied navies of the world were trying to use torpedoes against the
U-boats, but the torpedoes of the day were slow and released a
trail of bubbles that could easily be seen, so the torpedoes could
be avoided. Engineers realized that radio signals could be used to
steer the torpedo, but they also realized that if the radio signal
was detected it could be jammed, or worse yet, be overcome and
direct the torpedo back to the Allied ship that launched it!
"My hero had given a lot of thought to this, and reasoned that
the signal for the torpedo could be sent over several frequencies.
If there was some way to coordinate both the transmitter and the
receiver, so they were both using the same frequency at the same
time, yet keep changing the frequency in some secret, complex
pattern, then the radio signal would be a lot harder to detect and