Back in 1986, the United States Navy applied for and received U.S. Patent 4,617,530 for a pseudo-random noise generator, or PRNG.
It was a relatively simple electronic circuit, as shown below:
Two (by now long-obsolete) noise generator ICs, four opamp stages, and a transistor, plus some resistors, and that’s basically it. How it differs from what had come before is its “arbitrarily long repetition rate”… where “arbitrarily long” is apparently “on the order of minutes”.
If that sounds suspiciously vague to you, it’s because this PRNG was not intended for cryptographic use, but to drive a jet-noise simulator. Why?
“…to trigger anti-aircraft bombs planted on runways and for deception of sensors used for simulators.”
This immediately caught my attention, because I couldn’t think of a cold-war era (or any, really) anti-aircraft weapon that was triggered by jet noise. A couple hours of Google searches later… I still don’t know of any such weapon, made by any country.
That’s where I hope you, dear reader, come in. Does such a thing exist? It seems a relatively logical idea, when you think about it, but perhaps everyone dismissed it as too obvious? There are numerous patents for the audible detection of aircraft, but none (that I can find) anywhere around 1986, and none that (IMO) seem obviously adaptable to integration into a weapon. We may well never know…