Hedy Lamarr's Secret Radio System for Torpedo Guidance

This patent, US 2292387, proposes a method for secure radio communication by rapidly switching the transmission frequency. The core idea is that both the transmitter and receiver jump between many different frequencies in a synchronized, unpredictable pattern. The original design, as commonly understood, used a pair of synchronized piano rolls to control these frequency changes. One piano roll would dictate the sequence of frequencies for the transmitter, and an identical, synchronized piano roll would guide the receiver to listen on the correct, matching frequency. This made it very difficult for an enemy to listen in or block the signal, as the frequency was constantly changing.

The clever bit

The truly novel aspect was the idea of synchronizing rapid frequency changes between a transmitter and receiver using a mechanical system, like a piano roll. This made the signal appear as random noise to anyone not knowing the precise hopping sequence, making it incredibly difficult to intercept or jam.

What it does not cover

• ×Does not cover communication systems that use a fixed radio frequency without any changes.
• ×Does not cover spread spectrum techniques that do not involve frequency hopping, such as direct sequence spread spectrum.
• ×Does not cover systems where the frequency changes are not synchronized between the transmitter and receiver.
• ×Does not cover purely electronic or digital methods for generating and synchronizing frequency changes, as the original patent describes a mechanical system.
• ×Does not cover systems where the frequency hopping pattern is easily predictable or static.