Hedy Lamarr's Secret Radio System for Torpedo Guidance

Hedy Lamarr and George Antheil's 1942 patent describes a secret communication system that rapidly changes radio frequencies to prevent enemies from jamming or eavesdropping on torpedo guidance signals.

What it covers

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.

What it doesn't 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.

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.

Why it matters

Although not widely adopted during World War II, this patent laid foundational groundwork for modern wireless communication. Its principle of frequency hopping spread spectrum is crucial for technologies that need to be robust against interference and secure. This concept is now central to many everyday devices and military applications.

Real-world examples

• 1.Wi-Fi (some standards)
• 2.Bluetooth
• 3.GPS (Global Positioning System)
• 4.Military radio communications
• 5.Cordless phones (older models)