Together, starting from Lamarr's idea, they devised a pre-digital way to synchronize a radio transmitter and a receiver so that they can simultaneously switch from frequency to frequency while communicating a message. Signals transmitted on a single frequency can be easily located and jammed, but not a signal that is jumping randomly among, say, a few dozen different frequencies. (This technique, which Lamarr dubbed "frequency hopping," is now subsumed within the broader term "spread spectrum.")
