Prior to the World War II the Germans had deployed the Lorenz blind-landing aid at many airports and equipped most of their bombers with the radio equipment needed to use it.

Lorenz worked by placing two radio transmitters close to each other, near the end of the runway. The antennas were fairly directional, projecting their signals slightly to each side of the runway centerline, one slightly left, the other slightly right. One broadcast a series of widely separated dots, the other dashes, so the pilot could tell on which side of the runway centerline they were on by the sound of the signal. Key to the system was an area in the middle where the two beams overlapped, where the dots of the one signal "filled in" the dashes of the other, resulting in a steady tone known as the equi-signal.

Lorenz could fly a plane down a straight line with relatively high accuracy, enough so that the aircraft could then find the runway visually in all but the worst conditions.

See also

• Battle of the beams
• radio navigation