The first instrument approaches were conducted using the A-N range. These four-legged navigation stations produced either a Morse code A or N in the quadrants between legs or a tone when the aircraft was situated on one of the legs—and thus the origin of the term: "on the beam." There was no CDI and the pilot flew on course by sound in the headset. Due to the large antenna array required, these stations were placed off airports but near enough to provide a let-down capability. They also established an airway system across the country.
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