The airborne radio receivers—initially simple Amplitude Modulation (AM) sets—were tuned to the frequency of the low-frequency radio ground transmitters, and the Morse code audio was detected and amplified into speakers, typically in headsets worn by the pilots.[1] The pilots would constantly listen to the audio signal, and attempt to fly the aircraft along the course lines ("flying the beam"), where a uniform tone would be heard. If the signal of a single letter (A or N) became audibly distinct, the aircraft would be turned as needed so that the modulation of the two letters would overlap again, and the Morse code audio would become a steady tone.[2] The "on course" region, where the A and N audibly merged, was approximately 3° wide, which translated into a course width of ±2.6 miles when 100 miles away from the station.[4]