Based on a network of radio towers which transmitted directional radio signals, the radio range defined specific airways in the sky. Pilots navigated using low-frequency radio by listening to a stream of automated "A" and "N" Morse codes. For example, they would turn the aircraft to the right when hearing an "N" stream ("dah-dit, dah-dit, ..."), to the left when hearing an "A" stream ("di-dah, di-dah, ..."), and fly straight ahead while hearing a steady tone.[1][5]