Jim Stewart wrote:

You are describing, almost exactly, an early instrument
flying technique. Flying down the center of the beam,
the pilot hears a steady tone. Drift right or left and
he hears a Morse "r" or "a" character in the headset.

Not quite. a = dot dash
the opposite side was n
n = dash dot
r = dot dash dot

...lew... ( former W3SLX )