You might not be aware, but aircraft landing systems that locate the aircraft on the center of the runway use that same system to modulate an RF signal that is received by the aircraft from transmitters on either side of the runway.
It’s been a long time, but IIRC, the tones are 120 and 90 Hz. Too far to one side and you get solid freq of one or the other. Right down the middle you get the mixture of the two at equal amplitudes...........
https://en.wikipedia.org/wiki/Instrument_landing_system
I remember about that.