This is an early version of the localizer from the more modern instrument landing system (ILS). The localizer uses two tones (90 and 150Hz) modulated on a carrier (108.1-111.95 MHz) and transmitted on a wide aperture antenna array, this can be seen at almost all airports at the end of the runway. The strengths of which (or Difference in Depth of Modulation) are detected by electronics in the aircraft and present the pilot with an indication to fly left or right and indeed up or down with a co-located glidepath (328.6 to 335.4 MHz) equipment.