An aircraft with an on-board ILS receiver within the capture area of an ILS, (glideslope and localizer range), will detect varying depths of modulation according to the aircraft's position within that airspace, providing accurate positional information about the progress to the threshold.
The ILS uses two radio frequencies, one for each ground station (about 110 MHz for LOC and 330 MHz for the GS), to transmit two amplitude-modulated signals (90 Hz and 150 Hz), along the glidepath (GS) and the course (LOC) trajectories into airspace.
It is this signal that is projected up from the runway which an aircraft employing an instrument approach uses to land.
The difference between the two signal modulation depths is zero when the aircraft is on the correct course and glidepath on approach to the runway—i.e.
No difference (zero DDM), produces no deviation from the middle indication of the instrument's needle within the cockpit of the aircraft.