Plain English Meaning
A row of antennas placed past the far end of the runway that beams a signal back toward approaching aircraft. The signal tells the aircraft whether it is lined up with the runway centerline or drifting off to one side.
Full Definition
The set of transmitting antennas on the ground that produce the localizer signal for an Instrument Landing System (ILS). The array is positioned on the centerline of the runway, beyond the stop end, and radiates two overlapping signals on either side of the extended runway centerline. An aircraft's ILS receiver compares the strength of these two signals to determine whether the aircraft is left of, right of, or on the centerline during an .