Localizer

Instrument landing system localizer (short: localizer [LOC or LLZ]) is a system of horizontal guidance in the instrument landing system, which is used to guide aircraft along the axis of the runway.

Each radio station or system shall be classified by the service in which it operates permanently or temporarily.

In aviation, a localizer is the lateral component of the instrument landing system (ILS) for the runway centreline when combined with the vertical glide slope, not to be confused with a locator, although both are parts of aviation navigation systems.

A localizer (like a glideslope) works as a cooperation between the transmitting airport runway and the receiving cockpit instruments. An older aircraft without ILS receiver cannot take advantage of any ILS facilities at any runway, and much more important, the most modern aircraft have no use of their ILS instruments at runways which lack ILS facilities. In parts of Africa and Asia large airports may lack any kind of transmitting ILS system. Some runways have ILS only in one direction, this can however still be used (with a lower precision) known as back beam or "Back Course" which is not associated with a glide slope.

Two signals are transmitted on one of 40 ILS channels. One is modulated at 90 Hz, the other at 150 Hz. These are transmitted from co-located antennas. Each antenna transmits a narrow beam.

Localizer (LOC) and glide slope (G/S) carrier frequencies are paired so that the navigation radio automatically tunes the GS frequency which corresponds to the selected LOC frequency. The LOC signal is in the 110 MHz range while the G/S signal is in the 330 MHz range.
LOC carrier frequencies range between 108.10 MHz and 111.95 MHz (with the 100 kHz first decimal digit always odd, so 108.10, 108.15, 108.30, etc., are LOC frequencies and are not used for any other purpose). See Instrument Landing System (ILS) Frequencies on even-numbered TACAN channels from 18X to 56Y.

The localizer indicator is (on most aircraft manufactured from the late 1950s) shown below the Attitude Indicator, but is still a part of this instrument together with the glideslope indicator and the cross in the center of the instrument which is called Flight Director.

...
Wikipedia