The localizer is used to provide lateral guidance to the aircraft and thus allows for tracking the extended runway centerline. The localizer information is typically displayed on a course deviation indicator (CDI) which is used by the pilot until visual contact is made and the landing completed. The localizer radiates on a carrier frequency between 108 to 112 MHz with 50 kHz channel spacing. This carrier is modulated with audio tones of 90 Hz, 150 Hz, and 1020 Hz. The 1020 Hz tone is used for facility identification.

The localizer antenna array radiates two different signals, carrier plus sideband (CSB) and suppressed carrier sideband only (SBO). The CSB signal consists of the RF carrier amplitude modulated (AM) with equal amplitudes of 90 Hz and 150 Hz tones. The SBO signal is similar except that the carrier is suppressed. The localizer radiation patterns are normally arranged so that the course sector of the proportional guidance sector is symmetrical around the runway centerline (see figure).

If the aircraft on approach is aligned with the runway centerline, the CDI will display no difference in the depth of modulation (DDM) between the 90 Hz and 150 Hz audio tones; therefore, the CDI needle is centered.

If the aircraft is to the right of the centerline, the 150 Hz modulation will exceed that of the 90 Hz and produce a deflection on the CDI towards the left. Conversely, if the aircraft is to the left of the centerline, the 90 Hz modulation will exceed that of the 150 Hz and produce a similar but opposite deflection. This deflection corresponds to the direction the pilot must fly to be aligned with runway centerline and is proportional to the angular displacement from centerline.

The CDI has a full-scale deflection of 150 microamperes where the DDM equals 0.155 in both the 90 Hz and 150 Hz directions. The angular displacement, or proportional guidance sector, that corresponds to this full scale deflection is known as the localizer course width. This width is typically tailored for a full-scale CDI deflection to occur at 350 feet from runway centerline at threshold.

When the aircraft is outside this course guidance sector, the CDI is required to provide full scale deflection. This region is known as the clearance sector. The FAA requires that this region extend from the localizer course edge out to 35 degrees on both sides of centerline.

Reflected or scattered signals that come from hangars and buildings historically and today pose the greatest concern for establishing a localizer. These reflected signals cause quality derogation of the on-course signal as seen by the aircraft. To minimize these reflections, the common technique is to use larger array apertures that narrow the localizer course beam and thus reduce the quantity of signals incident on the reflecting surface.

In some cases, to maintain the ±35-degree clearance coverage, a separate RF carrier, offset from the course frequency by 8 kHz, is radiated. Since these two signals fall within the passband of the ILS receiver, the stronger of the signals is "captured" by the receiver and is used for the guidance. These two-frequency localizer arrays are called dual-frequency and are primarily used to support Category II/III operations.

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