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Localizer as component of an ILS (KMEZ runway 27, Mena, Arkansas).

In aviation, a localizer (LOC) provides runway guidance to aircraft. It is not to be confused with a locator, although both are parts of aviation navigation systems. The localizer is a component of an instrument landing system (ILS) for the runway centreline when combined with a glide slope transmitter. When the glideslope is unserviceable, the localizer element can often be conducted as a separate non-precision approach, abbreviated to 'LOC'. A standalone instrument approach installation without an associated glidepath carries the abbreviation 'LLZ'. In some cases, a course projected by localizer is at an angle to the runway (usually due to obstructions near the airport). It is then referred to as a localizer type directional aid (LDA). The localizer system is placed about 1,000 feet from the far end of the approached runway. Usable volume extends to 18 NM for a path up to 10° either side of runway centerline. At an angle of 35° either side of runway centerline, the useful volume extends up to 10 NM. Horizontal accuracy increases as distance between the aircraft and localizer decreases. Localizer approach specific weather minimums are found on approach plates.


  • Course line (CL) is where difference in the depth of modulation (DDM) is zero in the horizontal plane.
  • Course sector (CS) is the sector in the vertical plane bounded by DDM = 0.155.
  • Displacement sensitivity (DS) is the change in DDM per meter at ILS reference datum (the runway threshold) and should be 0.00145 (hence CS is 106.9m at threshold).
  • Clearance: from the angle where the DDM is 0.180 to 10° from CL, the DDM shall not be less than 0.180; from 10° to 35° from CL the DDM shall not be less than 0.155.
  • Coverage: at 25 nmi (46 km) within 10° from CL; at 17 nmi (31 km) within 10° and 35° from CL; at 10 nmi (19 km) outside 35° if coverage is required.
  • Reduced coverage: at 18 nmi (33 km) within 10° from CL; at 10 nmi (19 km) within 10° and 35° from CL.
  • Mean course line at ILS reference datum (threshold) shall be adjusted and maintained within:
    • CAT I: ± 10.5m (15μA)
    • CAT II: ± 7.5m (11μA) (recommended ± 4.5m (6.4μA))
    • CAT III: ± 3.0m (4.3μA)
  • The DS shall be adjusted and maintained within:
    • CAT I: ± 17%
    • CAT II / III: ± 10%

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