||It has been suggested that this article be merged into instrument landing system. (Discuss) Proposed since May 2012.|
In aviation, a localizer (LOC) is the component of an Instrument Landing System (ILS) that provides runway centerline guidance to aircraft, but not the glideslope information. It is not to be confused with a locator, although both are parts of aviation navigation systems.
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). Localizers also exist in stand-alone instrument approach installations and are not always part of an ILS. The localizer system is placed about 1,000 feet from the far end of the approached runway. Useable volume extends to 18 NM for a path up to 10 degrees either side of runway centerline. At an angle of 35 degrees 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 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° of CL, the DDM shall not be less than 0.180; From 10° to 35° of CL the DDM shall not be less than 0.155.
- Coverage: at 25 nmi (46 km) within 10° of 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 course line; 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 (15uA).
- CAT II: +/- 7.5m (11uA) (recommended +/- 4.5m (6,4uA).
- CAT III: +/- 3.0m (4.3uA).
- The DS shall be adjusted and maintained within:
- CAT I: ± 17 percent.
- CAT II / III: ± 10 percent.
See also 
- Andrew Alford
- Instrument approach
- Instrument landing system
- Simplified Directional Facility
- Localizer Type Directional Aid