page_title: Instrument landing system localizer
section_title: Specifications
hierarchical_section_title: Instrument landing system localizer / Specifications
caption_attribution_description: English: Limits of Localizer Coverage
context_page_description: An instrument landing system localizer, or simply localizer, is a system of horizontal guidance in the instrument landing system, which is used to guide aircraft along the axis of the runway.
  See also
context_section_description: 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 is measured from the angle where the DDM is 0.180 to 10° from CL, the DDM will be no less than 0.180; from 10° to 35° from CL the DDM will not be less than 0.155.
  Coverage will be set 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 will be set 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) will 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%
caption: Limits of Localizer Coverage
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