localizer-type directional aid

localizer-type directional aid (LDA)

A NAVAID (navigational aid) used for non-precision instrument approaches with utility and accuracy comparable to a localizer but which is not a part of a complete ILS (instrument landing system) and is not aligned with the runway. The LDA course usually provides a more precise approach course than the similar simplified directional facility (SDF) installation, which may have a course width of 6° or 12°. The LDA is not aligned with the runway. Straight-in minimums may be published where the alignment does not exceed 30° between the course and runway. Where this alignment exceeds 30°, only circling minimums are published.
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The localizer beam must track within three degrees of the runway centerline (or else it's called a localizer-type directional aid, LDA) and is surveyed to provide at least 250 feet of obstacle clearance in the final approach area.
There are only 25 or so Localizer-Type Directional Aids in the National Airspace System, according to the Instrument Procedures Handbook.