Outer Marker
What is outer marker?
The outer marker is a radio beacon located on the final approach course of an instrument landing system (ILS), typically 4 to 7 miles from the runway. As you fly over it, your cockpit marker beacon receiver gives you a blue light and a series of low-tone audible dashes, telling you that you've reached the point to begin your final descent (glideslope intercept). Think of it as the "you're lined up and starting down" checkpoint on an ILS approach.
Also known as: OM, outer marker (OM)
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