Christianbothe From Germany, joined Jan 2000, 124 posts, RR: 0 Reply 1, posted (11 years 8 months 3 weeks 3 days 13 hours ago) and read 1033 times:
Yes, there are outer, middle and inner markers. These are beacons, the outer marker being a Non-directional beacon that could also be used as a navigation fix. When the aircraft passes over the outer marker, about half way through the ILS approach (about 6 miles), the pilot gets a sound and a light going on. The middle marker is about 1 or 2 miles from the runway threshold, and the inner is just a few seconds before touchdown.
So when you reach the outer marker, you know it's about time to get on the glidepath...
Check out this photo: The lights for the markers are the white, red and blue lights at the very left edge of the picture.