inner marker beacon
A marker beacon used in conjunction with an ILS (instrument landing system) and located between the middle marker and the threshold of the runway where an ILS approach is being made. It is usually located at a distance where aircraft height would be 100 ft above the touchdown point elevation on a Cat II approach. It transmits a 75 MHz fan-shaped radiation pattern modulated at 3000 Hz and keyed at six dots per second. It indicates to the pilot, both aurally and visually, that he or she is at the designated decision height (DH), normally 100 ft (30 m) above the touchdown zone elevation, on the ILS CAT II approach. It also marks progress during a CAT III approach. Also called an inner marker beacon. See compass locator.