All ILS Marker beacons operate on 75 MHz VHF (thus no frequency selections are necessary for the pilot) and radiate a fan shaped field pattern giving to the pilot an indication of range from the threshold.
The purpose of the markers is to provide range information while on the approach.

They transmit an almost vertical beam.
Almost all installations are equipped with an outer marker and a middle marker.
Category 2 or 3 ILS may be equipped with an inner marker as well.

Audio- and visual signals in the cockpit will indicate when the aircraft is passing overhead.
In many installations, marker beacons are being replaced or supplemented by the use of a DME associated with the ILS.

The Outer Marker is located approximately 3.9 nautical miles from the runway threshold and is aligned across the front beam of the localiser.

Its purpose is to provide height, distance and equipment functioning checks to aircraft on final approach.
It is modulated at 400-hertz and keyed to transmit dashes (“- - - - - ”) continuously at a rate of 2 per second.

The Middle Marker is aligned across the front beam of the localiser and is situated approximately 1050 metres from the runway threshold.
Its purpose is to indicate the imminence, in low visibility conditions, of visual approach guidance.

This marker is modulated at 1300-hertz and keyed to transmit alternate dots and dashes (“• - • - • - ”).
The rate is 2 dashes and 6 dots per second.
An aircraft on the glide slope over the middle marker should be roughly 200 feet above the touchdown zone elevation.

The Inner Marker is modulated at 3000-hertz, identified by a keyed continuous signal of 6 dots per second (“• • • • •”) and is located 75–450 metres from the runway threshold.

Summary:

• Outer Marker: Identifies glideslope intercept or the Final Approach Fix (light flashes blue)

• Middle Marker: Identifies decision height (light flashes amber)

• Inner Marker: Identifies decision height for a CAT II ILS (light flashes white)