by Jonathan M. Stern
Marker Beacon Receiver
Marker beacons are components of an instrument landing system. There are up to three marker beacons on any ILS. They are an outer marker, middle marker, and inner marker, and their purpose is to provide range information relative to the runway threshold.
The marker beacons are directional transmitters situated on the ground along the final approach course of an ILS approach. They transmit an elliptical pattern upward. One thousand feet above the antenna, the pattern of the signal being transmitted is approximately 2,400 feet wide and 4,200 feet long. An aircraft passing directly overhead at 120 knots groundspeed receives the beacon signal for approximately 12 seconds.
When an aircraft flies over the top of the marker beacon, the airborne receiver does two things. First, it emits a flashing light specific to the type of marker being overflown. It also emits an aural Morse Code tone that corresponds to the type of marker. Table 11.5 shows the light and sound signals and placement for each type of marker beacon. In real aircraft, the marker beacon receiver can be turned on and off and set to high or low sensitivity. With Flight Simulator, the marker beacon receiver always remains turned on and at a high sensitivity setting. The use of the marker beacon receiver is more fully discussed in Chapter 18 "Instrument Approach Procedures."