Page 3-6
GMA 340 Installation Manual
Rev. L
190-00149-01
3.1.13 Marker Beacon Receiver
The marker beacon is part of an ILS system, and in certain instances, used to identify an airway. In addition to
the normal marker beacon functions, the GMA 340 provides an audio muting function. The LEDs illuminate, and
an associated keyed-tone is heard (when MKR audio is selected), when the aircraft passes over a 75 MHz marker
beacon transmitter.
The lights and audio keying for ILS approach operation are summarized below.
Audio
Frequency
Audio Keying
LED Actuated
400 Hz
Blue (Outer)
1300 Hz
Amber (Middle)
3000 Hz
White (Airway/Inner)
The marker beacon audio level is aligned at the factory to produce its rated audio output. However, the output
level is adjustable through an access hole in the top cover of the unit (see figure 2-3).
The GMA 340’s marker beacon receiver controls are located on the left side of the front panel [(1) through (4)].
The SENS button (4) selects either high or low sensitivity as indicated by the HI or LO LED being lit. Low
sensitivity is used on ILS approaches while high sensitivity allows operation over airway markers or to get an
earlier indication of nearing the outer marker during an approach.
The marker audio is selected initially by pressing the MKR/mute button (2). If no marker beacon signal is
received, then pressing again will deselect the marker audio. This operation is similar to selecting any other audio
source on the GMA 340. However, if the second button press occurs while a marker beacon signal is received,
then the marker audio is muted but not deselected. The button’s LED will remain lit to indicate that the source is
still selected.
The GMA 340’s
SmartMute
TM
function then monitors the marker signal and automatically unmutes the audio
when the current marker signal is no longer being received.
In all cases, the marker beacon LEDs operate independently of any audio selection and cannot be turned off. The
GMA 340 can drive external marker lamps if required. Maximum source current is 125 mA (8 Vdc max).
