DME & Marker Beacons

 What is DME & How does it work?

DME, shortcut for ‘Distance Measuring Equipment‘ provides the Pilot with distance information
To/From a VOR Station IF the station has a DME antenna

      single DME antenna                                                                                                   VOR/DME Station

The airplane sends out a signal, also known as an ‘interrogation’ directly to the Station from which it will be sent back (after ~50 microseconds) to the airplane immediately. The interrogation can also be understood as a signal request from the aircraft to the VOR Station. The airplane’s receiver will calculate the time it takes for the signal to travel back and forth and will then convert it into distance information (nm).

Since the airplane is flying at an altitude, the signals sent by the airplane travel a ‘diagonal’ way,
which is also called ‘Slant Range‘.

Therefore, as you get closer to the station, the indicated distance will usually be a little bit further than the actual (horizontal) distance. Otherwise, DME should be accurate within +/- 0.2 nm

The biggest error in indicated distance will be at higher altitudes directly over the station.
At high 
altitudes, your DME shows your height in nm.


If you cross a station at 7000 ft, the distance indicated will be around 1.2 nm.

DME Error illustration

Where else may DME be used?

a) instead of an OM
b) as a back course (BC) Final Approach Fix (FAF/FAP)
c) to establish other fixes on the localizer course
d) as the initial approach segment of an ILS DME ARC

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What are Marker Beacons?

A Marker Beacon is a certain type of VHF radio beacon that helps to identify positions,
usually along the Final Approach Path of an ILS Approach.
They have a power output of 3 watts or less.

Where are Marker Beacons used?

As already mentioned in the answer above, they are typically located along the
Final Approach Path of an ILS approach to audibly confirm the Pilot specific positions.
For position information, just take a look at the examples below.

There are 4 main types of MB’s available:

a) the Outer Marker (OM)

‘located about 4 to 7 miles and intercepts the Glideslope/path and designates the FAF/FAP’
(DME may be used instead of an OM)

b) the Middle Marker (MM)

‘positioned at about 0.5 – 0.8 nm (~3,500ft) on the extended RWY CTL from the threshold.
‘for CAT-1 ILS approaches the MM tone appears at a height of ~200ft AGL (DH/DA)’

c) the Inner Marker (IM)

‘positioned at or closed to the threshold  (@ DH) for CAT 2 & 3 a+b ILS approaches

d) the Back Course Marker (BC)
‘normally indicates the FAF/FAP of an ILS back course and thus the point where descent is started.’

AND, the…

d) Compass Locator
the CL can either be a MM or OM co-located with a low powered NDB (output < 25 watts), which enables the pilot to navigate directly to the beacon (NDB-station). It reaches out to at least 15 miles from it’s location. 
The CL usually acts a the FAP/FAF of an ILS.
Some of them have a higher power (up to 400 watts) and broadcast a
Transcribed Weather Broadcast (TWEB) information.


Where does the name Compass Locator come from?

Click here to listen to the sound of a Marker Beacon