Electronic Navigation SC

Localizer and Marker Beacons

How does a Localizer work?

A localizer is just like a VOR, however instead of different radials, it is simply transmitting one long straight line and telling whether you are left or right of that line.  It DOES NOT matter what the OBS knob is set to when you have a localizer frequency programmed into your NAV radio (same place you would normally put your VOR frequency).

There are several parts that make up the ILS (instrument landing system), and the localizer is just one of those parts.  The localizer for a runway is generally always going to be located at the far end of the runway you are landing on (as show, below landing right to left).  The localizer works by simply transmitting to signals, one at 150Hz and the other at 90Hz.  The receiver in your airplane just tells you which signal is stronger (think of it as your NAV radio listens to that localizer frequency you tuned in and just listens to see which signal it hears louder), and if you are perfectly on course in the center, the signal strengths should be equal.

localizer antenna

The course width in degrees of the localizer varies but is usually somewhere around 5 degree wide total.  The course width in degrees is ALWAYS adjusted so that going full-scale deflection all the way from the left to the right on the CDI would equal 700′ (350′ either side of centerline) when the aircraft is crossing the runway threshold.

Extreme caution must be used when approaching an airport and outside of the “receiving cone” for the localizer or glideslope.  Especially when approaching an airport in hilly or mountainous terrain, it is common to receive “false glideslopes” and “false localizers”.  You must always stay alert to the position of the aircraft, and have enough situation awareness to know if you are in fact receiving the correct signal, or if you are so far right or left of course, or high or low, that you are receiving a reflection of a signal and possibly navigating towards an unprotected area of the approach.  The best way to combat this is to back up an ILS or localizer approach with some sort of other “aid” (like an iPad or panel mounted GPS unit).  Also, flying more of a “straight-in” approach and starting from an IAF (initial approach fix) will decrease the likelihood of this happening compared to getting vectors to final.

localizer range

Marker Beaconsmarker beacons ils approach

Marker Beacons are another part of the ILS.  They are not installed at every airport or on every ILS approach, but where they are installed they are denoted by the “eye” shaped ovals along the final approach course as shown on the right.

They come in three flavors:

  • Outer
  • Middle
  • Inner

When combined with an NDB, we call the marker beacon a “Compass Locator” (so an Outer Marker becomes a Locator Outer Marker, as shown on the plan view of the chart on the right, LOM, with a frequency of 368).  The Marker Beacons are ground-based stations that transmit up in a “fan” or “cone” shape that at 1000′ agl is about 2,400′ wide and 4,200′ long.  The power is about 3 watts at 75MHz.

The purpose of the Marker Beacons are to give you an idea where you are on the approach.  With an airplane without any sort of GPS or DME, the Outer Marker lets you know you are crossing the FAF (final approach fix), generally 4-7nm from the runway.

marker beacon ils


The Outer Marker (if installed on the ILS), is also situated so that one dot deflection on your CDI needle would indicate that you are 300′ off course from the centerline.  The Middle Marker is much closer to the runway (not halfway between the outer marker and the runway by any means).  Being one dot deflection off course on the CDI when over the Middle Marker means you are 100′ off centerline.  The Middle Marker is also installed roughly 3,500′ from the runway threshold, which means if you are on glideslope when crossing the Middle Marker, you will be about 200′ above touchdown zone elevation (generally near the minimums for the ILS approach).

All Marker Beacons give off an Aural Tone when you are flying directly over them and generally light up something in the cockpit (either an annunciation on a glass panel or generally flashing lights on the audio panel).  The outer marker has a slower “beep” tone to it (2 dashed per second), with the middle marker being a faster “beep” or tone (95 dash/dot combinations per minute), and the inner marker gives off a very fast tone.  (note: inner markers are located typically where you would be 100′ above touchdown zone elevation if you were perfectly on glideslope.  Inner Markers are used in CAT II ILS approaches, not anything you’ll be doing in your Cessna).

You generally have to “turn on” the ability to hear the marker beacons in your headset the same way you would select to “monitor” another radio or morse code identifier on a VOR.  If you happen to leave the switch or button selected to listen to these, and you’re taking up a passenger on a nice VFR flight, they may get a little nervous that as you get closer to the runway on landing they hear a faster and faster “beep” in their headset.  Just might be a good idea to give them a heads up or mute the marker beacons (I know from experience of scaring passengers).