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Principles Of VOR  
User currently offlineJzucker From United States of America, joined Nov 1999, 100 posts, RR: 0
Posted (12 years 1 month 3 weeks 1 day 18 hours ago) and read 5989 times:

Can anyone explain exactly how VOR's work?
I am tired of hearing the lighthouse analagy, and am really concerned with where the "phase difference" actually occurs between the reference and variable...

7 replies: All unread, jump to last
User currently offlineSkyguy11 From , joined Dec 1969, posts, RR:
Reply 1, posted (12 years 1 month 3 weeks 1 day 17 hours ago) and read 5945 times:

Well if you understand the lighthouse analagy... the real thing isn't much different:

The variable signal (the rotating 'beacon') rotates round at 1800RPM, your equipment only 'sees' it when it is shining right toward it.

The reference signal is sent out everytime the variable signal moves through north (360*).

Your VOR reciever just measures the difference between the two signals, and therefore can figure out what radial you're on:

User currently offlineDufo From Slovenia, joined May 1999, 846 posts, RR: 4
Reply 2, posted (12 years 1 month 3 weeks 1 day 9 hours ago) and read 5875 times:

Skyguy11 this has got to be the best (and yet the simplest) graphical presentation of VOR. Thanks!

I seriously think I just creamed my pants without any influence from any outside variables.
User currently offlineSkyguy11 From , joined Dec 1969, posts, RR:
Reply 3, posted (12 years 1 month 3 weeks 1 day 8 hours ago) and read 5852 times:

Thanks! I agree it is very good, got it from a friend.

User currently offlineDc-10tech From United States of America, joined Jun 2001, 298 posts, RR: 1
Reply 4, posted (12 years 1 month 3 weeks 19 hours ago) and read 5765 times:

"Skyguy11 this has got to be the best (and yet the simplest) graphical presentation of VOR. Thanks!"

Great post skyguy11. That is the same graphic that was in our Air Force training manuals at Com/Nav Technical Training.

To simplify: The VOR receiver measures the phase difference of the two signals it is recieving, which then tells it where it is in relation to the station, it then inverts this signal to represent the azimuth to the VOR station. (ok, not that simplified  Smile )

In the diagram. Lets say your aircraft is east of the station. ( 90* relative to the station) and heading north. The aircraft VOR rcvr sense a 90* phases difference, inverts this and position the needle on the RMI to point to 270*. This is the direction to the VOR station.

User currently offline707cmf From France, joined Mar 2002, 4885 posts, RR: 27
Reply 5, posted (12 years 1 month 3 weeks 12 hours ago) and read 5725 times:

Sorry for the double post, but hey, Skyguy11, great great explanation. Thanks.


User currently offlineKlaus From Germany, joined Jul 2001, 21652 posts, RR: 53
Reply 6, posted (12 years 1 month 3 weeks 11 hours ago) and read 5728 times:

I´ve had trouble understanding exactly how the signal encoding would work to transmit both the reference and the variable-phase signal...

I´ve finally found an excellent source here:


The section that deals with VOR/DME/ILS is "Aviation - en route" with the long explanation here:


The respective overhead sheets provide a good illustrated summary as well (can´t link to them due to the freakin´ a.net HTML conversion.  Angry)

Apparently, both signals are transmitted on the same carrier, with the variable-phase signal being amplitude-modulated on the main carrier at 30 Hz, and the reference signal FM-encoded with a 9960Hz sub-carrier on the same signal. (In between, there´s still space for audio transmission of the beacon´s ID).

Pretty neat!  Big thumbs up

The documents also explain the Doppler VOR system (with reversed signal encoding) and ILS, among other things.

User currently offlineFly2hmo From , joined Dec 1969, posts, RR:
Reply 7, posted (12 years 1 month 2 weeks 5 days 22 hours ago) and read 5623 times:

Get yourself a scanner and you can hear the funny alien-like-antimatter-gun sound they make and the Morse code, some even have voice, like ATIS. You need to get close to them though, at ground level their range for crystal clear sound is less than 3 miles (from my own experience). It's best when you have them in direct line-of-sight (no obstructions) for hearing them. You can distinguish the two tones mentioned if you listen closely. A fast repeating cyclical tone (reference signal) in the foreground and a constant tone in the background (rotating beacon signal).

cheers  Smile

[Edited 2004-04-14 02:34:37]

[Edited 2004-04-14 02:37:03]

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