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Inner-workings Of A Vor?  
User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Posted (12 years 4 months 3 weeks 5 days 14 hours ago) and read 3238 times:

Could someone discuss the inner-workings of a VOR staton? How is the signal radiated and how does the 360 degree radial system apply to that? The other day it occured to me that I use them almost everyday, but I do not know much about the actual technical side of one. Thanks a Ton!

14 replies: All unread, jump to last
 
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 1, posted (12 years 4 months 3 weeks 5 days 10 hours ago) and read 3151 times:

You can think of it as a lighthouse, sweeping a full circle 30 times every second. On top of the lighthouse is a strobe, flashing every time the emitted beam of light points due north. By timing the delay between the strobe flash and the flash from the light beam, you know what radial you're on.

Technically, you have your carrier wave, the frequency you dial in on the receiver. The directional signal is amplitude modulated with 30 Hz. To get the "strobe", you amplitude modulate the carrier with a subcarrier at 9660 Hz. This 9660 Hz subcarrier is then frequency modulated at 30 Hz to give the strobe. Then there are doppler VORs but the basic principle is the same.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Reply 2, posted (12 years 4 months 3 weeks 5 days 3 hours ago) and read 3115 times:

So is the NAV reciever actually the one doing the "timing" after the revolving mechanism crosses due north?

User currently offlineCosync From Mexico, joined Nov 2001, 556 posts, RR: 0
Reply 3, posted (12 years 4 months 3 weeks 4 days 23 hours ago) and read 3110 times:

yes
i went for a flight today and used actually got to dial in perth VOR and see what radial im on.
it was my friends dad flying and i was copilot (sort of). im a flight simmer and understand VORs but today was my first time at actually using it in realk life. i felt quite special.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 4, posted (12 years 4 months 3 weeks 4 days 22 hours ago) and read 3105 times:

Dg,
yes, the receiver does the "timing" or rather phase difference detection.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 5, posted (12 years 4 months 3 weeks 4 days 11 hours ago) and read 3084 times:

FredT, huh?

No time measurement is done in a VOR system. Timing is used in systems that use triangulation like Loran or GPS, and in DMEs.

The VOR station transmitter radiates a signal that is composed of a 30 Hz reference signal that is FM modulated on a 9960 Hz sub-carrier and a 30 variable signal that is AM modulated on the VOR frequency. (108-117.95). This is done so that the two signals can be easily separated.

The phase of the 30 Hz variable signal compared to the reference 30 Hz signal is relative to your bearing to the VOR station. (0 to 360 degrees shifted)

Inside the VOR receiver, the 30 Hz reference and variable signals are separated from the VOR signal. The reference signal is routed off to your Omni Bearing Selector (sometimes called the course selector) where it's phase is shifted relative to the selected course.

The reference signal is then routed to a phase comparator that compares it to the 30 Hz variable signal. If there is a difference in phase, the comparator supplies a voltage to drive the deviation needle. The needle centers when you turn the OBS until the shifted reference phase signal is the same phase as the variable signal. The OBS dial displays the radial you are on. Of course the To/From flag will tell you if the course will take you to or from the station.



User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Reply 6, posted (12 years 4 months 3 weeks 4 days 9 hours ago) and read 3065 times:

Ok I'm lost.

User currently offlineAaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2
Reply 7, posted (12 years 4 months 3 weeks 4 days 7 hours ago) and read 3061 times:

dgpilot, without an electrical engineering degree, what goes beyond the most simple explanation is probably more than 99% of pilots could understand anyway... if you understand how to use VOR navigation and its limitations/errors, be content with that...


In addition to what airplay has posted, the VOR signal is actaully composed of 4 signals... For your masochistic pleasure, here is the general equation for the VOR signal in space.



Omega = angular frequency of the 30 Hz AM and FM

Omega' = 9960 Hz subcarrier

Omega'' = voice band

Omega''' = 1020 Hz identification signal

m1 through m4 = modulation depth (m1-3=30%, m4 <10%)

k = frequency deviation of the subcarrier (16 ± 1 Hz)

Theta = 30 Hz phase shift equal to the azimuth



Just on note to clarify the post of Airplay, no timing is done in the nav receiver, it just measures the phase difference between the modulated signals. I'm sure there are a hundred sites that people can link to that will show you the phase relationships for the cardinal azimuth locations; It's quite simple actually.


If you want to know how the ground stations are constructed, I can post that for you as well...


aaron


User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Reply 8, posted (12 years 4 months 3 weeks 4 days 7 hours ago) and read 3048 times:

So, in a very large nutshell, the signal is ommited by a revolving mechanism within the VOR (30 revolutions/sec), and the NAV reciever picks this signal up. This signal is actually composed of 4 different waves--one for each cardinal direction. Then, the NAV reciever determines the bearing/radial by analyzing the phase (shift?) difference between the waves?

Is the service volume (T, M, or H) simply controlled by the amount of power (AC?) being sent to the output device, like the transciever part? Just guessing...

Yes, aaron, I would be interested in the station contruction as well. Thanks a ton.


User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1644 posts, RR: 1
Reply 9, posted (12 years 4 months 3 weeks 4 days 6 hours ago) and read 3051 times:

Gee, thanks folks. I've been happily navigating with VORs for 43 years but, now, I don't think I'll be able to even look at an OBS without experiencing terminal brainlock  Big grin

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 10, posted (12 years 4 months 3 weeks 4 days 1 hour ago) and read 3042 times:

Airplay,
if you notice the quotes around "timing" and read my previous explanation, things will become clear to you. If you measure the phase difference and know the frequency, you are indeed measuring the time between peaks.

Dg,
stick with the lighthouse analogy. It says it all really. The actual modulation of the radiowave is of very little concern to you as a pilot. I added that part just in case someone wanted a slightly more in-depth explanation. Now, with hindsight being 20/20, I probably should have left it out.  Smile

The different wave formats haven't got anything to do with cardinal directions I'm afraid. The phase difference between two of the submodulations is what varies depending on what radial you're on.

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1644 posts, RR: 1
Reply 11, posted (12 years 4 months 3 weeks 3 days 13 hours ago) and read 3022 times:

Thank you again FredT; that part about submodulations cleared it all up for me.  Nuts

User currently offlineDG_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Reply 12, posted (12 years 4 months 3 weeks 3 days 12 hours ago) and read 3013 times:

LMAO....I'm kind of sorry I asked.  Wink/being sarcastic

User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 13, posted (12 years 4 months 3 weeks 3 days 12 hours ago) and read 3016 times:

Haha, sorry... asking an engineer is like buying one of those boxes full of various size bolts. Most of the time you get the one you're looking for but you have to search throgh a bunch of otherwise useful bolts that you don't need at the moment first! Big grin

I'll spare you the technical definition of a bolt, just this once since I am rather tired.  Big grin

Cheers,
Fred



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1644 posts, RR: 1
Reply 14, posted (12 years 4 months 3 weeks 3 days 10 hours ago) and read 3005 times:

I never buy a bolt; I just take one out of something that has two of them and looks like it will hold with just one. Thanks FredT.

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