Dg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 3 Posted (11 years 2 months 2 weeks 2 days 20 hours ago) and read 2393 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!
FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 1, posted (11 years 2 months 2 weeks 2 days 17 hours ago) and read 2306 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.
Dg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 3 Reply 2, posted (11 years 2 months 2 weeks 2 days 10 hours ago) and read 2270 times:
So is the NAV reciever actually the one doing the "timing" after the revolving mechanism crosses due north?
Cosync From Mexico, joined Nov 2001, 556 posts, RR: 0 Reply 3, posted (11 years 2 months 2 weeks 2 days 6 hours ago) and read 2265 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.
Airplay From , joined Dec 1969, posts, RR: Reply 5, posted (11 years 2 months 2 weeks 1 day 18 hours ago) and read 2239 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.
Dg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 3 Reply 6, posted (11 years 2 months 2 weeks 1 day 15 hours ago) and read 2220 times:
Aaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2 Reply 7, posted (11 years 2 months 2 weeks 1 day 14 hours ago) and read 2216 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...
Dg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 3 Reply 8, posted (11 years 2 months 2 weeks 1 day 13 hours ago) and read 2203 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.
ThirtyEcho From United States of America, joined Dec 2001, 1634 posts, RR: 1 Reply 9, posted (11 years 2 months 2 weeks 1 day 13 hours ago) and read 2206 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
FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 10, posted (11 years 2 months 2 weeks 1 day 8 hours ago) and read 2197 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.
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.
ThirtyEcho From United States of America, joined Dec 2001, 1634 posts, RR: 1 Reply 11, posted (11 years 2 months 2 weeks 20 hours ago) and read 2177 times:
Thank you again FredT; that part about submodulations cleared it all up for me.
DG_pilot From United States of America, joined Sep 1999, 856 posts, RR: 3 Reply 12, posted (11 years 2 months 2 weeks 18 hours ago) and read 2168 times:
FredT From United Kingdom, joined Feb 2002, 2184 posts, RR: 26 Reply 13, posted (11 years 2 months 2 weeks 18 hours ago) and read 2171 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!
I'll spare you the technical definition of a bolt, just this once since I am rather tired.
Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
ThirtyEcho From United States of America, joined Dec 2001, 1634 posts, RR: 1 Reply 14, posted (11 years 2 months 2 weeks 16 hours ago) and read 2160 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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