What does a VOR look like?
 

Another thread ('How do you get from A to B') got me thinking about this.

My usual method of route planning is to draw lines on the map and do a PLOG, but to go via VOR's/NDB's where possible/practical. Although using the electronics, I always try to visually identify the beacons - after all I am navigating visually!

Some of the VOR's I fly over (well, not over, to the right of - traffic avoidance) are easy to spot - big circular thingies with sticky-up bits around them, painted a light colour and out in the open so I can spot them easily. GAM - Gamston is the one I see most often - and they even put it on an airfield of the same name so I'd be able to find it easier! Others I never seem to be able to spot (POL - Pole Hill springs to mind).

Question is, are all VOR's the same or are some different and thus harder to spot? I read somewhere that some are classified as 'high level' and some are 'low level' - does this make a difference to their physical appearance?

[Apologies in advance to the more technically-minded for the 'big circular thingies with sticky-up bits around them' bit, but, well - they are!]

Good question Knobby,

The only ones I've seen look the same. When I first saw one near Heathrow in a field before I started flying I thought we'd been invaded by Aliens :\

No, VORs are not all the same.

A VOR transmits two signals, both at the same frequency. One is frequency modulated, and the other is amplitude modulated. Because the information that's being transmitted on each of the two carriers is the same frequency, it's possible to do phase comparison of the two signals, and that's exactly what the VOR receiver in the aircraft does to establish what radial it's on.

Traditionally, a VOR would transmit the FM signal in all directions equally. The AM signal would be transmitted in such a way that its phase is different depending on which direction from the transmitter your aircraft is. This was a very simple mechanism, and didn't require much ground equipment. As such, the transmitters are difficult to spot from the air.

The problem with this type of transmitter is that it is particularly susceptible to interferance from obstacles which are located near to it. Some clever b@stard figured out another way of arranging the transmitter. Rather than rotating the AM signal, he fixed the AM signal, and use Doppler shift to rotate the FM signal. The resulting transmitter, the Doppler VOR, is much bigger, and is probably the one that you're familiar with, since all modern VORs are built this way.

As a user of the VOR, you don't really care which method it uses, since the receiver works exactly the same way. I don't know either of the two VORs you mention, but it could be that POL is a very old transmitter, which uses the old technology? Not sure how you could find this out other than find someone who actually knows... does that AIP list the type of transmitter? I'm not sure...

Hope that helps, anyway.

FFF
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G'day FFF,

At the risk of being pedantic, and whilst conceding that the rest of what you said gets it across to non-technical people well..: Isn't it in fact the case that one of the signals is phase modulated, whilst the other is continuous (CW)..? At least thats how I thought VORs worked back when I was an avionics engineer in the RAAF (about 20 yrs ago!) - but to be fair I was only ever focussed on the airborne equipment in my role, i.e. the receivers, so this might just be the appropriate description from the receiver's point-of-view.

I love talking about electronics and commnunications systems, so happy to stir this one up a bit ;) Perhaps someone has a link to some geeky paper online in PDF format which I can read for the fun of it!

Andy ;)

(currently a banker - and yes I know what it rhymes with(!) - but wishing I still worked in avionics...!)

P.S. Forgot to add, in my experience modern VORs all look very similar - as has been said and described above.

NDBs on the other hand take two different forms: either a vertical tower with a "capacitive hat" (I have a photo of one lurking somewhere and will post it later), a good example of this type is the Chiltern NDB - CHT - near Denham, which you can drive past near the M25. Alternatively they may be of the dipole or longwire types (usually inductively loaded due to the LF band on which they operate), which generally take the a wire strung between two vertical poles, usually centre-fed. I think from memory Westcott (WCO) might be this sort (?) and the NDB on the field at Wolverhampton space-port...

P.S.S. - thinking about it, FFF must be right about the AM bit at least for most VORs the CW bit (i.e. the phase reference signal) would presumably be amplitude in order to carry the ident tone and/or ATIS etc. But from point-of-view of a phase discrimator, it would seem CW... I really must look this up..!

Well, Andy, since you wanted to stir it up...

I'm not an avionics or electrical engineer, nor have I ever been. I haven't studied any electronics since I left school, except for my ATPL exams. But my understanding, which is a combination of A-Level physics and Bristol's notes, is as follows:

In the old-fashioned style of VOR, the "FM" signal really was frequency modulated, and transmitted in all directions. But in a Doppler VOR, it's more complicated. An unmodulated, relatively high frequency signal is transmitted through a number of discreet transmitters which are arranged in a circle. Initially it is trasmitted through the first transmitter, then the next, then the next, then the next. So, depending where you are located in relation to it, at any point in time the source will be either moving nearer to you or further away from you. This gives rise to a Doppler shift, and you see a slightly different frequency to that which was broadcast, with the frequency constantly varying as the source moves first towards you, then away from you. Thus it appears that you are receiving a frequency-modulated signal. The phase of this signal will depend on your QDM from the VOR.

As for the AM signal, in the Doppler VOR this is a simple transmitter. In the old-fashioned VOR, it is a combination loop/omnidirectional aerial, very similar to "traditional" direction finding equipment (of which the ADF is a derivative). This transmits a "limacon"-shaped signal, with some areas receiving a stronger signal than others. Again, the signal which is transmitted is unmodulated, relatively high frequency. The aerials are rotated, and as the rotate, the receiver will pick up first a high-amplitude signal, then a low-amplitude signal, as the limacon rotates and the receiver passes through different parts of the signal. Thus, it appears to the receiver as if an amplitude-modulated signal is being received. Again, the phase of this signal will depend on your QDM.

And that is about the sum of my knowledge, so I doubt I'll be able to add much more to this thread!

FFF
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So how about TACAN then - or VORTAC?

Interesting stuff FFF - I'll have to do an ATPL (!) or look up a paper...

Evo: TACAN is I think much like VOR, with compatible DME, but on different frequency band. I did work on TACAN receivers also - but can't remember much detail... The expression VORTAC, AFAIK, is used in the US to indicate a VOR with co-sited DME, but I am not 100% sure about this....

Now, here's a couple of photos of the "capacitive hat" style vertical NDB at Fort Carre, Antibe, on the approach to Nice:

http://members.lycos.co.uk/andyhardyuk/NDB/NDB1.jpg

http://members.lycos.co.uk/andyhardyuk/NDB/NDB2.jpg

(The detailed close-up shows it appears to have both capacitive and inductive loading elements near the top...)
Andy

TACAN, as Andy says, is a VOR on a frequency which us civvies can't use, and a DME which we can use. A VORTAC is a co-located VOR and TACAN. So civvies can use the VOR together with the DME part of the TACAN, which military guys can use the TACAN. I think.

FFF
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Doppler VOR array at YSSY (Sydney)
 

http://www.acay.com.au/~willt/yssy/e...mages/dvor.jpg

And a TACAN antenna, somewhere in the UK I think... Looks different to what I had expected:

http://www.ravl.co.uk/images/extras/tacan.jpg

... and a "dipole" sort of NDB antenna at Lord Howe Island (Pacific ocean, east NSW):

http://www.acay.com.au/~willt/lhi/images/ndbant.jpg

Hmmm... 'planespotting' is known to be a particularly British pastime - seems Andy is more into his Navaids :D - so are these photos from your personal collection?

There is a very good description of how a VOR works here :)

(Basically, the two signals are phase-shifted.)

RTFM: Thanks! Will read on the train this evening...

knobbygb: well [red-face] yes I suppose I am[/red-face]! The first pics (at Antibe) are from my personal collection, the others I just found using Google image search http://www.google.com/imghp

Andy :uhoh:

WOW! After years of using Google, I didn't know it had an image search function. :8

See... I can do it too:

http://www.airandinas.com/images/a4_t.jpg

GAM VOR - circle just above the triangle of trees to the left of the main runway.

Thanks. Now, anyone find a photo of Pole Hill?

VOR modulation..!
 

Couldn't wait so read the paper RTFM pointed out..!

We're kind of all right, in a sense, but I conced FFF was "more" right ;) And wow - the ground station equipment seems far more complex than the receivers! Of course, this is how it should be to keep things lighter and simpler on the airborne side (and perhaps because airborne avionics engineers are simpler too..!?!)

So as I read it:

1) AM is used to modulate the reference carrier, in order to carry voice and morse ident.

2) In a doppler type VOR installation, a pair of of ~10kHz sidebands are frequency & space modulated around the 48 antennas displaced around the array.

3) By being both space and frequency modulated, the doppler effect gives rise to an apparent simple phase modulation at the receiver!

I think....

So, I will still search for a more comprehensive paper, preferably one with diagrams!

Now: maybe I should do some work this afternoon..!

Andy :=

Not read the article, but isn't the "net result" in the Doppler VOR that the phase difference between the two signals provides the bearing information, without the need for moving parts in either transmitter or receiver

When I was a lad, and learning this stuff, Doppler VORs were new, and some of the older VOR receivers were prone to giving silly results with them. That was in the days when a 360-channel COM set was the height of luxury.

And yes, I confess that I and a certain other person around here took a great delight, years ago, in flying over and trying to photograph every VOR in the area. We gave up at something like 30.

Wonder where those photos are now...

I have always wanted to know what a VOR looked like close up after seeing pictures of them in books and flying over Compton (CPT) a couple of times.

You might think that I'm sad, but Last October I took my wife away for a week in Torquay, and as usual, I took my airband scanner and my bag of PPL study material along.

Whilst we were out sightseeing, It occurred to me that we couldn't be that far away from the Berryhead VOR (BHD), so after persuading my patient wife how important it was to me to see what a VOR looked like, we set out on a mission to find it.

We drove for about 12 miles until we came to a little fishing village called Brixham and decided to stop and view the Torbay harbour before asking a couple of the locals if they knew where we could find this VOR. Luckily, one of them knew exactly what I was talking about and directed me towards the Berryhead Fort where it was tucked away in the bushes. Still not satisfied with the placard that they erected, explaining to visitors what the structure was, we went through the bushes and stood right inside it.

When I do get a chance, and if you are interested, I will post a couple pictures that I captured on my digital camcorder.

I told you I was sad. :8 :D

TACAN is based upon the same principal as the conventional VORs using the rotating antenna to space modulate the carrier wave creating an AM signal. It rotates 6 times faster than the VOR to generate in theory, 6 times the accuracy. It is prone to a 60 degree error as the same phase relationship can occur at 6 different locations in one rotation. Because of the higher frequency used, it is not as prone to the propogation errors encountered on early VORs.

The range side of the TACAN is exactly the same as DME.

Some years back I also decided to have a close look at a VOR - Pole Hill as it happens as it is the closest to me. As I remember it, it looked exactly like the pictures above, but what I remember most was a police car drawing up within a few minutes of me climbing over the fence. I thought I was going to be in some trouble, and wasn't prepared for this, but my innocent explanation that I was just "interested in these things" seemed to be accepted at face value. In the light of the World Trade Centre incident I cannot see this being the case anymore.

I haven't been back to Pole Hill since - it is a bit remote on the Pennine hills, but back then it was all too easy to access. I would like to think it is a bit more substantially protected now, although I never knew whether the rapid arrival of the police was just the result of a fortuitous passing, or followed some form of active monitoring.

Fernau
 

Apparently some people from called Fernau Aviation in Luton manufacture DVOR ground stations systems (as well as DME, TACAN, NDB and DF equivalents). They have a spec for their DVOR ground station system online at http://www.fernau.com/library/items/2050_dvor_tech.pdf

The spec gives the impression that the RF side of things only generates amplitude modulation (AM):This seems to imply that the only other form of modulation would be the "space" modulation achieved by feeding the sub-carrier mod signals to the 48 antennas mentioned elsewhere...

Or maybe the spec is oversimplified..?

Keef: yes from receiver's point-of-view its all very simple!

Andy ;)

A simple explanation from avionics.com: So: 1) a simple AM reference signal, and 2) a ~10kHz subcarrier with FM modulated signal (whose phase relationship to the first signal varies according to radial position around the beacon).

I feel I owe FFF a beer ;)

Andy

Or here's BIG (Biggin Hill VOR) just a few metres above the mid point of the runway.

BUB VOR (Brussels - in the final approach)

DENVER VOR