Saturday, April 20, 2013

When Omni Was Futuristic

I'm VFR inbound to a controlled airport on a day with a high thin overcast. Beautiful day for flying: no glare, little turbulence, old snow still sparkling on the ground below, but the runways and taxiways perfectly clear. The controller tells me to call ten miles out and asks another aircraft to report "over the OMNI."

How retro. The OMNI? I think the term might be more common in the US than Canada, but it's still pretty old fashioned. Omni means all. A moment here for the non-pilots, because I think I've left them behind in a cloud of terminology a few times recently, and because I still remember the thrill of excitement I felt on first learning how the thing I'm about to explain works. It was like a grade nine physics class come to life and become useful. There's a thing in aviation called Very High Frequency Omnidirectional (Radio) Ranging. Very High Frequency is always and already abbreviated VHF, which is already a type of radio, hence the thing itself is called a VOR. ("Vee-Oh-Arr").

A VOR is two things, one that's on the ground and sends signals and one that's in your plane and receives, interprets and displays the meaning of the signals. The one the air traffic controller referred to was obviously the ground-based one. It sends a VHF radio signal on a published frequency. You know that a radio waves are depicted as squiggles, sine waves oscillating from start to finish, a little repeated radio shape at frequency times per second. That's what frequency means. Now this is the cool part. The same frequency signal is sent out twice. Once starting at the same point all the way around and the second time starting at the same point at north, but offset by one three-hundred sixtieth of the cycle for each degree of the compass away from north the signal is sent.

Now the percentage of you that know how VORs work are either bored or laughing at my obsession with these. It's probably the fifth time I've explained this now aged technology on my blog. And those of you who let your eyes glaze over at anything math or physics related are waiting for this paragraph to be over. But if there's one reader who didn't know this simple, brilliant navigation solution and is now about to grasp it, it's worth boring everyone else. Because the VOR receiver in the airplane measures the difference in phase (how many three-hundred-sixtieths, i.e. degrees, different the two signals are) and displays it as a needle swing. (Nowadays it can be displayed digitally, too, but that's not the super cool part). Do you see it? The difference between the two signals is the bearing from the ground station to the receiver.

I'm sorry if that was never the coolest thing you'd ever heard. I promise to find a more modern piece of technology to rave about soon. But nothing is as elegant anymore, now that everything new has a computer in it. Computers are amazing, but they don't have the simple brilliant simplicity of an airspeed indicator.

Hey look, this flight sim site agrees with me. "The VHF Omnidirectional Range navigation system, VOR, was probably the most significant aviation invention other than the jet engine." True! Jet engines are so amazing that they sound almost as impossible as this perpetual motion air car, but jets are real.

10 comments:

Ed Davies said...

The names of some nav aids seem very confusing to me. VOR: omnidirectional, well yes, it works in all directions; range, hmm, no it doesn't measure range.

NDB: non-directional beacon, again, yes, it's a beacon and it's not directional but what does it do? allows you to measure (relative) direction. So it is directional, sort of.

DME: distance measuring equipment: at least this one's better, it does what it says on the tin - it measures range. In any direction. On UHF. Maybe it should be called, UHF omnidirectional range - UOR. But that would be silly.

fatfred said...

Thank you for a very lucid explanation. I wondered as a non pilot what OMNI was.

Sarah said...

All I could think of from your title was this magazine. It did try to be futuristic, and now seems as dated as any other history of the future.

Nice concise explanation of how VOR works - I wonder if in 10 years VORs will be as rare as their predecessor, the "4 course radio range".

These days, I think how GPS works is more obvious. So the complications are regulatory: When, exactly, you can use GPS approaches with your equipment, to what minimums, and with what alternate requirements. Oh, and can you guarantee with RAIM that it will be working when you get there.

Aviatrix said...

Yay, someone got the title reference and someone learned something. Silly people: now you've given me leave to explain how to do range calculations from a VOR, how DME works, and why NDBs are non-directional compared to A-N beacons. Oh yes you will regret this.

nec Timide said...

Goodie! I was going to comment with an expanation how the word 'range' came to be used to describe a VOR (and the precursor) but I will leave you to it and not steal the thunder.

zb said...

How can VORs not be the coolest thing I've ever heard? With their elegance comes great reliability (read: safety). Navigation based on GPS may be more precise, but is it really as reliable, thinking about solar activity and just the fact that it is so much more complex? I don't have the deatails for North America, but in Germany, some VORs have been taken out of service during the recent years. I hope the remaining ones are still enough as a fallback layer in case GPS based navigation should fail.

By the way, here is an image of a VOR transmitter's circuit board. It is displayed at the Deutsches Museum in M√ľnchen, amongst quite a lot of other nice pieces of communication technology. (Sorry, this section of their website doesn't seem to be available in English.)

Feel free to embed the image, I own it and hereby declare it free. Sorry for the low contrast. Couldn't get a better shot through the glass. Anyhow, despite the mediocre picture quality, you can see a nice radio-frequency stripline in the center, and I like how all the nice metal-can transistors are laid out in a circle.

Cedarglen said...

Ha! Yes, I know how the VOR works. But as a non-pilot the periodic reminders are a good thing and I do not mind them at all. Your posts are fun reading Ms. Avx, so please, keep on writing about whatever you wish.. Thanks, -C.

A Squared said...

VOR: omnidirectional, well yes, it works in all directions; range, hmm, no it doesn't measure range.

Range in that context doesn't mean distance, it means a line along which you navigate. You also encounter the term in nautical navigation when speaking of "range markers" which also have nothing to do with distance. They are paired visual markers which are paired such that when the two appear to be aligned, you know that you are along a defined course, usually a channel. Aviatrix may remember a pair of lighted range markers in a park off the departure end of runway 32 at Anchorage.

grant said...

Equally ingenious was the radio range system prior to VOR. That the Morse code for A and N would signal which side of the intended track you were on, and would merge into one continuous hum when on course, is elegant simplicity, imho.

grant said...

re: "A Squared said...
VOR: omnidirectional, well yes, it works in all directions; range, hmm, no it doesn't measure range."

But there is a way to determine distance from the station, based upon the time to cross a radial splay... Not sure how often anyone ever had to do that, other than while preparing for the commercial pilot's licence test back in 'the day.'