VOR mechanics


Well-Known Member
I am interested in learning more about the mechanics behind VOR/ILS sytems and how they work. For instnace, how are the radials differentiated? How does the to/from indicator work? Basically, how do the ground portions of VOR's work?

Anyone have links or reccomended reading? The jepp texts and FAA publications don't seem to touch on this topic very much.
I thought your post title concerned the people that physically install/repair the things.

Never mind.
I don't have the book in front of me...but let me try this:

The VOR shoots at two signals...one variable and one fixed. The fixed signal is a reference to mag north, and the variable signal rotates at something like 18000rpm. The VOR reciever in the airplane "times" the difference between the variable and reference signal to determine what radial you are on.
Don't quote me, but I believe it is something like that.
I thought your post title concerned the people that physically install/repair the things.

[/ QUOTE ]

There are only two of them in the whole country!! One's named Tony, and the other one is named Paul. They get lots of free travel!!!!
On my previous job I used to work on VOR/ILS receivers, I could spend hours typing, trying to explain what I know about these receivers. Maybe THIS will help a little.

For the ILS system try THIS . It has some good theory,(even though it refers to rohde/schwarz equipment)

I can give you the name of some books, if you need more detail...
Thank you very much for those links Rodelu! I do have a few questions, but first, let me try to get a grasp on this (using my primitive knowledge of wave mechanics). The VOR emits two signals, one constant phase signal, and one variable phase signal. A special circular antenna allows the variable phase signal to "revolve" around the station (30 times per second?). The variable phase signal is out of phase with the constant phase signal by a different number of degrees depending on the direction it is emitting to. This is similair to what Cavok describes.

So on North, the signals might be in phase, at East the signals are 45 degrees out of phase, at south 90 degrees out of phase, etc? I guess the exact phase difference for each bearing is irrelevant, but I'm just curious.

Localizer works by using two different frequencies for each side of the course. Does the needle on the ILS indicator swing to the proper amount deflection because one signal becomes progressively more dominant as you go further off centerline, or is it some kind of phase interaction like the Vor's?

Also, how is the to/from flag controlled? Can the reciever pick up on Doppler effect (increasing frequency as you approach the station=to flag, decreasing frequency as you depart=from flag?)
Yes, you are correct about the VOR. That "revolving" is accomplished at the antenna (electronically) 30 times a second, as you said. Theoretically there are 360 degrees so the phase difference let's say EAST of the station will be 90 degrees; 180 at South etc. etc.
When it comes to the ILS, it uses a little different theory. You have 2 different modulation signals: 90 and 150 Hz for either Glideslope or Localizer. The detected signal strengths (or "proportions" if you will), will be compared and a voltage representation (+ or - for left/up or down/right) will be sent to the indicator/AP/etc etc
When it comes commercial receivers for the industry, the redundancy, self diagnostic and internal monitoring capabilities of some of these devices is pretty outstanding. In some cases it will flag you if the transmitter station is off freq. etc, etc.
I had a great time calibrating those boxes for the big carriers (mostly transport cat.). I felt a great deal of responsibility knowing that my "tweaking" could mean the difference between a great instrument approach/navigation and a not so good one...
Also, how is the to/from flag controlled?

[/ QUOTE ] I believe the TO/FROM flag is not controlled by the station. It is controlled by your equipment in the airplane.

Your VOR receiver can determine which (of 360) radials you are on. The TO/FROM indication is based on whether the course needle is within roughly 90 degrees (either side) of the radial you are on.