View Full Version : How do navaids work?

12th Dec 2001, 01:42
I was wondering if some kind soul can point me in the direction of some technical information about how common navaids work?

For instance, I am aware that a localizer emits 90Hz and 150Hz tones on either side of the centre-line, which presumably modulate the carrier. I would imagine that the receiver in the cockpit demodulates the signal and uses the amount of the measured tone at the two frequencies to move the needle.

I'm specifically interested in localizers, VORs and DME. I don't have any particular use for the info other than being curious from an engineering standpoint as to how both the transmitter and receiver portions work.

Many thanks!

12th Dec 2001, 02:34
after many years in the biz I am able to tell you the answer: smoke and mirrors or,as we say PFM. hope this helps. :rolleyes:

12th Dec 2001, 02:35

13th Dec 2001, 14:36
The way I was taught VORs was like this.
It's very basic, but it may give you an understanding.
Imagine the VOR station being a lighthouse with a normal rotating light but also a flashing beacon on top. The beacon only flashes when the rotating light is pointing north. It takes 360 seconds for the rotating light to turn full circle. So you're flying along, see a beacon flash and start counting. 90 seconds later you see the rotating light. you now know that you are east of the station. Do the same with another station and you have a fix. As long as you know where the stations are you now know your position. Of course the rotating light is a radio signal and in reality it takes a millisecond to rotate.
Well I said it was basic!!! :p

13th Dec 2001, 23:14
Appreciate the info guys.
I was really after something a lot more technical...
The FAA's AIM has some good info on ILSes and includes an illustration of a localizer and basically tells me that two beams come out of it, the left one modulated with a 90Hz signal, the right one modulated with a 150Hz signal, the carrier frequency being the localizer frequency. But there's no info on what happens from an avionics standpoint inside the aircraft receiver or indeed more technical detail on what the localizer is sending out.
Anyway, that's the sort of stuff I'm after - for no particularly good reason - its just been bugging me for a while and I wanted to try and understand the whole system :)

Squawk 8888
14th Dec 2001, 01:37
Tried the how stuff works site, no joy searching for "VOR" or "navaid" but "VHF omnirange" gave me a link to some data on viral hemoragic fevers :eek:

Since I don't have the details handy (I'm in the office and my textbooks are at home) I'll give the short version, which should inspire at least twenty other users to jump in and explain why I'm wrong :D

VOR and ILS both operate on the principle of phase displacement. When two signals of different frequencies are mixed, the interference between them leaves a "beat frequency" that can be measured. With ILS, the waveform of this beat will vary in shape depending on which of the two signals is stronger. For VOR, there are two signals, one constant and the other that shifts in phase around the station. The two signals are in phase north of the station, and the phase shift (which translates into a beat frequency) equals the bearing from the station, e.g. a 90-degree shift indicates that the receiver is east of the station.

DME is the simplest- it sends out a signal to the station and waits for a reply. Divide the response time by twice the speed of light and you have the distance.

14th Dec 2001, 03:12
Looks like the solution to my question is Principles of Avionics by Helfrick.
Anyone else that's interested can look at http://www.avionics.com/www/books/Principles%20of%20Avionics.htm for info on what it contains.

Cmdr Data
14th Dec 2001, 05:38
Author, R.B. Underdown, ground studies for pilots.ISBN 0-632-03601-X.
Vol 1 radio aids.
Ace book, got me through the CAA exams years ago. :p

16th Dec 2001, 15:08
Try the search facility on this site, theres some useful stuff here.