VORs are pretty interesting. At first i thought every plane had a small antenna array on board to check the direction to the VOR.<p>But the direction is just calculated by the phase shift between an omnidirectional and a directional signal. So it can be implemented very cheaply on every plane.
If you want some in-depth technical details on how VORs work, I found this video very helpful. It goes into many of the details of the analog radio engineering:<p><a href="https://www.youtube.com/watch?v=tZ2gG1v9Xg8" rel="nofollow">https://www.youtube.com/watch?v=tZ2gG1v9Xg8</a>
Cute.<p>He set up his receiver near the VOR, though. So he doesn't get any useful distance info from it. He can hear the aircraft's query and the fixed station's reply, but near the DME station, the difference will be constant, just the fixed delay.<p>The next step is to put the receiver far from the DME station. Then, the time delay measured will indicate the aircraft to DME station distance minus the aircraft to receiver distance. I think this lets you locate the aircraft somewhere on a hyperbola, similar to the way GPS and LORAN work off time differences. If you have two receivers at different locations, you should be able to get two hyperbolas and locate the aircraft.<p>This is really a 3D problem, because altitude. So you get quadric surfaces and need 3 receivers. Preferably four, because there are multiple solutions. Two is enough to get a rough aircraft location for test purposes.<p>This has potential as a ground backup for ADS-B. ADS-B tells you where the aircraft nav system thinks it is. This is telling you where it really is, if it's using a VOR/DME at the moment.<p>But not who it is. That's not in the DME poll.