CyberAirVA

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VOR Theory

If you studied our tutorial on VOR/VORTAC you will know how to use it, this tutorial is just to describe how it actually works.

The VOR receiver in the aircraft uses the principle of electronicly measuring an angle. Sounds difficult, just bear with me. Here is how it does it. The VOR beacon sends out two signals that the receiver catches. It first sends out a reference signal which is sent out in all directions, this signal contracts and expands 30 times a second. The other signal is a rotating one, which rotates 30 times, clockwise a second. Whenever the rotating signal passes the magnetic north direction the referenssignal is sent out.

Bolow you can see an illustration of the principle. The first picture illustrates the reference signal being broadcast, this happens every time the rotating one passes the magnetic north. The rotating signal is indicated by the purple bar and the radiowaves by the yellow rings.

The rotating signal then simply rotates, and sends a signal all the way around until it's back at the magnetic north at which point it triggers a new reference signal (see the illustration below).

Ok, so that seems pretty much straight forward. Here is the really smart thing about it. The receiver in the plane is really a timer. It starts timing when the reference signal is receive. and stops when the rotating one is received. Since it knows that the rotating signal rotates 30 times clockwise every second it can by timing the difference calculate on which radial from the VOR it is. The illustration below shows you the principle. At 1 the receivers timer starts, since it now receives the reference signal. At 2 it then receives the rotating signal. The receiver then calculates which radial it's on.

Let's play with the math a bit. The rotating signal rotates 30 times per second, that means that every rotation takes 33.333333 thousends of a second (1000/30). Every rotation contains 360 degrees, this means that every degree takes 0.0925925 thousends of a second to complete (33.333333/360). So in the case above the receiver timed the difference between the two signals to be 10.185176 thousends of a second which in turn indicates that we are on the 110 degree radial (10.185176/0.0925925). The math isn't really that hard once you understand the principles. Hope the theory behind the VOR has been clarified a bit.

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