Listener AR ratings - how they work?

[garthu]

I'm really curious as to how AR ratings work (audience listening/watching ratings)

My curiosity stems from a radio slot we did some time back and paid xyz based on the number of listeners.

Now they seemed to imply that they were able to technically tell how many people listened which i find extremely hard to believe as technically a radio does not transmit anything that says it is tuned into xyz frequency making this statement surely impossible..

But then how do they derive these figures that they have 10 000 listeners??