ADS-C is an over-the-horizon complement to ADS-B using satellite relay.
http://www.inmarsat.com/blog/ADS-C-another-step-forwardThe uplink is a narrow directed beam* from the aircraft to the satellite since the aircraft knows where the satellite is. This is the most power-efficient form of transmission..
But for the next step, the downlink relay, most commercial satellites don't used a directed beam despite the pretty diagrams showing a signal directed towards a ground station. They don't discriminate as to the type and destination of the transmission, they assume that all the receivers in their coverage area want to receive everything**. So they basically act as big convex radio-mirrors which spread the downlink signal over the chunk of Earth which they cover.
If you're anywhere in the coverage zone you can receive and decode*** the transmission regardless of the location of the aeroplane.
* In olden days it used to be a big parabolic dish steered by actuators, hence the huge bump on some Sheikh's 747SP. Nowadays it's a phased-array antenna that steers elecronically, hence the pert little bump on the back of airliners.
** Sometimes they're divided into spot-beams, and can relay specific transmissions through specific spots. I don't know if that's used for ADS-C.
*** Assuming presence of suitable hardware and software etc.
