I'm not sure where the Center keeps their equipment (might be able to find out on all-knowing Google), but, as far as the ability to hear transmissions, it has to do with line-of-sight -- airband frequencies require a somewhat direct line between the tranmitter and receiver, and is susceptible to interference. So, the antenna almost always has line-of-sight to planes in the air, but any number of things can mess with transmissions from people on the ground (heavy equipment, power lines, weather, etc.)