Satellite communications are always line of site. So are regular radio waves, however due to the distance the signal must travel, in spite of the broadcast power from the satellite, the receiving antenna must have a clear "view" of the sky in order to receive that signal. When that clear view is obscured by anything the radio will cut out. I compare this to driving around in a parking garage. Your regular radio will usually work fine, however your satellite radio will not necessarily work fine.

Part of this is also due to the difference in transmission. Regular radio is analog, so if the signal becomes weaker the radio will continue to broadcast what it receives even as sound quality reduces. Satellite is digital and as such if the information stream coming from the satellite must be decoded by the receiver and then turned into sound. If the data stream is missing information because reception is spotty the radio will not have enough data to complete the conversion to sound and will just stop converting altogether.

I wouldn't let underpasses and parking garages and the occasional spot of bad weather keep you from getting XM. It works as advertised.

It has been said:
Hybrid drivers come in 3 flavors, greenie, techie and cheapie. Pick any 2.
2005 Prius, Melinium Silver over gray, package 5 (AI)