I was picking up a friend from AIH airport and sitting in a line of traffic that was under the cover of concrete bridges, overhangs and such. Just like anytime the sky is blocked for more than a few seconds my satellite radio lost signal. It never happens when among big buildings unless the southern sky is blocked. Using the power of observations it seems to me the most reasonable explanation is that the signal comes from very high in the southern sky. And since no towers exist that could make this work the logical explanation is that there are satellites in geostationary orbits that deliver the signal.
The longest road trip I have taken since I had satellite radio in a car was around 1200 miles. Not once in that time was the signal weak or sketchy in any way. (A side point is that the GPS predicted my arrival within 10 minutes door to door.)
What other reason would this happen?