We had an XM portable we used in the cab of our Kodiak and now our E450. The problem was heading north from Florida - no reception with the antenna on the dash as both satellites are well to the south. I'd tried mounting it on the mirror and that helped, but it could disappear when going around a bend. Also ran it up to the roof on one occasion and wrapped it around the radio antenna and that worked well.
We switched to Sirius and the problem is far less and almost non-existent. XM has two satellites around the equator as does Sirius but Sirius also has one farther north in a figure 8 like pattern and is overhead the US 16 hours a day. That makes a huge difference with tall buildings. 2 of the 3 are broadcasting at all times.
In many built up communities they have local repeater units to keep buildings from obliterating the signal.
Our two autos have Sirius now as does our MH with the portable. I doubt we'd ever go back to XM. My guess is that XM will eventually die out. No way they could abandon either system at the time of the merger.
