The XM satellites are in geostationary orbit over the equator, some handling the western US and some for the eastern US. (I don't recall how many are on each side.) Geostationary means they don't move in relation to the Earth, so they are always hovering over the same spot on the planet at all times. From our perspective on Earth, a geostationary satellite just stays in one place in the sky and doesn't move. Since they are so far south (over the equator), the further north you go the lower in the horizon they get. This means your signal strength drops the further north you go. I would run into this with a Pioneer XM unit in one of my previous cars. I installed the antenna on the dash up by the windshield rather than on the roof. Down here in southern California it worked fine. Traveling up to northern Cali and Oregon, though, would cause it to start dropping out more often the further north I went.
I later moved the antenna to the roof of the car. That helped a lot, but I'd still get more cut-outs the further north I'd go.
An interesting note is that XM has ground-based repeater units in many urban areas, especially those with very tall buildings that would otherwise block the satellite signals. It's possible that those of your who've always had good signal strength happen to live in an area with repeaters, or you live in the southern US where signal is the strongest.
Sirius works differently, though. It doesn't use geostationary satellites over the equator like XM does, but rather it uses 3 satellites that are constantly orbiting the planet. They carefully time each satellite's orbit so that at least one of them is always over the US.
I don't know how they do it now that XM and Sirius merged, though. Do they simply pump out the same content through *all* the satellites, so you get the same content whether you have an XM or a Sirius receiver? Anyone know which one the 4runner uses?
