Geostationary satellites are above the equator and in orbits that are more than 24,000 miles away. Therefore, power budgets being what they are, one needs a high gain dish antenna to receive their signals.
Sirius' birds are in very low earth orbit. Thats how you get away without having to aim a dish antenna at them. Since they are in low orbit, they move across the sky very quickly. As they move through the sky, the Sirius receiver switches to the one with the strongest signal, sort of the way a cell phone switches to the strongest cell tower....only with Sirius, its the "cell tower (i.e. sats)" that are doing the bulk of the moving....I suspect that XM sats work in a similar manner.
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