Sirius uses three SS/L-1300 satellites in an elliptical geosynchronous orbit, instead of a geostationary orbit. A satellite in an elliptical geosynchronous orbit will appear to oscillate in the sky from the viewpoint of a ground station.7 This type of orbit allows the satellites to appear higher in the sky than XMs, cutting down on the potential for a listener to be out of range of a satellite signal — and allowing Sirius to have a much smaller number of repeaters.8 Like XM they broadcast their radio programs from a ground station to one of the three satellites, which transmits the signals to the ground where it can be picked up by your receiver. They also have ground repeaters in urban areas.
XM uses three Boeing 702 satellites placed in geostationary orbit. A geostationary orbit is a circular orbit directly above the Earth's equator, and it orbits in the same direction and speed as the Earth's rotation. Geostationary Earth orbit is about 22,223 miles above Earth, and is the type of orbit most commonly used for communications satellites. A satellite at geostationary orbit stays in a fixed position relative to a point on Earth's surface
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