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You bring up a good point though about altitude: If SIRI and XMSR use geosynchronous orbits and SIRI's orbit is elliptical, its apogee must be higher than XMSR's sats. This can be seen in the previous link: SIRI's apogee is about 47,000 km, vs XMSR's 36,000 km. I'm sure this extra distance doesn't help SIRI, but I really don't know enough to say by how much.

Electromagnetic signals lose power in proportion to 1/(distance squared). So you can see the received signal has very low power for both in general.

It's not all about the satellites. You have to have good engineering in the antennas and receivers, too. It's amazing how much talk there is about the satellite paths when that is only part of the picture of signal reception. If you've cut too many corners and built a crappy receiver or too "noisy" (in electrical terms) of an antenna, you get a big loss in signal quality.

Though I don't know anything about the comparative quality of the receivers for XM and SIRI. For now it seems that most people who have subscribed to XM are pretty happy with their reception.

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