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The SIRI system uses three satellites that are in low earth orbit. They are NOT in geosync. orbit. This means they are constantly traveling across the USA.

First, SIRI's satellites are in geosynchronous orbit. They are not in geostationary orbit. Satellites in geosynchronous orbit orbit Earth once every 24 hours. Satellites in geostationary orbit orbit Earth once every 24 hours over Earth's equator.

SIRI uses three satellites in a style of geostationary orbit called a Molniya orbit. They orbit the earth every 24 hours, but are inclined so that at its apogee (farthest and slowest part of the orbit) it is high over the United States. The period of which the satellite is in the best position to transmit is approximately eight hours, requiring three satellites to cover the United States 24 hours per day.

This is important because the receivers in cars must switch between the strongest satellite. That makes the chipsets or brains for these radios VERY complex and difficult to produce.

The XMSR system uses two satellites that are parked in geosync. orbit just like Direct TV. The receivers in the cars pick from the strongest satellite and actually use what is called a diversity system. In other words, they use the best signal.


Correct me if I'm wrong, but, from what you said, SIRI's chipset is far more complicated than XMSR's because it has to choose the strongest signal from three signal sources instead of two?

Since SIRI satellites time-share the United States, at most, only two satellites would be visible at any time, since the third would be over the other hemisphere.

The reason that SIRI is rolling out service in 4 large towns is because they use ground antennas to support their service where large buildings block the satellite antennas.

HERE IS THE BIG QUESTION: When SIRI customers drive into another city and into the country on Feb. 14, will the satellites deliver music to their cars? I don't think they will.


XMSR does the same thing. In fact, this is where SIRI should have an advantage. Their use of Molniya-orbiting birds is supposed to result in far less signal shadow becuase, during the eight-hour period that each satellite is overhead, they are much higher overhead than XMSR's satellites.

In addition, to the best of my knowledge, XMSR is using far more repeaters than SIRI to fill in the gaps in signal. One person said (I can't remember who) once said while SIRI is a satellite radio system using repeaters to fill in gaps, XMSR is a ground system using satellites to fill in gaps. That's surely an exxageration, but it does illustrate the differences in signal shadow each system might have.

All of this might be moot, since XMSR's reception seems to be very good, but I would have bet SIRI would have XMSR beat in terms of reception.

Craig
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