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There are several errors above. First you claim that SIRI's satellite's are geosynchronous and in the next paragraph you claim they are geostationary.

Oops! I certainly goofed. I meant "SIRI uses three satelllites in a style of geosynchronous orbit..." Glad I included the definitions in the previous paragraph. :-)

You then claim that the satellites are in a molniya orbit. Molniya orbits are NOT any kind of geo-orbit. A geo-orbit has a period of 24 hours. A molniya orbit has a 12 hour period.

You are correct. Molniya orbits do have 12 hour periods. SIRI's orbits have been called Molniya-like because they use highly elliptic and and highly inclined like Molniya-orbiting sats have, but they are not in Molniya orbits. My mistake.

Far anyone interested, here's a link to a site with great graphics illustrating the differences in orbits. They include geosynchronoous and Molniya orbits, but not specifically Siri's sats:

http://www.geocities.com/CapeCanaveral/Hangar/1668/orbits.htm

SIRI's sats do indeed have 24 hr periods, not 12 hour periods that Molniya-orbiting sats have.

http://spaceflightnow.com/proton/sirius3/001130sirius.html

>during the eight-hour period that each satellite is overhead, they are
>much higher overhead than XMSR's satellites.

I would tend to hypothesize the opposite. The molniya orbit is only 5000 km higher at apogee (40080 km vs 35790). In addition, it is not at that height for the entire 8 hours. It is only higher for approximately 4.5 hours of the 12 hour orbit.


What I meant by "higher overhead" is that it is at an angle higher above the horizon than geostationary sats. I should have worded that more clearly.

From the previous link, SIRI's satellites are useful when they are north of the equator, and each satellite is above the equator for 16 out of 24 hours. Since the satellites are spaced 8 hours apart, as satellite #1 drops below the equator, satellite #2 is at its apogee (highest angle above the horizon) and satellite #3 is just rising above the equator. Pretty cool the way it works out...

Here's another article detailing the orbit scheme of SIRI:

http://www.spaceandtech.com/digest/flash-articles/flash2000-090.shtml

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.

Thanks for the critique -- its keeping me honest. :-)

Craig




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