Sirius Radio Satellite Orbit Needed?
 

Sirius uses 3 satellites in a geosynchronous orbit.
XM uses 2 satellites in geostationary orbit.

Anyone know where I can find pictorials of these orbit paths?

wrote in message ...
Sirius uses 3 satellites in a geosynchronous orbit.
XM uses 2 satellites in geostationary orbit.

Anyone know where I can find pictorials of these orbit paths?

http://www.cashflowbusiness.net/siri_orbit.htm

Note that three times a day both active Sirius sats are at the
crossover point of the figure-8. There won't be much of any spatial
diversity for an hour or two around this these times, ie: the two
acitve sats won't be much better than just one.

The two-line orbital elements can be obtained from:
http://celestrak.com/NORAD/elements/other-comm.txt
You can then use a program like NOVA and have your own display of the
orbits with a display of time and direction to each satellite. (XM-3
is the first replacement sat for XM).

I'm highly suspicious of that graphic, RJ. Something about the way that
tiny little Sun peels around Earth. Do we know the graphic's origin?

For something straight from Sirius, try

http://www.siriusradio.com/servlet/C...=1038414951381

If that doesn't work, Sirius.com -- About Us -- Image Library --
Miscellaneous -- Control Room

...then download and unzip the big image to see what's really going on to
create that Figure 8 effect. Three technicians in the control room; the large
view of the satellites in space and the four views of the ground track.

I see three orbits at right angles to one another. The ground tracking
matches in both graphics, but is it possible that the actual bird heading south
over MN is *not* the one that was heading north immediately prior? And that,
while the satellites' paths cross, they don't necessarily cross over Central
USA?

Jerome



"Robert J Carpenter" wrote in message
...

wrote in message ...
Sirius uses 3 satellites in a geosynchronous orbit.
XM uses 2 satellites in geostationary orbit.

Anyone know where I can find pictorials of these orbit paths?

http://www.cashflowbusiness.net/siri_orbit.htm

Note that three times a day both active Sirius sats are at the
crossover point of the figure-8. There won't be much of any spatial
diversity for an hour or two around this these times, ie: the two
acitve sats won't be much better than just one.

The two-line orbital elements can be obtained from:
http://celestrak.com/NORAD/elements/other-comm.txt
You can then use a program like NOVA and have your own display of the
orbits with a display of time and direction to each satellite. (XM-3
is the first replacement sat for XM).

"Cooperstown.Net" wrote in message
...
I'm highly suspicious of that graphic, RJ. Something about the
way that
tiny little Sun peels around Earth. Do we know the graphic's
origin?

I'm sure it was created by the NOVA program from the official NORAD
orbital elements.
That is the subsatellite point of the sun - not the sun itself.

For something straight from Sirius, try


http://www.siriusradio.com/servlet/C...=1038414951381

You will note that the images on the four lower screens show orbits
which are identical to the one in the graphic I linked to.

...then download and unzip the big image to see what's really
going on to
create that Figure 8 effect. Three technicians in the control room;
the large
view of the satellites in space and the four views of the ground
track.

Done, that, see above comment.

I see three orbits at right angles to one another. The ground
tracking
matches in both graphics, but is it possible that the actual bird
heading south
over MN is *not* the one that was heading north immediately prior?

You agree then about the ground tracks. Satellites can't change
direction instantly without a lot of energy (propellant) being
expended. Thus each one continues to rat-race around it own figure-8
orbit.The north-heading sat has to go around the upper part of the
loop before it can head south. In the case of Sirius the three orbits
are 8 hours apart which accounts for the upper picture, I suppose.
You'll note that each sat takes exactly 24 hours to get around its
orbit, otherwise everything would change from day to day, which it
doesn't.

And that, while the satellites' paths cross, they don't necessarily
cross over Central
USA?

Their 4 images show they do. Each sat spends 1/3-rd of its time north
of the crossover and 2/3-rds of its time to the south. My guess is
that if the orbits weren't "exactly" 8 hours apart two sats wouldn't
be at the crossover at the same time.

I have faith in the accuracy of NORAD and NOVA, especially when they
seem to agree with Sirius..

The Sirius program ground stations must actively track the sats.
Unless each sat has its own ground station it would be Really Fun when
the feed was switched from the western sat to the eastern sat three
times a day. The FCC would never permit a nondirectional feed from the
ground.

I conclude that the sat's own antenna must have a broad beam (low
gain) since they surely have to accept its default pointing to a fixed
point in space no matter where in the orbit. No sat could carry
enough fuel to continually optimally point its antenna. This probably
explains the lower field strengths of Sirius vs XM.

bob c.