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figure eight, geo-synchronous orbit???



 
 
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  #11  
Old December 30th 03, 05:45 AM
Glenn Shaw
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Default figure eight, geo-synchronous orbit???

On Sun, 28 Dec 2003 22:10:26 -0500, Roy Smith wrote:

The Sirius Satellite Radio (www.sirius.com) says:

"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"

I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?


"Figure eight" describes the apparent path that the satellite makes in the
sky, as well as its ground track (the path that the satellite traces out on
a world map). The Sirius birds trace a figure-eight ground track because
their orbits are inclined at roughly 63.5° relative to the plane of the
Equator.

HTH. HAND.
--
Glenn Shaw
Indianapolis, IN USA
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  #12  
Old December 30th 03, 08:47 AM
Matti Anttila
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Default figure eight, geo-synchronous orbit???

Roy Smith wrote:
The Sirius Satellite Radio (www.sirius.com) says:


"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"
I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?


Ground-track makes "figure eight", see:
http://celestrak.com/columns/v04n07/


Matti Anttila
--
email:
home:
http://antti.la/
  #13  
Old December 30th 03, 03:13 PM
Oren Tirosh
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Default figure eight, geo-synchronous orbit???

Roy Smith wrote in message ...
The Sirius Satellite Radio (www.sirius.com) says:

"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"

I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?


A geosynchronous orbit does not have to be equatorial. If it is
inclined the satellite will appear from the ground to make "figure
eight" motion. For zero inclination this figure eight will shrink to
zero size and you get a conventional geostationary orbit.

This figure eight orbit is useful for getting better coverate at
higher lattitudes. Satellite radio needs good reception in deep urban
canions and a geostationary satellite may be too low in the southern
skies to be visible. To get continous coverage you need more than one
satellite at different phases in the same orbit, though.
  #14  
Old December 30th 03, 05:10 PM
Carsten Nielsen
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Default figure eight, geo-synchronous orbit???

Roy Smith wrote in message ...
The Sirius Satellite Radio (www.sirius.com) says:

"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"

I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?


Maybe an inclined orbit at geosync. altitude ?

That would trace a figure 8 pattern as seen from Earth, but why
someone would do that now, I don't know.

The first geosync. sattelite did that, but any geostationary don't,
and you don't want it, because you will have to track the satelite.

The whole point of geostationary satellites is that you don't have to
track, they stay in the same point of the sky.

Regards

Carsten Nielsen
Denmark
  #15  
Old December 30th 03, 09:32 PM
ed kyle
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Default figure eight, geo-synchronous orbit???

Roy Smith wrote in message ...
The Sirius Satellite Radio (www.sirius.com) says:

"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"

I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?


The Sirius satellites are geosynchronous because they
orbit in sync with the rotation of the earth. That is,
they complete one revolution per day. They differ from
traditional geosync comsats that orbit above the earth's
equator because they are in 24 hour elliptical orbits
(Sirius 1 is in a 24663 x 46906 km x 62.9 deg orbit, for
example) that are inclined about 63 degees from the
equatorial plane. These orbits trace a distorted
"figure-eight" pattern on the planet's surface, with a
smaller top loop in the northern hemisphere above North
America and a larger bottom loop in the southern
hemisphere above South America. The three Sirius
satellites take turns spending more of their time in the
little (apogee-side) part of the "figure-eight" above
North America than they do in the bigger (perigee-side)
loop above South America. The inclined orbits are
supposed to provide better RF propagation look angles for
receivers in urban areas.

- Ed Kyle
  #16  
Old December 31st 03, 03:26 AM
Matthew Jessick
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Default figure eight, geo-synchronous orbit???



ed kyle wrote:
Roy Smith wrote in message ...

The Sirius Satellite Radio (www.sirius.com) says:

"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"

I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?



The Sirius satellites are geosynchronous because they
orbit in sync with the rotation of the earth. That is,
they complete one revolution per day. They differ from
traditional geosync comsats that orbit above the earth's
equator because they are in 24 hour elliptical orbits
(Sirius 1 is in a 24663 x 46906 km x 62.9 deg orbit, for
example) that are inclined about 63 degees from the
equatorial plane. These orbits trace a distorted
"figure-eight" pattern on the planet's surface, with a
smaller top loop in the northern hemisphere above North
America and a larger bottom loop in the southern
hemisphere above South America. The three Sirius
satellites take turns spending more of their time in the
little (apogee-side) part of the "figure-eight" above
North America than they do in the bigger (perigee-side)
loop above South America. The inclined orbits are
supposed to provide better RF propagation look angles for
receivers in urban areas.

- Ed Kyle


Inclinations near 63.4 deg also minimize the rotation of the
argument of perigee due to the Earth's oblateness.
  #17  
Old December 31st 03, 07:24 AM
Greg D. Moore \(Strider\)
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Default figure eight, geo-synchronous orbit???


"Jorge R. Frank" wrote in message
...
Roy Smith wrote in news:roy-
:

The Sirius Satellite Radio (
www.sirius.com) says:

"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"

I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?


A geosynchronous orbit with a non-zero inclination angle will have a
groundtrack resembling a figure eight. See:

http://celestrak.com/columns/v04n07/



Hmm, this is a great page on this. To be honest, I'd never really thought
about what a geosynch orbit would look like to an observer on the ground.
Thanks Jorge.


  #18  
Old January 7th 04, 07:48 AM
lcopps
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Posts: n/a
Default figure eight, geo-synchronous orbit???

XM and Sirrius satellite systems use a low gain non directional antenna.
So they don't need to be tracked by users. I assume the uplink uses a
directional dish, unless they receive their signal from other birds.

Carsten Nielsen wrote:

Roy Smith wrote in message ...

The Sirius Satellite Radio (www.sirius.com) says:

"our three state-of-the-art satellites that rotate in figure eight,
geo-synchronous orbits around the earth"

I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?



Maybe an inclined orbit at geosync. altitude ?

That would trace a figure 8 pattern as seen from Earth, but why
someone would do that now, I don't know.

The first geosync. sattelite did that, but any geostationary don't,
and you don't want it, because you will have to track the satelite.

The whole point of geostationary satellites is that you don't have to
track, they stay in the same point of the sky.

Regards

Carsten Nielsen
Denmark

 




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