figure eight, geo-synchronous orbit???

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?

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?

You're probably thinking of geostationary orbits, where the satellite
orbits in the equatorial plane at exactly the right distance, so that it
appears to be stationary from the point of view of an observer on the
ground. Geosynchronous orbits, though, simply mean an orbit which has a
period equal to the rotation period of the Earth, e.g., 24 hours,
although often the latter is used colloquially to mean the former.

An elliptical, inclined geosynchronous orbit can indeed have the "figure
eight" properties you describe; every 24 hours it will return to the
same point in the sky relative to an observer on the ground, but in the
meantime it can meander.

--
__ Erik Max Francis && && http://www.alcyone.com/max/
/ \ San Jose, CA, USA && 37 20 N 121 53 W && &tSftDotIotE
\__/ Bachelors have consciences, married men have wives.
-- H.L. Mencken

"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?

These satellites orbit at the geosynchronous distance, but their orbits are
inclined slightly relative to the equator. Viewed from Earth's surface,
they appear to move in a figure-eight pattern as a result.


Jim McCauley

possibly this a description of a geo satellite with more than a little
inclination and a non-circular orbit that will look like it traces a figure
8 with respect to a ground user.


"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?

Once in geosynch, they drift a bit in their relative position, as a way to save
fuel from "absolute" stationkeeping. Ever look at an old Earth globe in your
school or library? See the little figure-eight shape that describes the sun's
annual wobble?

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/

(Many people use the term geosynchronous when they really mean
geostationary, which can cause confusion when dealing with orbits with non-
zero inclination or eccentricity.)


--
JRF

Reply-to address spam-proofed - to reply by E-mail,
check "Organization" (I am not assimilated) and
think one step ahead of IBM.

In article ,
Roy Smith wrote:
I know what a geo synchronous orbit is, but what's the "figure eight"
part all about?

A satellite in geosynchronous orbit (i.e. one that completes an orbit in
exactly one Earth rotation) only appears to be stationary if the orbit is
circular and inclined exactly along the equator.

If it isn't, the satellite appears to move eastward as it travels faster
around perigee, and westward as it slows around apogee. Differences in
inclination account for apparent north and south movement.

The combination of the two sets of apparent movements describes a figure
of eight path above an observer on the ground.

Stationary satellites tend to acquire such orbits when they are left to
their own devices, e.g when propellant runs low, or are put in such
orbits deliberately, allowing them to continue to be used for odd jobs
after being replaced.

-- don

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?

24-hour period, but not 0 inclination. As it moves up and down in
latitude, the ground track is a figure 8.

Brett

In article ,
Roy Smith wrote:
"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?

This is probably a reference to the sort-of-figure-eight ground track
(centered on the equator) traced out by a spacecraft in an *inclined*
geosynchronous orbit.
--
MOST launched 30 June; science observations running | Henry Spencer
since Oct; first surprises seen; papers pending. |

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?

If you are at a non-zero inclination, but with the proper period
for a synchronus orbit, the orbit ground track will trace a figure
8 pattern.

As the satellite crosses the equator, because its orbit
is inclined, its velocity relative to the surface is less
than needed to keep up with the ground (if it was at zero
inclination, remember, it would be just enough to keep up).
So the satellite appears to drift westward.

At higher latitudes, the satellite catches up again as its
Earth relative eastward velocity component is a maximum
and the surface's eastward velocity component reduces as the
surface point becomes closer to the Earth's rotation axis.

Hence the figure 8.