Re: [amsat-bb] strange orbit

[Franklin Antonio <antonio@xxxxxxxxxxxx>]

At 03:00 PM 11/19/2004, G8IFF/KC8NHF wrote:
>Would someone care to explain a figure of eight geo-synchronous orbit 
>around the earth that doesn't crash twice every orbit?

The explanation is that the shape of the orbits of the Sirius satellites 
are elliptical, not "figure 8".  I know they keep calling them "figure 8" 
but that doesn't mean that the shape of the orbit is a figure 8.

Most communication satellites are in circular orbits, because only circular 
orbits can be geostationary (which means stays at the same place relative 
to the surface of the earth).  Unfortunately these circular geostationary 
orbits only work at 0 degrees inclination, meaning they orbit around the 
equator.  For folks on the equator that's great, but for folks at high 
latitudes the darn things appear at low elevations, ie down near the 
horizon which can be problematic.

Now if you live at a high latitude, like the Russians, you might look for a 
different solution.  If you're building a mobile satellite system where you 
want cars to have a good chance of having a clear path to the satellite, 
not blocked by buildings, you might look for a different solution.

A different solution is an elliptical orbit, which is tilted at a nice high 
inclination, so that when the satellite is the farthest from earth (at 
apogee) and therefore going around the center of the earth the slowest, it 
will be sitting above some nice high latitude (like where you live) instead 
of over the equator.  Because satellites in elliptical orbits spend most of 
their time near the apogee, the thing will appear to hover there when 
viewed from earth, meaning it is usable a lot of the time.  Maybe not all 
the time, but a lot of the time.  If you have more than one of these 
satellites you can get them timed so that one or the other of them seems to 
be hanging above you all the time.  A lot like juggling.  This is just 
about as good as a geostationary satellite, but obviously has some 
different properties.  Its better for people with higher latitudes.  That's 
good.  Geostationary satellites appear to stand still from the viewpoint of 
a human on the surface of the rotating earth, but  elliptical orbit 
satellites appear to move.  That might be bad for providing TV to homes, 
for example, because people might need their dishes to move to stay pointed 
at darn satellites.  If you are building a system for which people are 
gonna use omnidirectional antennas which don't need pointing (on their cars 
for example) then you don't care that the satellite appears to move from 
the viewpoint of the human standing on earth.

So if you look up at this geosynchronous elliptical orbit satellite from a 
fixed point on the rotating earth, what do you see?  If your eyes were good 
enough you'd see the satellite move around a figure eight kinda 
thing.  That's why they call 'em figure eight orbits.  I don't call 'em 
that, but you gotta admit "figure 8" is probably a better term to put in 
marketing literature than "elliptical geosynchronous".

Interestingly, real figure 8 shaped orbits are possible, but you can't make 
'em by only orbiting around one thing.  The apollo missions used 'em to go 
to the moon, so that they would head back toward earth automatically if for 
some reason they couldn't slow down when they were around the moon.  I used 
to fly figure 8 orbits around the sun and an orbiting planet in a space war 
game I built in college which was a long long long time ago.  I usually 
crashed after one or two times around the figure 8.
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