[APML] Satellite orbital taxonomy

["Jon Kolb" <jkolb@datawest.net>]

Hi Paul,

Maybe it's just late, but you've got me confused.
 
>       I mean just that:  they remain over nearly the same longitude
> but stay within the _Ecliptic_ as it tracks north or south across 
> that longitude.  One such satellite I tracked actually followed a 
> narrow, analemma-like pattern in the sky.  

The ground trace you are describing sounds like a typical geosynchronous
orbit (not to be confused with "geostationary") with some inclination
and maybe a bit of eccentricity.  If they stay "over nearly the same
longitude" then they've got to be in nearly 24 hour orbits.  If they're
tracking north and south with the ecliptic, which is at about 23.5
degrees inclination, they will make the narrow analemma-like pattern on
a ground trace.  Add some  eccentricity, and they take on skewed shapes
with fatter but unequal north and south lobes.  If you really mean
"within the ecliptic," then I guess that could mean anything from 0 to
23.5 degrees inclination, but if you mean at the inclination of the
ecliptic, then it's got to be around 23.5.  Operationally, all geosynchs
make the analemma-like ground trace, since they can't maintain perfectly
0 inclination or perfectly 0 eccentricity.

To stay near the same geographic longitude at 23.5 degrees inclination,
these guys must have to maneuver quite frequently - much more frequently
than geostationary sats (close to 0 degrees inclination).  Otherwise,
the inclination is going to precess the ascending node, and hence the
orbital plane, westward in pretty short order.  The inclination also
compounds the problem by making the subsatellite point move away from
the desired geographic longitude.  Sounds like an expensive, short-lived
bird.

> I also tracked Russian
> Molniya's which had highly elliptical -- but in no way geosynchronous 
> -- orbits.  They were quite low and moved very rapidly across US 
> territory.

I'm well familiar with Molys, which are in 12-hour orbits, and have an
eccentricity of about .7 as I recall.  They are at an inclination that
causes the precession of their argument of perigee to be very nearly
zero, and are oriented so that apogee stays in the northern hemisphere
all the time.  The spend just about 11 hours north of the equator, and
go screaming through descending node to ascending node in just about an
hour.  When over US territory, they move pretty slowly however, since
we're north.  The Molniyas are (or were) the main Soviet/CIS/Russian
communications satellite network.
 
>       I don't think so, but I don't have the distance of the
> antisolar 
> Lagrange point from the center of the Earth and so don't know if a 
> satellite orbitting at that distance would yield a "geosynchronous" 
> track.
 
Since you're talking about staying close to one geocentric longitude,
that rules out heliocentric orbits like Lagrange point positions.  The
Lagrange points are substanially farther out than geosynch altitude, so
it would be pretty hard to do any spying from there anyway.  Most
Earth-orbiting satellites at supersynchronous distances are also highly
eccentric and make a wild ground trace as the Earth rotates under the
track while near apogee, but then the sat speeds through perigee to
"catch up" to itself.  
 
> Definitely not.  I described the orbit above.
 
Hence my confusion as described above and again below.
 
>       Actually, there is;  and that was why this location was such a
> good one in which to park a spy satellite.  To find a geosynchronous 
> satellite, you park a telescope at a specific altitude and azimuth, 
> don't move it and make lots of integrations.  During those 
> integrations, stars will track through your field and possibly ruin 
> an exposure of a fixed target.  The more stars, the better.  Couple 
> trailing stars with a faint object moving orthogonally to the stars 
> and you create a spy satellite that is very difficult to find.  Then 
> add orbit changes to this motion and you end up with a devilishly 
> difficult object to find...snip...

Finding dark, deep sky objects without knowing the element set is tough,
indeed.  Add some maneuvers, and sure, you can almost forget about
finding it unless by chance.  Maybe I'm just dense tonight, but this
still sounds like a bird at about 23.5 degrees inclination, with a
roughly synchronous period, and that makes a boatload of maneuvers to
stay near the same geographic longitude because of the inclination
messing with ascending node (hence the very short lifespan you mentioned
in a later message).  What am I missing? Do you remember any satellite
numbers?

Sincerely,
Jon Kolb
Adventures in Astrophotography
http://home.datawest.net/jkolb/
jkolb@datawest.net





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