Mir Predictions again

Philip Chien (kc4yer@amsat.org)
Wed, 1 Oct 1997 06:39:08 -0400

Alistair Grant <agra1@ihug.co.nz> said:

>I read a post the other day that talked about how important it is to have
>the most recent tle. Is say a week acceptable for MIR since it is in a
>stable orbit? And as for the shuttle I assume that while its docked to MIR
>it is also in a "stable" orbit? And when the shuttle is not docked I
>assume that you need the newest?

depends - what are you going to do with your predictions?

As a rule there are two primary things which change Mir's orbit.  It's
large size, especially the solar arrays and low altitude give it a fairly
high drag factor which gradually decreases its orbit.  And a Progress or
Soyuz spacecraft can be used to lower Mir's orbit (for operational
purposes) or raise it's orbit (to counteract the drag).  The third
possibility, that CNN is correct with its colorful description of Mir
spinning out of orbit and out of control when its attitude control computer
failed, belongs in the category of bad science fiction.

The natural decay to Mir's orbit is fairly steady over relatively short
periods (e.g. under a year).  Orbital adjustments happen on a periodic
basis and make fairly significant changes to the orbital altitude/shape.  I
usually look for a change in the eccentricity or mean motion.

However regular keps are very useful for other purposes.  The keps are only
approximations of a spacecraft's mean orbit - how everything averages out.
And keps have a limited number of significant digits.  Over time keps
become 'stale' as little errors build up.  All other things being equal the
closer you are to the epoch the more accurate your calculations are.

So, depending on how much accuracy you need for your calculations (in
increasing order of difficulty: omni antenna, naked eye visibility, beam
antenna, visibility within binoculars, determining whether or not it's
within danger range of another spacecraft, location for a
rendezvous/docking mission) you should figure out how often you need to
update your keps.

Personally I update my keps once a week whenever I receive them via the
AMSAT keps list (amateur radio satellites, polar weather satellites, HST,
GRO and UARS) which I'm subscribed to.  However when a shuttle is flying a
dynamic mission (e.g. one with a lot of maneuvers like a rendezvous
mission) I'll make sure to update my keps as soon as practical after a
maneuver has taken place.  If it's a SAREX flight I'll try to update my
keps about an hour or so before I try to contact the shuttle unless I'm
sure that the shuttle hasn't maneuvered since my most recent update.

Philip Chien [M1959.05.31/31.145//KC4YER@amsat.org]