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Here's an image of the Hubble Space Telescope passing through
Sagittarius Friday evening.  Same imaging conditions:  0.5 second
exposure, handheld.

It was a lucky shot, the best of about a dozen.  Still, with enough
exposures, lucky things happen.

As for internal astrometric consistency, I think it's better than the
6 arcsec I mentioned before.  I have a few images of the field around
Polaris with the camera sitting on a tripod (15 second exposures).
Hundreds of stars are visible, so astrometry.net has sources from all
over the field to work with.  After telling it to use SIP polynomials
of order 4 (up from the default of 2), it now says 1.7 arcsec rms.  I
think it's now taking out the last remaining bits of barrel distortion
from the lens system.  (I don't think it's cheating to use a tripod
shot to cook up a lens-distortion model to be used with handheld
shots, which are much sparser.)

So, bottom line:  suppose we have a system capable of 3 arcsecond
accuracy using, say, half a dozen selected handheld photos.  That's
the equivalent of 5 or 10 meters on the ground for objects in LEO and
a couple of hundred meters for objects in MEO.  The problem seems to
be that the published orbits (and the orbit models themselves) are not
nearly that good.  The Spacetrack report says that TLEs degrade to
hundreds or even thousands of meters before the next update, and
that's with objects that get frequent updates.  (The reference-frame
situation is also confusing.)  I didn't see data on how much error is
typically cross-track and how much along-track.

If we abandon the SGP and SDP models, estimate our own state vectors
for all objects of interest, and do full-bore numerical integration
with a current gravity model, what is the inherent unpredictability of
typical LEO objects assuming no maneuvers?  If the game is to have a
system usable onboard ship, autonomously, for a duration of several
weeks or months, then we need to pick objects for which this
predictability is better than our accuracy goal.

Cheers,
Peter

ps: this photo was taken at a grassy field with a better view to the
south:  37.43326 N 122.18227 W, ellipsoidal height 1 meter.

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