Re: aircraft vs. satellite

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Mon, 5 Jan 1998 17:25:18 -0500, Stupendous Man <richmond@p674p06.isc.rit.edu> wrote:
*>
*>  Aha -- now I understand.  When I looked at the individual
*>"dots" in the trail, I found that their JD times differered
*>by a few minutes.  That's why I concluded they were due to a
*>satellite -- an aircraft would move more than 3 degrees in a
*>single minute.
*>
*>   But, thanks to Nick's message, I understand -- all the 
*>points came from a single image, and were really made at
*>the same time ... but the TASS software _gave them times
*>which were several minutes apart_ (incorrectly).  
*>The software said, "Oh, here's a point source -- it must
*>be a star, and since it's at this _position_, it must have
*>gone through my field of view at this _time_."  For true
*>stars, position _is_ related to time.  But for airplane
*>trails, it's not.
*>
*>  Okay, I get it now.
*>
*>  Is this good or bad?  I think neither -- it's just the way
*>things work out.  From now on, if we see a trail of dots
*>which moves diagonally across a field, and the dots are each
*>a few minutes apart, we'll know it was probably an aircraft.
*>
*>  And, has Nick points out, especially if there are several 
*>parallel tracks :-)
*>
*>                               Michael

I'm interested in any TASS comments about apparently moving objects, due
to my interest in asteroids. Seems to me that "trail of dots" begs
the question. If all we have is the star list data, how do we as
end users determine a difference between a "trail of dots" and an
asterism of stars in an apparent line? My casual recollection is that
there is a coefficient or two of "shape" in the database: would these
parameters provide useful information? If all the stars in a "line" are
"sausages" in the direction of the line.....Or, do some of the star
extraction programs remove "trails" beforehand? At typical asteroid rates of
tens of arc minutes an hour, there would be no trails of solar system
objects in TASS cameras. And we are not tracking satellites, so eliminating
trails is probably OK.

In any case, your observations above must be documented, so they don't have
to be rediscovered every six months. Whatever "documentation" means....

Herb Johnson


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Herbert R. Johnson                      voice/FAX 609-771-1503 day/nite
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