Re: Decaying Objects Questions

Alan Pickup (
Sun, 16 Mar 1997 16:32:36 +0000

In message <>, Ron Lee
<> writes
>>OIG modified
>>1 24316U 96054B   97075.22151452 1.29999999 -24605-4  47693-2 0   895
>>2 24316  25.2920  85.7328 1985484 273.3537  64.6432 11.93874537  9057
>I just used the elset provided by Bjoern to see what is visible here
>tonight (early 17 Mar).  After a low pass at 03:31, I get an error
>from Skymap about ndot or drag being too large.   I was not able to
>use the elsets in STSPLUS due to similar problems.
>Granted the object may not be in orbit after 331 UT (17 Mar), but
>I would like to observe it again if possible.

Unfortunately, I doubt if it will still be in orbit at that time - see
my other posting(s).

>Is there another program that will work with elset or should I wait
>for an elset with a lower drag term that is acceptable to Skymap?

One option is to take a SatEvo elset for the current rev and decrease
the ndot2 term to something your software will accept. Any time error
introduced by an incorrect ndot2 is probably small over the period
between the equator crossing and visibility from your site.

>This is an interesting object.  Alan's comment about the object being
>lower if earlier ( higher if later ) was right on.  Will have to
>visualize the orbit to understand the viewing changes.  But it 
>makes future obs of this nature easier to undertake since I will be 
>able to make adjustments for likely elset uncertainties.
>Is the earlier/lower, later/higher adjustment for near apogee objects
>a general rule, or might that be reversed under different conditions?

The critical issue here is that the period and apogee height was/is
changing rapidly from one rev to the next and that a time error for this
object implies a noticeable period and height error. For most other
objects, with very much lower drag and a longer interval between elset
epoch and observation, a time error may not imply a significant height
error. In such cases, a more important consideration is the rotation of
the orbital plane between the predited time and the time of appearance -
the track will be further east if early and further west if late by
(near enough) one degree of longitude per four minutes of time. The same
rotation effect is true, of course, for #24316 but an E-W shift of the
track for this object _from_your_vantage_point_ (well N of the track)
has less effect on its path against the stars than the change in its
height. On the other hand, for someone directly underneath the orbit and
near the equator, a change of height has no effect on the track but the
rotation of the orbital plane does.

 Alan Pickup | COSPAR site 2707:  55d53m48.7s N   3d11m51.2s W  156m asl
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