Re: epochs in the future?

Mike McCants (mikem@fc.net)
Tue, 11 Mar 1997 21:47:10 -0600 (CST)

Alan Pickup wrote:

>Of course someone (are you there Mike?) might suggest that some
>tles are "predicted", being based entirely on observations prior to the
>epoch. Does anyone have the answer?

Only NORAD knows.  :-)

Of course our (Alan and me) past discussions were concerned with
decaying objects.  The question was: does the elset of the decaying
object (almost always, but not always, at its south->north equator
crossing) represent observations made subsequent to that time
or could they possibly represent an extrapolation into the future?
I believe that our conclusion was that they are never extrapolations
into the future.  The decay of the Delta rocket near Austin was
an interesting case because it went through the NavSpaSur fence
in north Texas on its way down and the last elset had an amazingly
high mean motion:

1 23852U 96024B   97022.37676412  .99999999  24109-5  41939-3 0  4309
2 23852  96.5767 344.6986 0013684 128.9053 231.6615 16.61056074 42086

When I first saw this elset, the mean motion was so large that the
question was: was this a normal representation or could this be an
extrapolation into the future?  Now it is clear that it was a normal
case.

Vladimir Agapov wrote:

>Yes, some of TLEs are "predicted" indeed. I guess such TLEs produces for
>some object which are hard to track due to their orbits and probably
>due to SGP4/SDP4 theory that is using on tracking stations couldn't
>be use to propagate correctly orbital motion for such satellites.
>I know that such procedure is normal for 13901,19688,20413,21139,
>22653,23331,23632,23686,23842 etc.

But some of these are "ordinary" geosynchronous objects and I do not
believe that I have ever seen "forecasted" elsets for such an object.

>For example, currently you can found in OIG BBS
>database TLEs for 13901 with epoch 97073.26759740, for 23632 - 97072.95002340,
>for 20413 - 97071.56690600 despite of it is approx. 97071.0000000 now.

Yes, of the really high objects, most are lost, but the following
5 remain:

1 13901U 83020A   97073.26759740 -.00002521  00000-0  00000+0 0  7610
2 13901  44.6340  16.4330 8621978 323.9560  26.0900  0.24286200  2084

1 14069U 83041C   97059.79946914 -.00000175  00000-0  10000-3 0  7733
2 14069  14.6030  44.2924 1682813 232.6849 110.8604  0.84341462  8846

1 20413U 83020D   97071.56690600 -.00002293  00000-0  00000+0 0  9614
2 20413  40.4400  22.3730 8757140 318.3160 335.3250  0.24727000  2099

1 23414U 94079B   97069.32196990 -.00000386  00000-0  00000+0 0  1879
2 23414  29.5800 346.9640 8907593 271.3370  11.7880  0.50008300  5833

1 23632U 95039A   97072.95002340 -.00000242  00000-0  00000+0 0  1204
2 23632  70.8580 248.0170 7561561 327.9680  42.3720  0.26415200  1564

Since the lunar/solar perturbations are so large, NORAD regularly
generates "forecasted" elsets for objects like these so that the
appropriate sensor will have a better chance to acquire the object.

But if that sensor fails to acquire it, I don't think that NORAD
will generate another extrapolation.  If this conjecture is true,
then the forecasted elset is never more than a fraction of an
orbital revolution in the future from an actual observation.

Mike McCants
mikem@fc.net