Re: USA 133r Obs, USA 69r Question

Bjoern Gimle (b_gimle@algonet.se)
Sat, 3 Jan 1998 11:39:10 +0100

>
>Since USA 69r is running late, how would I adjust my elset based
>on these observations?
>
If the observation was on 98001 = 97366 and the elset was
Lacrosse 2 Rk
1 21148U 91017B   97347.25322987  .00022500  00000-0  32388-3 0    01
2 21148  67.9780  43.0088 0067000 183.4440 176.5560 15.53105488    05
i.e. age = 19 days, the effect of the drag term is 19*19*.00022500
=  0.081225 orbits, or about 7.5 minutes.

So, if the rocket was 2.5 minutes late (5 min. drag effect), you should
reduce the drag terms by 1/3, or new ndot2=.00015000 and new B*=21592-3
(assuming the relation between B* and ndot2 was correct) :
Lacrosse 2 Rk
1 21148U 91017B   97347.25322987  .00015000  00000-0  21592-3 0    01
2 21148  67.9780  43.0088 0067000 183.4440 176.5560 15.53105488    05

Check the new elset with your prediction program. If you have QuickSat,
this checks ndot2, with SkyMap you can check both; most other programs
use B* (SGP4 model)

If you don't intend to post the improved elset, just get better predictions
for a few days, you can add to the error to the Epoch. 0.001 = 1.44 min.
so this would give  97347.25496598, instead of correcting ndot2/B*.

Finally, for an old elset for an object with a very small drag, it is wiser
to change MM. Compute the error in no. of orbits ( 2.5*15.53/1440 here)
and divide by the age (19) to get the decrease in MM.

These changes also affect the check digit at the end of the elset line,
but few programs complain. You can run it through Orbitel, Adj2line,
Vec2tle or som other elset manipulation program. Or compute by hand:
In the example, I replaced 225 and 32388 by 150 and 21592.
2+2+5+3+2+3+8+8=33 1+5+0+2+1+5+9+2=25. So the check digit (1) should be
decreased by 8 (increased by 2) to 3 

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