Re(2): Question on #12986 Searches

Bjoern Gimle (
Thu, 15 May 1997 20:56:21 +0200

>Ron wrote:
>>This decaying object is in a 62 degree inclination orbit.
>>The morning passes are moving to the south-southeast.  If the 
>>object is early should I assume that it will be west of the 
>>predicted path.  Also I assume that 1 degree per 4 minutes early 
>>is the correct displacement.
I answered, without paying attention to "early".

>Yes, this is correct. It is very slightly over 1 degree, since the
>orbit progresses westward 1.2 + 0.98 degrees/day.
The position of the orbit plane in space is affected very little by
a four minute change. But since Earth rotates to the East below it,
the satellite will appear one degree further East if four minutes early.

My mental arithmetic on orbital heights and position vs. errors in decay
rate is not necessary - just take the latest elset close to reality, and
(change the Bstar/ndot2 values to make predictions agree with the nearest
SatEvo orbit, and then) change the Bstar/ndot2 values +/- 10% and/or 20%
and see what the effect on predictions is.

This will compensate for most of the effects of errors in orbit and time,
and show the displacement of the track in az and el, or among the stars,
not only the EW motion.

For Thursday morning UT, a change of -11% on ndot2 and B* agreed quite well
with Alan's elset for both SGP and SGP4/SDP4, but there was about 0.2
degrees change in position, because the original and adjusted OIG sets
use SDP4 for predictions, but Alan's falls below the 225 min. limit !!

At 3:34 UT in az. 347, the track was pretty stable among the stars,
changing at about 3:50 to lower declinations again.
In Az/El, the elevation decreased smothly by 1 degree/20 minutes

For Az/El on Friday you will find a quite stable position near az 12,
el. 51.

The -11% elset gives about 03:14 UT there, Alan's 135.24 elset 02:47,
much closer to the original elset's 02:31. (-21% gives 03:54)

I haven't yet picked up any new SatEvo evolutions, or OIG elsets
since 0500 UT today, when there were none.


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