Re: epochs in the future?

jamesv@softcom.net
Tue, 11 Mar 1997 22:30:30 +0000 

Mike McCants wrote:
 
> 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.

> Only NORAD knows.  :-)

Not exactly... there's some unclassified stuff published on what they 
do.

Tracking stations measure the early/late error and the off-track 
(plane) error compared to predictions made from their own ephemeris 
software, not elsets.  If the errors are acceptable, the satellite is 
considered "correlated."  If the errors are too large or the object 
is completely unknown, it is considered an uncorrelated target (UCT).

Once a UCT is found, ground stations attempt to obtain tracking data 
for a minimum of 5.5 percent of one orbital period.  This is their 
rule of thumb for the minimum data needed to generate a good set of 
elements.

Correlated objects are always observed from multiple ground stations 
to sample different parts of the orbit.  The observed track is far 
less than is done for UCT's.  The mulitiple observations from 
multiple stations are used to correct the previous element set using 
differential corrections.  These working 'elements' are state vectors 
plus perturbation terms.  Two-line mean elsets are made after the 
state vector elements are produced.

The only exception to the use of multiple ground stations is for 
objects near decay.  Then they use what observations they can get 
(the Austin landfall object being an example).  

For most objects in the catalog there is no correlation between the 
2-line elset epoch and any given ground observation.  In a sense you 
could say that most elsets are 'predicted' because the elset position 
at epoch is never the raw observed position.  The near decay objects 
appear to be highly correlated to ground observations only because 
one or two stations are contributing to the analysis.


Jim Varney
jamesv@softcom.net