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Re: Predicting Final Orbit



> Is there any program around that can predict using 'current' keps 
when a         > satellite will decay into earth's orbit?

AFAIK the simple answer is no.  I spent a lot of time studying this 
during the decay of OSCAR-9 and Salyut-7, when we had 'Chicken 
Little' competitions to predict the date of re-entry.  

What I did was to correlate the decay of the orbit with the density 
of the of the upper atmosphere using the existing data.  Then I would 
predict how the decay would vary until re-entry.  The density of the 
upper atmosphere varies with height, solar flux, and time of year.  
The decay of a satellite also varies with it's mass and geometry.  
Small satellites appear to decay much faster than larger ones. 

Although I had some success with this approach, the predicted 
re-entry date did vary considerable, as each new value of mean motion 
was processed. In other words the results didn't converge accurately 
towards the final re-entry date, especially for times of more than a 
few weeks before re-entry.

Even if the method of processing the data is 100% correct, you still 
have to predict the solar flux.  However, as the satellite height 
decreases, the solar flux has less effect on the density of the atmosphere.

The formula that John mentioned was widely used at the time, but gave 
very unreliable results, especially at more than a few days before 
re-entry.  The derivation of this formula was never revealed.  I 
suspect that it related to a particular satellite, under particular 
conditions, and may have been the final stage of lenghty calculations 
for that satellite.

IIRC the SATEVO program scans a file of Keplerian elements, and picks 
those with high mean motion, ie. those near to re-entry.

-- 
73     

         Clive    G3CWV

         Hitchin, North Hertfordshire, UK.       

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