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# Re: Predicting Final Orbit

• Subject: [amsat-bb] Re: Predicting Final Orbit
• From: Clive Wallis <clivew@xxxxxxxxxxxx>
• Date: Thu, 27 Apr 2000 09:53:04 +0100

```> 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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