[amsat-bb] ARISSat-1 (37772) decay

DeYoung James deyoung_james at yahoo.com
Tue Nov 15 09:29:31 PST 2011

First, thank you Mineo for reading the AMSAT Journal and making several of my papers available
on your web site.
My AMSAT Journal
paper published in the March/April 2011 issue is actually still fairly valid
for the scenarios shown in the paper. The solar flux has turned out to be
somewhat higher than was used/predicted in the paper. This has caused the
atmospheric densities to be higher which results in higher decay rates. When I
wrote the paper I had this nagging feeling that stopping the 
release height
scenarios at 370-km was not going to be high enough. We are very fortunate that 
the ISS was boosted to such a height before release of ARISSat-1 and not after
There is a
valuable lesson, I think, to be made with respect to predicting satellite decay
dates far into the future. The future state of the atmosphere, i.e. the
atmospheric density that the satellite will pass through is poorly predictable
in the long-term, say starting greater than a week or two into the future.
Predictions of satellite decay dates months in the future should be
evaluated with the understanding that your date of prediction errors may be
large. The errors 
are due to the future uncertainties of the orbital path which
grow quickly with time in a prediction. The atmospheric density is not the only
source of error. Your orbit model, the integrator, and the accounting of the gravitational and
drag forces among others will affect your results.
Predictions of
satellite decay dates are not do-and-forget. The general process is to make a
prediction, get new measured observations of the height in the future, and at
some point re-do your prediction when the errors become significant to you. 
With that all
said here is my current prediction using the same tools used in the AMSAT J.
paper and produced as of 2011 November 13th. The decay of ARISSat-1 (37772)
will happen nominally on 2012 January 30th with a 10% rule-of-thumb error
allowance of 18 days around this date. The 
errors may be larger than the rule-of-thumb indicates!
Jim, N8OQ

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