[amsat-bb] Re: TACSAT 3 Question

Sat May 12 05:41:53 PDT 2012


As usual, Paul has covered the important points.  I would suggest, if you
are really interested, getting an account on Space Track and looking at the
archived values for TLE of decayed satellites.  Looking at the decay rate
will clearly give you an indication to within a day or so.  Normally it is a
value of something like .00000X, but in the final days that number increases
rapidly.  Empirically, when it gets close .01 the end is near, and the last
data always shows something like the .15 Paul mentioned.  Alternatively, if
you are willing to look at two Keps, you can calculate the actual descent
rate.  I recall ARISSat-1 was descending at something like 20km/orbit at the
end.  I expect that if you pick 5-10 satellites and study the results, more
detailed criteria will suggest themselves.  There is no doubt a good AMSAT
Journal article here.  

>From the ARISSat-1 Chicken Little contest, I learned that there are already
many predictive models, but it should be relatively easy to decide from
current Keps when the satellite is down.  For my own use, I usually know
from reports the status of any satellites which interest me.  If a tracking
program throws an exception, I check Space Track for recently decayed
satellites, and here to see if there was a glitch in the Keps.

73s and have fun,



-----Original Message-----
From: amsat-bb-bounces at amsat.org [mailto:amsat-bb-bounces at amsat.org] On
Behalf Of Paul Williamson
Sent: Saturday, May 12, 2012 3:17 AM
To: Thomas Doyle
Cc: AMSAT-BB at amsat.org
Subject: [amsat-bb] Re: TACSAT 3 Question

On May 11, 2012, at 12:59 PM, Thomas Doyle wrote:
> There must be a better way to detect re-entry than simply than throwing an
> unhandled exception.
> You guys seem to be the experts - any suggestions on how to do it ?
> It seems to be related to the BSTAR drag value. Some say when the BSTAR
> drag value gets "too high" the sat has re-entered or is about to re-enter.

I don't have access to SatPC32 internals, but most amateur tracking programs
ignore BSTAR completely. BSTAR doesn't even appear in the so-called AMSAT
format element sets. It might not be the element you're looking for.

Mean motion, the number of revolutions the satellite makes per day, is a
nice simple value to look at to detect re-entry, but you can't just look at
the mean motion in the element set. That value, like all the other values in
the element set, is a snapshot as of the epoch time. Mean motion is actually
changing all the time, and we model that change rather simplistically as
linear. The value called "Decay rate" in the AMSAT format is basically the
rate of change of the mean motion, revolutions per day per day. To compute
the value of mean motion at the present (or target) time, you have to
multiply the decay rate by the time (in days) elapsed since the epoch time
of the element set, and add that to the mean motion from the element set.

The threshold to compare against would be the mean motion (number of
revolutions per day) of a satellite in circular orbit on the threshold of
re-entry. This would be somewhere between 16 revs/day (the familiar
90-minute orbit approximation) and about 17 revs/day (the mean motion a
satellite would have if orbiting at sea level).

For TACSAT-3 as shown in the current Keplerian elements bulletin, today (day
133 of 2012):

16.45 + (133-121.7) * 0.152 = 18.17

a clearly subterranean mean motion. If you compare TACSAT-3's values to
those of other satellites in the bulletin, you'll see that both mean motion
(16.45) and decay rate (0.15) are outliers, much higher than any of the

It might or might not make sense to add a filter like that to a program like
NASAWASH. The element set is not invalid in itself. It's just invalid if
propagated to the present time.

I know I've glossed over a few details in the above analysis. I'm sure one
of the experts who actually understands orbital mechanics will correct me if
I've fouled it up entirely.

73  -Paul
kb5mu at amsat.org

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