[amsat-bb] Re: TACSAT 3 Question
tomdoyle1948 at gmail.com
Sat May 12 10:19:54 PDT 2012
One of the great things about AMSAT is the willingness of people like Alan
to share information and answer questions. I followed Alan's chicken-little
contest and recall someone had done some graphs related to the decay. I
searching for the postings but could not find them or remember who posted
These days remembering what I had for breakfast is an occasion worthy of
If anyone is willing to share their chicken-little contest secrets I would
be most interested
73 W9KE tom...
On Sat, May 12, 2012 at 7:41 AM, Alan P. Biddle <APBIDDLE at united.net> wrote:
> 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
> value of something like .00000X, but in the final days that number
> 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
> 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
> > 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
> I don't have access to SatPC32 internals, but most amateur tracking
> 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
> the element set, is a snapshot as of the epoch time. Mean motion is
> 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
> 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
> 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
> I've fouled it up entirely.
> 73 -Paul
> kb5mu at amsat.org
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