# [amsat-bb] Re: TACSAT 3 Question

Paul Williamson kb5mu at amsat.org
Sat May 12 01:16:38 PDT 2012

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 others.

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