One of the great things about AMSAT is the willingness of people like Alan and Paul 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 tried searching for the postings but could not find them or remember who posted them. These days remembering what I had for breakfast is an occasion worthy of celebration. If anyone is willing to share their chicken-little contest secrets I would be most interested and appreciative.
73 W9KE tom... On Sat, May 12, 2012 at 7:41 AM, Alan P. Biddle <apbid...@united.net> wrote: > Tom, > > 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, > > Alan > WA4SCA > > > > -----Original Message----- > From: amsat-bb-boun...@amsat.org [mailto:amsat-bb-boun...@amsat.org] On > Behalf Of Paul Williamson > Sent: Saturday, May 12, 2012 3:17 AM > To: Thomas Doyle > Cc: AMSAT-BB@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 > 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 > kb...@amsat.org > > > _______________________________________________ > Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. > Not an AMSAT-NA member? Join now to support the amateur satellite program! > Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb > > > > -- Sent from my computer. tom ... _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
