Subject: Re: Delta 2 R/B listed To: Frank@reednavigation.com 1) It was a secret mission, that is why neither decay date nor TLE are listed by SpaceTrack. I don't use other sources than SpaceTrack, H-A, prismnet.com/~mmccants/tles/classfd.zip and CalSky. mmcants has data for the satellites ("STSS Demo"). 2) Yes and no. The test is fairly accurate, but only when it has reduced its eccentricity to about .005 or below. This still corresponds to a difference of about 130 km from perigee to apogee, so it spends more than half its time with less than 1/3 the drag of perigee ! 0.2 at a mean radius of 8000 km would make the perigee at 6400 km ("instant death") and apogee at 9600 km ! Eccentric orbits have practically all drag at perigee, so the energy loss causes the apogee to decrease, with perigee nearly constant. For these orbits a better estimate is (16.55-Mean Motion)/2/ndot2 However, highly eccentric orbits, like with apogee over ~20000 km, can be severely disturbed by Moon (and Sun) gravity. I have seen Molniya orbits oscillate between 90 km and 700 km ("saved by the bell"), so this formula must also be used with caution. The remedy can be NPOE or Mike McCants' Int2/Int3 integrating programs, but I don't know how well the latter model the atmosphere. 3) There is no useful limit for a wide class of objects. For those with a consistent (and positive) drag term (ndot2) the common error estimate is 0.2 * ndot2 * (age days^2) * 1440/MeanMotion. But operational scientific, telecommunications and classified satellites make occasional or regular orbit changes, so errors can be up to ~10 min/day ! The attachments list predictions for IGS 5 rocket with three elsets ages 34, 84, 118 days (also "postdictions" gives six tests). With average ndot2=0.000036 and MM=15.035 I get corrresponding errors 0.8 4.9 9.6 mins, but the actual errors still vary a lot from these estimates. SpaceTrack Satellite Situation Report : 2009-052C 35939 US NO ELEMENTS AVAILABLE N/A DELTA 2 R/B Launched (09/25/2009) 2009-052B 35938 US NO ELEMENTS AVAILABLE N/A STSS DEMO 2 (USA 209) Launched (09/25/2009) 2009-052A 35937 US NO ELEMENTS AVAILABLE N/A STSS DEMO 1 (USA 208) Launched (09/25/2009) http://www.calsky.com responds to a search for 35938 or 35937, but not 35939 : Name: *USA 208 / STSS Demo SV-1* Launched: 25 Sep 2009 Military Sat.: *Together with SV-2 (USA 209) the satellite will monitor missile flights using infrared sensors. STSS Demo stands for Space Tracking and Surveillance System Demonstrator mission.satellites were built by Northrop Grumman for the US Missile Defense. Mass of each spacecraft is around 1000 kg.* Brightness: 6.0 mag (at 1000 km, 50% illuminated) 5.9 mag (at perigee, full illumination) Mean magnitude estimated from object size RCS: 6m2 (Radar cross section) USSPACECOM Nr: *35937* Internat. Designator: *2009-052* <http://www.calsky.com/cs.cgi/Satellites/1?&satid=2009-052>*A* Orbit: 1335 x 1354 km, 112.6min Inclination: 58.0° Age Elements: 2.5 days (Orbit from amateur sources) 2013/9/7 <Frank@reednavigation.com> Bjorn Gimle, you wrote: > "In this case, the Epoch (10043.72050712 = day 43 of 2010) is extremely > old. > When the date is more realistic, and the orbit is nearly circular, I use > 0.05 / ndot2 (0.02340000) as an estimate of remaining days in orbit (ie > about 2 days)" > > Thank you. I'm trying to understand two things: > 1) Why is this satellite still listed in a current set of TLE data? Is > this a heavens-above anomaly, or is it happening somewhere else? > 2) Would the calculation that you suggest combined with the orbital epoch > provide a good test to exclude a satellite? That is, if Epoch+0.05/ndot2 > yields a date before today's date, should it be safe to tag it as "probably > decayed" (for eccentricity<0.2 maybe)? And if the difference between that > calculated decay date and today's date is, let's say, greater than ten days > (a significantly larger time difference, in other words), is there any way > that it might not yet be decayed? I understand your point about higher > eccentricity orbits so might it be safer to use a test based on calculated > perigee? Maybe if the perigee is below 50km I can safely drop it from a > prediction list?? > 3) What is the maximum age that we should consider even remotely > acceptable for a TLE set for current predictions? > > -FER > > -------------- next part -------------- > An HTML attachment was scrubbed... > URL: > http://mailman.satobs.org/mailman/private/seesat-l/attachments/20130906/2f55f3de/attachment.html > _______________________________________________ > Seesat-l mailing list > http://mailman.satobs.org/mailman/listinfo/seesat-l > -- ---------------------------------------- Björn Gimle, COSPAR 5919 59.2576 N, 18.6172 E, 23 m Phone: +46 (0)8 571 43 312 Mobile: +46 (0) 704 385 486 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://mailman.satobs.org/mailman/private/seesat-l/attachments/20130907/08f53276/attachment.html _______________________________________________ Seesat-l mailing list http://mailman.satobs.org/mailman/listinfo/seesat-l
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