In general the effect of drag is given by the ndot2 value (here -.00000134) * age(days)**2 orbits Today, 10 days after day 04067 this would be .000134 orbits One orbit is 1440/14.1268 minutes, or 103 minutes. So the net effect would be 0.0138 minutes, or less than a second. It would take 200 days for this elset to reach 5 minutes in total, and the error might be 20% or so. If you load the elset in Rob Matson's SkyMap, it computes this approximation. However, this object is one of a few special cases, where the mean motion of 14.12 causes a resonance with the Earth's gravitational anomalies. For normal objects, where drag is the overwhelming effect, the ndot2 value is positive (and the rule-of-thumb is usually reasonable), but for Cosmos 1833 I have seen ndot2 values as low as -.00000320, though the average since 2000 has been about +0.00000080 Cosmos 1833 1 17589U 87027A 04067.89627470 -.00000134 00000-0 -46528-4 0 8510 2 17589 70.9095 294.3979 0003131 111.1350 249.0106 14.12684325875418 ----- Original Message ----- > Hi Geoff, > > The differance can be easily explaned. > How old were your TLE's? > When they are a bit outdated a difference of several minutes is quite well > possible > depending on how high the orbit of the sat is. > Lower sats are more effected by the earth's atmosphere than higher ones. ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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