Andreas Vossinakis wrote: > Hi, in these pages with the tle, I guess we must use the tle > which is near the time we want to observe. It is a bit more complicated than that, but not too complicated. Here is the rule for selecting trajectory data on the NASA pages in question: http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html http://spaceflight1.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/SHUTTLE/SVPOST.html To make accurate predictions for your specific time (or period) of interest, ignore the epoch of the TLEs, and instead find the coasting arc that includes your time (or period) of interest, and use the TLE for that coasting arc. The remaining discussion explains the need to follow this rule. > What is the meaning of these lines?: > > The mean element set is posted at the UTC for which position is > just north of the next ascending node relative to the above > vector time The trajectory data on the NASA pages in question is presented for what are called Coasting Arcs, i.e. periods when the spacecraft is not manoeuvring. The state vectors on the NASA pages in question correspond to the time of the start of the coasting arc (aka the vector time); however, in accordance with common practice, the epoch of TLEs correspond to an ascending node, i.e. time of passage over the equator, north-bound - in this case the first ascending node following the vector time. This practice can lead to confusion, especially when a manoeuvre will occur prior to the spacecraft reaching the first ascending node after the vector time, as is the case with the circularization burn that occurs about half an hour after MECO (main engine cut-off), and during the final stage of rendezvous, when there are frequent small manoeuvres. > I used yesterday the tle near the time of docking but the > program (orbitron) showed the shuttle way ahead of ISS. > What did I do wrong? I am not familiar with Orbitron, and I do not have the actual data that you used, so I can only guess. Shuttles catch up with ISS by orbiting behind it, in a lower, therefore, faster orbit. As they move closer to ISS, they make manoeuvres to raise their orbit to match that of ISS. Since your prediction showed the shuttle ahead of ISS, my guess is that you selected a shuttle TLE that was close in epoch to the planned time of docking, but that belonged to an earlier coasting arc, when the shuttle was still in a lower orbit. Therefore, by the time of docking, that elset would have been invalidated by one of more shuttle rendezvous manoeuvres. Ted Molczan ------------------------------------------------------------------------- Subscribe/Unsubscribe info, Frequently Asked Questions, SeeSat-L archive: http://www.satobs.org/seesat/seesatindex.html
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