RE: Request for NASA info

From: Ted Molczan (
Date: Mon Jun 11 2007 - 10:33:35 EDT

  • Next message: Vossinakis Andreas: "Re: Request for NASA info"

    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:
    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
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