Re: OIG TLE harangue

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Fri Jun 18 2004 - 11:16:01 EDT

  • Next message: Thomas Fly: "Re: OIG TLE harangue"

    > I update the satellite and Sun positions every 1/4th of a second in the last
    10 seconds.
    
    You only need to compute the ISS position at the reference points (i.e., the 1
    second tick marks).
    
    The Sun only moves about 1 in the sky each day, or 1' of arc in 24 minutes, so
    updating the Sun's position every 30 seconds gives about 1" of accuracy!
    
    The SGP4 computation wants UTC, which is basically Greenwich sundial time, while
    the computation of the position of astronomical bodies wants TT (or Terrestrial
    Time - which isn't particularly descriptive, as nomenclature goes), which is
    atomic clock time.
    
    delta-T is the difference between TT and UTC (or technically UT, Uniform Time).
    Because the Earth's rotational period has been slowing down in recent years,
    this difference is currently about 68 seconds, so TT is ahead of UT.
    
    I doubt that's the source of the problem, however.  Perhaps this evening, I can
    send you some intermediate results, such as ECI positions for the ISS, Sun, and
    my observation point at transit time.
    
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