Re: 2.2' of arc discrepancy between CalSKY and SkyMap ISS tracks

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Tue Jun 29 2004 - 23:54:54 EDT

  • Next message: Ted Molczan: "TJM obs of 2004 Jun 30 UTC"

    I received the following information from Curtis Haase, which I've summarized
    below.
    
    When I get a little time, I'll burrow into the internal workings of my own
    transit track computation and dig out the critical intermediate results (e.g.,
    the ECI position of the ISS at 10:09:17 UTC), and post them for anyone who may
    find them useful, or can find any significant error in them.
    
    In the particular case of the simultaneous ISS / Venus transit, as observed by
    Tomás Maruska, failing to use UT1 in place of UTC would not make much difference
    in the predicted track- which was essentially west-to-east across the disk of
    the Sun- but instead would manifest itself as a small timing error (were the
    TLE, used for the prediction, itself accurate).
    
    ------------------------------------------------
    
    In the transformation from an earth-fixed reference frame to a celestial
    reference frame, the proper "time" to use is UT1.
    (see page 162 of EXPLANATORY SUPPLEMENT TO THE ASTRONOMICAL ALMANAC, ed. by P.
    Seidelmann, 1992 -- this is the authoritative
    reference. The Astrodynamics text by Vallado is also very useful).
    
    The current value of UT1-UTC can be found in the IERS Bulletin A from the USNO:
    ftp://maia.usno.navy.mil/ser7/ser7.dat
    The current value of UT1-UTC is about -0.47 sec, so as you guessed, this makes a
    considerable difference in the position of the observer in a space-based frame.
    
    For an online reference on the coordinate transformations see
    http://maia.usno.navy.mil/conv2003.html
    Chapter 5 has the discussion of the transforms.
    
    Bottom line: for precise work involving earth-based observer, satellite, and
    planet, you should use:
    
    -- UTC for the satellite state vector
    
    -- UT1 to calculate the earth's rotation angle. Specifically,
       use UT1 to calculate Greenwhich Mean Sidereal Time (GMST);
       then add the equation of the equinoxes (EQEQ) to GMST to
       get Greenwhich Apparent Sidereal Time (GAST):
       GAST = GMST + EQEQ
       GAST is the "angle from Greenwhich" that is used the coordinate
       transformation.
    
    -- Dynamical time (i.e., apply the "delta T") for planetary
       positions.
    
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