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

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Sat Jun 26 2004 - 18:26:12 EDT

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    Hi Arnold,
    
    > that's too much of an error.
    
    My point exactly.  Also, I'm not sure that I understand it, unless it's just
    that I never tried before to compare my results with yours, to this level of
    accuracy.  Particularly interesting is the fact that your results suggest that
    the MCC TLE was extremely accurate in this case.
    
    > deltaT = ET-UT = 64.5 sec
    
    I calculate deltaT in my WorldView program, but at present, I don't actually do
    anything with it.  Rob Matson's SkyMap program gives a value of 67.74 sec for
    deltaT.
    
    deltaT, of course, should be used to adjust UTC in the computation of the
    position of the Sun & Moon, etc., whereas (unadjusted) UTC should be used for
    the SGP4 computation.
    
    > 208m above WGS84 ellipsoid (164m above sea level)
    
    There's not a huge amount of difference between 164 and 208 meters (44 meters *
    cos(63.2)= 20 meters or ~ 1/6 Venus perpendicular to the line of sight), but 208
    meters was what I used in SkyMap.
    
    > Are you also using topocentic coordinates for Sun and Venus?  I'm currently
    using DE404, and may include DE406 soon (difference in
    position I think will be smaller than 1")...
    
    My WorldView program computes the ground track from the ECI position of the ISS
    (from the SGP4 computation) and the ECI position of Venus (in this case).  For
    the computation of the positions astronomical bodies, I use Mark Huss' Java
    AstroLib, which is a translation from Bill Gray's Project Pluto C code, and is
    based upon VSOP.  In a test of accuracy based upon the most recent total eclipse
    of the Sun by the Moon, I have high confidence in its results.
    
    I don't know the internal workings of SkyMap, but SkyMap generally confirms my
    computed ground track to high accuracy, given the same TLE and my computed
    observer site & time.
    
    At the very least, the relative positions of Venus & the Sun on this date are
    known to very high level of accuracy, so it shouldn't be difficult to determine
    the source of the discrepancy.
    
    Tom
    
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