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

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Thu Jul 01 2004 - 16:27:21 EDT

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    Moritz Heger wrote:
    
    > These constants should not be changed because the OIG TLEs are generated to
    give best calculation results with the WGS72 constants.
    
    Thanks, though I use MCC generated TLEs for my predictions- though in fact, they
    also may be optimized for the WGS72 ellipsoid (it's a bit annoying to anyone who
    claims to have an ounce of scientific integrity that these details are either
    not documented, or not easily uncovered!).
    
    In fact, that concern was largely why I resisted making any such change.  My
    initial instinct- which is reinforced by your information- was to use WGS72
    constants in the SGP4 computation itself, but WGS84 constants when converting
    the ECI value for the Earth intersection point to a latitude/longitude/elevation
    (although, as a practical matter, use of the WGS84 ellipsoid appears basically
    equivalent to adding 2 meters to the elevation, compared to using the WGS72
    ellipsoid).
    
    The bottom line here is that there appears to be no practical difference between
    the two, for this application, though for "mathematically aesthetic" reasons,
    the approach I just described is appealing.
    
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