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

From: Thomas Fly (
Date: Thu Jul 01 2004 - 11:13:41 EDT

  • Next message: Moritz Heger: "Re: 2.2' of arc discrepancy between CalSKY and SkyMap ISS tracks"

    I just noticed that
    gives deltaT (as "Ephemeris Time" or Universal Time - UTC = 64.18) as well as
    UT1 - UTC, which it gives as 0.00 sec.
       Lift off time (UTC)  :  N/A
       Area (sq ft)  :  3000.0
       Drag Coefficient (Cd)  :   2.35
       Monthly MSFC 50% solar flux (F10.7-jansky)  :   111.5
       Monthly MSFC 50% earth geomagnetic index (Kp)  :   3.53
       ET - UTC  (sec) :   64.18
       UT1 - UTC (sec) :    0.00
    The actual current value for UT1 - UTC is about -0.47:
    If the TLEs are generated based upon an assumption of UT1 - UTC = 0, then it may
    be a mistake to attempt to "correct" for the difference, when computing a ground
    or sky track.
    Contrary to an earlier post, I in fact do apply deltaT in my computation of the
    positions of the planets, etc.  Also, last night I switched from the WGS72
    constants in my Java translation of the ancient SGP4 FORTRAN (URGENT ASSISTANCE
    NEEDED...) code to WGS84 constants.  Although I didn't do a character by
    character comparison, the transit reports I generated for myself appeared
    I give latitude & longitude to 4 decimal places, which works out to a resolution
    of about 10 meters.  The WGS84 ellipsoid increases the Earth's radius by 2
    meters, relative to the WGS72 ellipsoid, and increases the polar radius by about
    1.8 meters, so in fact there's very little difference between the two.
    I can't find any Geoid height code written in a MODERN (i.e., readable)
    language, so in order to adjust GTOPO30/SRTM30 MSL elevations to elevations
    above the WGS84 ellipsoid, I'm probably going to have to translate some more
    FORTRAN code:
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