Apparent difference in reference systems used by OIG and MCC TLEs

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Mon Aug 09 2004 - 15:25:42 EDT

  • Next message: Leo Barhorst: "LB obs 2004 Aug 08"

    Ted Molczan had noted some while back that:
    
    "the OIG elset closest in epoch to that of the MCC had a slightly lower RAAN...
    the RAAN of subsequent OIG elsets moved toward that of the MCC elset."
    http://www.satobs.org/seesat/Jun-2004/0258.html
    
    Tomás Maruska's extraordinary video of the simultaneous ISS / Venus transit
    provided a basis for a detailed analysis of predictions based upon these 2
    brands of TLEs - it was particularly fortunate that the epoch of one OIG TLE
    represented a time delay of only about one minute, relative the transit itself.
    http://www.satobs.org/seesat/Jun-2004/0313.html
    (subsequent, more careful measurement revealed the discrepancy to be about 1.64'
    of arc, rather than 2.2')
    
    I did a significant amount of analysis of Tomás' photo here:
    http://iss-transit.sourceforge.net/MissionAccomplished.html
    
    Using Rob Matson's SkyMap 6.6 (which confirms my current WorldView ground
    track), the OIG TLE "ISSd," whose epoch was nearly coincident with the transit
    time, predicted a path that was nearly dead-center from Tomás' GPS-measured
    position; the predicted ground-path center line corresponding to that TLE was
    about 78 meters north of the actual ground path.
    
    Not coincidentally, it turns out, CalSKY's prediction, using the appropriate MCC
    TLE, is virtually identical to that OIG path:
    http://eclipse.astronomie.info/transit/venus/isstransit/historicalimage.html
    
    The most frustrating aspect of developing my WorldView program has been
    collecting all the bits and pieces of arcane knowledge that are required to
    accurately complete the puzzle.  From
    http://celestrak.com/columns/v04n05/index.shtml
    (and reiterated in
    http://www.stk.com/pdf/STKandSGP4/STKandSGP4.pdf )
    
    "What is the reference frame of the resulting coordinates?
    
    "This question is a bit more technical than most we have covered. To be precise,
    the reference frame of the Earth-centered inertial (ECI) coordinates produced by
    the SGP4/SDP4 orbital model is true equator, mean equinox (TEME) of epoch.
    
    "In layman's terms, this simply means that the Cartesian coordinates produced by
    the SGP4/SDP4 model have their Z-axis aligned with the true (instantaneous)
    North pole and the X-axis aligned with the mean direction of the vernal equinox
    (accounting for precession but not nutation)."
    
    In fact, it appears that OIG TLEs assume True Equator / True Equinox, while MCC
    TLEs assume Mean Equator / Mean Equinox.
    
    Taking this into consideration, and given the small error of the resulting
    predictions, then even the wandering of the earths' axis becomes significant
    (i.e., true vs. mean equator):
    http://www.iers.org/iers/earth/rotation/polmot/figure1.html
    (1" of arc in latitude is 1/60th nautical mile, or about 100 feet)
    
    As an aside- now that WAAS-enabled GPS units compute positions allegedly
    accurate to a few meters- there is a question as to where the earth's poles are
    assumed to be!
    http://www.defenselink.mil/photos/Feb1996/960120-N-1259S-002.html
    http://www.npr.org/news/healthsci/antarctica/southpole.gallery10.html
    http://www.confluence.org/photo.php?visitid=808&pic=ALL
    
    Another consideration is that the MCC TLE page
    http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html
    assumes that the difference between UT1 - UTC = 0.00 seconds
    (i.e., UTC doesn't differ from International Atomic Time:
    http://tycho.usno.navy.mil/systime.html )
    
    One thing that's been lacking, in the relatively few good opportunities provided
    by Tomás, John Locker, and a handful of others* to fully facilitate the analysis
    of transits is precision timing.  But Christmas isn't far off...
    http://www.geocities.com/kiwi_36_nz/kiwi_osd/kiwi_osd.htm
    
    * not including myself!... evidently, I'm permanently jinxed by Murphy's Law
    when it comes to capturing the ISS on video!
    
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