Re: ISS Lunar Transit Recorded from Livermore, CA

From: Thomas Fly (tfly@alumni.caltech.edu)
Date: Sun Feb 01 2004 - 14:12:31 EST

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    > The TLE I used is the one from the NASA spaceflight website which I got
    > about 2 hours before event time:
    >
    > ISS
    > 1 25544U 98067A   04030.38641742   .0002000  00000-0  20000-3 0  9012
    > 2 25544  51.6275 312.2703 0007708 107.1260 253.0743 15.66615930 16547
    
    That's the Mission Control Center TLE that my prediction was based upon:
    
    http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/orbit/ISS/SVPOST.html
    
    There is the claim, at least, that:
    
    "You also have access to a Mission Control Center, or MCC, ephemeris during
    space shuttle missions and for the International Space Station. An ephemeris is
    simply a collection of state vectors in chronological order generated in the
    MCC. The state vectors that are generated in the MCC are far and above the most
    accurate available, and their use is highly recommended."
    http://spaceflight.nasa.gov/realdata/sightings/SSapplications/Post/JavaSSOP/SSOP_Help/Vector.html
    
    According to Richard Clark of NOAO (who's monitoring 7 sites!):
    
    "In the absence of maneuvers those predicted TLEs should be considerably
    better than an SGP4 based prediction. Their models are more elaborate in
    both a more complete Earth gravity representation, and better atmosphere
    models. They are a numerical integration instead of an analytic solution.
    As long as the mission keeps to the timeline they're pretty good. But I've
    only used them in the context of shuttle missions. I always forget that
    they also do this for ISS. I've never ground truthed them (in the
    absence of maneuvers) with what eventually gets release from OIG."
    
    In other words- as had been my suspicion- the MCC does a numerical integration,
    then "dumbs the results down" to SGP4 model TLEs, for programs like mine to use.
    
    > Interestingly enough, there was a TLE from heaven's above which was even
    > closer to event time.
    >
    > ISS_HA
    > 1 25544U 98067A   04030.84074947  .00015969  00000-0  15407-3 0  1037
    > 2 25544  51.6300 309.9574 0007833 109.2562 295.0015 15.66638907296617
    
    Most likely, that came from OIG - NASA's Orbital Information Group:
    http://oig1.gsfc.nasa.gov/scripts/foxweb.exe/app01?
    
    > The ISS path from this TLE was not as accurate as the
    > one from NASA (although the time of the transit was closer -- 21:43:40.7 vs
    > 21:43:40.2 -- My estimate based on WWV was that the transit occurred very
    > close to 21:43:40.7):
    
    That seems to confirm my own impression- that the MCC ephemeris tends to give
    better positional data, while the OIG TLE (which is evidently based upon radar
    measurements made several times each day) will give better timing accuracy.
    
    It seems to be a bit of a paradox, since the timing should have a pronounced
    effect upon the position of the transit path!
    http://www.satobs.org/seesat/Jan-2004/0263.html
    
    Arnold Barmettler- the creator & operator of CalSKY- uses the MCC ephemeris only
    for "long range" predictions, and the OIG data for near-term predictions.
    
    I hope to do some after-the-fact comparisons, using photos such as yours, in the
    next day or so, to see if I can finally resolve the issue of MCC vs. OIG
    accuracy, in regard to both timing & position.
    
    Tom
    
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