Re: Which is correct?

From: Mike McCants (mmccants@io.com)
Date: Wed Mar 22 2006 - 15:12:37 EST

  • Next message: Lutz Schindler: "Observation 22.03.06"

    Greg posted:
    
    >HIGHFLY:
    >  Time  alt azi
    > 23:40   37 260
    >
    >GUIDE 8 (SDP4)
    >Date           Alt  Azim
    >20 Mar 23:40  39.8 265.2
    
    Note that although these two predictions differ by 3 degrees
    in altitude and 5 degress in azimuth, this difference is really
    a difference of about 50 seconds in the time of prediction.
    In other words, a Highfly prediction for 50 seconds later
    would match the Guide prediction or a Guide prediction
    for 50 seconds earlier would match the Highfly prediction.
    
    I loaded SkyMap 6.6 onto my work computer and verified that
    its SDP4 prediction for 21:40 UT on March 20 for Greg matched
    this Guide prediction.  Then I used an option for SkyMap
    and changed the prediction model from SDP4 to SGP.
    Then I verified that the SkyMap prediction matched the Highfly
    prediction.  So my conclusion is that SGP (also SGP4) differ
    from SDP4 in this case by about 50 seconds in time.
    
    Since this object passes by the same place in the sky at
    nearly the same time every night, it was easy enough for me
    to change the date of prediction from 3/20 back to 3/12.  This
    allowed me to compare SGP and SDP4 for an time very near the
    epoch of the elset.  The 50 second difference between the
    two models remained the same.  So it appears to be a fundamental
    discrepancy between the two models.
    
    I have never used SDP4 - I always use SGP or SGP4 - even for
    the objects that are higher than the SDP4 limit.  The particular
    elset for 90027 that Greg used was an elset that I generated
    with my SGP4 programs.  But if Spacecom was using SDP4 to generate
    elsets for eccentric objects and if a 50 second discrepancy between
    Spacecom elsets and the model in Highfly was to be expected, you
    would think that I would have noticed this some time in the last
    15 years.  :-)
    
    Mike McCants
    
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