Re: significantly different predictions by different programs

From: Mike McCants (mmccants@io.com)
Date: Mon Oct 30 2006 - 18:15:47 EST

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    Ed Cannon posted:
    
    >It has come to my attention that among the programs being used for
    >predictions, there are some significantly differing results for
    >at least some objects.  A specific case is ETS 6 (94-56A, 23230).
    >
    >Orbitron and Heavensat gave predictions that were something like
    >one hour of RA different from what Highfly predicted...
    
    I have installed and tested Heavensat and I believe that its
    computations are correct, but I found that one of its displays
    could be misleading.
    
    In particular, I tested a case where ETS 6 was low in my south
    rising towards apogee:
    
    30.3N  97.8W  BCRC 2006 Oct 28 Times are UT 23230 ETS 6 94 56A
     Hrs Min   Alt   Azi    Hgt  Range    R  A     Dec (2000)
       2   0  27.8 152.6   9643  11298  23 41.36 -26.48
       2  10  26.6 148.0  10447  12190   0 10.02 -25.53
    
    The SeeSat program and Ted's ObsReduce produced very similar answers.
    
    When I ran Heavensat and asked it to find a passage near a star
    during this 20 minute interval, it produced a prediction that the
    satellite would pass about 1/10 of a degree from the star SAO 192166
    at 02:00:51.  The altitude was 27.5 and the azimuth was 152.0.
    When I changed the field of view to 2 degrees, the mag 6 star was
    just above the plotted satellite trajectory.
    
    When I ran Heavensat in the "passes prediction" mode, the output
    was the entire 14 hours that ETS 6 was above my horizon (even though
    the duration was set to 20 minutes).  The [***] was around the maximum
    altitude of the pass - alt 62, azi 153, time 12:22.  So this was 10
    hours after the 20 minute "duration" of the prediction.  I could
    zoom in on the 02:00 part of the trajectory and the alt/azi was
    correct, but the 6th magnitude star did not appear.  It seems that
    the stars are in their proper places for 12:22 UT, not 02:00 UT.
    There are no RA and Dec labels on this display.  This is confusing.
    So I can see how you could select a part of the trajectory that
    was one or more hours off of the time of prediction and the stars
    would not be in the right place for that part of the trajectory.
    
    Mike McCants
    
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